Diatom Cultivation for Biofuel, Food and High-Value Products E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Serie: Diatoms: Biology and Applications
- Sprache: Englisch
This unique book examines the techno-economic prospects of diatom cultivation, the design and implementation of algal reactors, and the potential of diatoms as a source of biofuel and other value-added products.
Diatom Cultivation for Biofuel, Food and High-Value Products covers the scientific, economic, and practical aspects of using diatoms for multiple purposes. It explores an integrated approach to diatom cultivation, including discussions on techniques, harvesting methods, and innovative technologies. The book discusses the potential of these techniques for improving the efficiency and yield of diatom-based biofuels, as well as the challenges and ethical considerations associated with genetic engineering.
Readers of the book will discover a wealth of information including:
- The adaptation of chitosan-based harvesting methods for microalgae flocculation; the trends, scope, and techno-economic prospects of diatom cultivation, including the design and implementation of algal reactors and the potential of diatoms as a source of biofuel and other value-added products.
- Advanced applications and innovative techniques in the field of diatoms and microalgae such as an in-depth analysis of the pigments and proteins found in Phaeodactylum tricornutum; the nature and applications of diatom cell walls, including their purification processes and industrial uses; the biochemical engineering of diatoms for health and biorefinery concepts, highlighting the potential of diatoms in producing biofuels and other high-value products; the metabolic and transcriptomic stress and engineering of diatoms to enhance lipid production, exploring the stress conditions that can increase oil yield; explores the genetic engineering techniques, such as CRISPRCas9 and RNA interference.
- The environmental and industrial applications of diatoms for low-value products, such as diatom as a prospective green anode material; diatom cell disruption and milking via a nano biorefinery for biofuel production, utilizing techniques like pulsed electric fields, high-pressure homogenization, ultrasonication, etc; genetic engineering and metabolic engineering in diatoms for oil production; the use of diatoms for heavy metal bioremediation, exploring the mechanisms of heavy metal uptake by diatoms, including biosorption and bioaccumulation; the transesterification of diatom oil and parameters for optimization; diatom harvesting for lipid production like bubble wrap (Bubble Farming).
Audience
The book serves as a guide for researchers and scientists in phycology, biology, ecology, environmental science, biofuels, bioengineering as well as nutritionists and dieticians who design functional foods and nutraceutical products.
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Seitenzahl: 743
Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Series Page
Title Page
Copyright Page
Preface
Acknowledgements
Part I: Culture Methods
1 Adaptation of Chitosan-Based Harvesting Methods for Flocculation of Microalgae
1.1 Microalgae
1.2 Microalgae Cultivation and Challenges
1.3 Microalgae Harvesting: Technological Limitations and Needs
1.4 Harvesting Methods
1.5 Chitosan as Natural Flocculant
1.6 Chitosan in Conjunction with Other Physicochemical Methods
1.7 Comparison of Different Harvesting Methods
1.8 Conclusion
References
2 Diatoms Cultivation: Trends, Scope and Technoeconomic Prospects
2.1 Introduction
2.2 Cultivation Strategy and Production
2.3 Design and Implementation of a Prototype Algal Reactor
2.4 Potential of Diatoms as a Source of Biofuel with Value-Added Products
2.5 Industrial Aspects of Diatoms as a Source of Biofuel
2.6 Economic Feasibility Assessment
2.7 Biochemical Composition
2.8 Feedstock Availability Assessment
2.9 Scope of Diatoms in Biorefinery
2.10 Conclusions and Future Prospects
Acknowledgment
Human and Animal Rights and Informed Consent
References
3 Biochemical Compounds in
Phaeodactylum tricornutum
3.1 Introduction
3.2 Biochemical Compounds
3.3 Demand, Valorization and Biotechnological Applications
3.4 Conclusion
References
Part II: High-Value Products
4 Diatoms: A Natural Resource of High-Valued Products and their Future Prospective
4.1 Introduction
4.2 Biosilicification and Silicification as a Crucial Application in Bone Repair
4.3 Effect of Metals as a Therapeutic Application on Diatom Frustules
4.4 Successful Deposition of Metals on Diatom Frustules
4.5 Biomedical and Environmental Applications
4.6 Deposition of Different Metal Nanoparticles for Various Applications
4.7 Interaction of Diatoms with Peptides and Their Plausible Applications
4.8 Diafuel: A Diatom Application with the Most Potential
4.9 Conclusion
Acknowledgments
References
5 Diatom Cell Wall: Nature Engineered Nanostructures
5.1 Introduction
5.2 Nature of Diatom Cell Wall
5.3 Purification of Diatoms
5.4 Nutritive and High-End Product
5.5 Biofuel Industry
5.6 Factors of Diatom for Producing Biofuel
5.7 Biomedical Industry
5.8 DE Silica for Tissue Engineering
5.9 Nanotechnologically Derived Smart Drug Delivery System
5.10 Future Perspective
5.11 Conclusion
References
6 Biochemical Engineering of Diatoms for Health Benefits
6.1 Introduction
6.2 Chemical Composition of Diatom Biomass
6.3 Microalgae as Hidden Treasure of Novel Drugs for Good Health
6.4 Microalgal Drugs in Preventing Viral Pandemics
6.5 Conclusions
References
7 Metabolism and Transcriptome Stress in Diatom
Phaeodactylum tricornutum
for Value-Added Products
7.1 Introduction
7.2 Commercial Market Value
7.3 Metabolic Pathways and Mechanisms for Synthesis of High Value Added Products in Diatoms
7.4 Light Stress in Diatoms and Fucoxanthin Biosynthesis
7.5 Transcriptomics in Diatoms
7.6 Conclusions
References
8 Terraforming Mars with Microalgae, Especially Diatoms
8.1 Introduction
8.2 Instrumentation to Artificially Simulate Life on Mars
8.3 Diatoms for Long-Term Space Missions
8.4 Potential Diatoms for the BLSS: Taxa Tolerant to Extreme Conditions
8.5 Testing Diatom Growth Under Microgravity Conditions
8.6 Life Support Systems for Space Missions
8.7 Management of the Culture Vessel and Elements
8.8 Conclusions
Acknowledgments
References
9 Diatom: Source of Biofuel and Active Green Anode Material for Advanced Energy Storage Application
9.1 Diatoms – Microalgae with Unique Structure and Properties
9.2 Biofuel Application
9.3 Diatom Silica: Material for Li-Ion Battery Anode
9.4 Conclusion
Acknowledgment
References
Part III: Low-Value Products
10 Milking of Diatoms: A Realistic Approach to Serve the Biorefinery Concept
10.1 Introduction
10.2 Cell Disruption Methods
10.3 Concept of Milking Cells for Value-Added Compounds
10.4 Economic Perspectives of Biofuels and Cell Disruption
10.5 Prospects and Challenges of the Milking Process
10.6 Conclusions
References
11 Dissection of Gene Expression Pattern and Metabolic Profile Under Enhanced Oil Production Conditions in Diatoms
Abbreviations
11.1 Introduction
11.2 Generalized Pathway for Lipid Biosynthesis in Diatoms
11.3 Stress Conditions (Metabolites) Helping to Increase Oil Production
11.4 Changes in Gene Expression in Diatoms During Stress Conditions
11.5 Structural and Functional Aspect of Candidate Genes/Enzymes of Lipid Biosynthesis Pathway
11.6 Role of rDNA Technology in Improving Diatom Strains for Enhanced Lipid Production
References
12 Implications of Diatoms for Heavy Metal Bioremediation
12.1 Introduction
12.2 Mechanism for Heavy Metal Removal by Diatoms
12.3 Bioremediation and Biosorption of Heavy Metals
12.4 Challenges
12.5 Advantage of Diatoms Over Other Techniques and Algae
12.6 Production of Diatoms on a Commercial Scale and Its Application
12.7 Future Aspects
12.8 Conclusion
References
13 Optimizing Bioenergy from Diatoms through Biofilms
13.1 Introduction
13.2 Different Configurations of Biofilm Cultivation Systems
13.3 Surface Materials for Biofilm Cultivation
13.4 Microalgal Biorefinery
13.5 Conclusion and Future Perspectives
Acknowledgments
Funding
Research Ethics
Animal Ethics
References
14 Diatoms Characteristics and Mass Processing of Lipids for Biofuel Production
14.1 Diatoms
14.2 Reproduction
14.3 Ecology and Distribution
14.4 Morphology and Identification
14.5 Diatom Age, Diversity and Ecological Functions
14.6 Biofuel Production and Types of Biofuels
14.7 Different Methods of Lipid Extraction for Biofuel
14.8 Benefits of Diatoms
14.9 Genetic Engineering and Metabolic Pathway Engineering
14.10 Future Prospects
14.11 Conclusion
Acknowledgment and Funding
Data Availability
References
Index
Also of Interest
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Biomass and lipid productivity under different modes of cultivation ...
Table 2.2 Different diatom species and their relative lipid content in dry cel...
Chapter 3
Table 3.1 Biochemical compounds from diatom their yield, use and type of photo...
Table 3.2 Accumulation of biochemical compounds under stress conditions.
Chapter 4
Table 4.1 General features of oil bodies in diatoms.
Table 4.2 Applications of diatoms in different fields [4.16] [4.25] [4.26] [4....
Chapter 5
Table 5.1 High-value products from microalgae.
Chapter 7
Table 7.1 Fucoxanthin production achieved in some of the diatoms and microalga...
Table 7.2 Genes in
P. tricornutum
and their function.
Chapter 8
Table 8.1 Different diatoms that thrive under different extreme temperature co...
Chapter 9
Table 9.1 Various types of feedstocks and lipid content in diatoms.
Chapter 10
Table 10.1 Cell disruption methods with lipid yields of diatoms.
Chapter 11
Table 11.1 Impact on lipid level in different diatomic species using various s...
Table 11.2 Lipid contents and productivity levels of different microalgae and ...
Table 11.3 List of diatom strains and their gene modification to enhance lipid...
Chapter 12
Table 12.1 Species of diatoms used in the remediation of heavy metals.
Chapter 13
Table 13.1 Analysis of variance table of data collected from different station...
Table 13.2 Eigenvalues of principal components.
Table 13.3 Variation in operating parameters considered for the study.
Table 13.4 FAME composition of biomass harvested from different stations.
Chapter 14
Table 14.1 Fatty acids profiles and components of fatty acids in different dia...
List of Illustrations
Chapter 1
Figure 1.1 Methods for microalgae cultivation.
Figure 1.2 Commercial production of chitosan from shrimp and crab waste.
Figure 1.3 Chemical interaction of chitosan and diatom’s cell surface protein ...
Chapter 2
Figure 2.1 Probable bioactive compounds extracted from diatoms along with appl...
Chapter 3
Figure 3.1 Extraction of proteins, valorization of residual biomass for the ma...
Chapter 4
Figure 4.1 Diatom lipid classification [4.50] [4.60] [4.110].
Figure 4.2 Flowchart depicting plausible applications of peptide-loaded diatom...
Chapter 5
Figure 5.1 Application areas of biosilica.
Figure 5.2 Diatoms: Nature’s engineers for nanotech, drug delivery, filtration...
Chapter 7
Figure 7.1 Value-added products produced diatoms.
Figure 7.2 Flow chart showing commercial value of
P. tricornutum
in various se...
Figure 7.3 Chemical structure of the fucoxanthin molecule.
Figure 7.4 Model of the oxidative and reductive pentose phosphate pathways and...
Figure 7.5 Flowchart showing the biosynthesis pathway of carotenoids
P. tricor
...
Figure 7.6 Schematic representation of transcriptomics in diatoms.
Chapter 8
Figure 8.1 Overview of the SpaceQ chamber. (a) This figure shows the entire se...
Figure 8.2 Time evolution of the simulated environmental parameters inside the...
Figure 8.3 List of some microalgae experimented on space missions. Copyright (...
Figure 8.4 (a) Single loop for cell culture and (b) Open-source cell culture c...
Chapter 9
Figure 9.1 Transmission electron microscope (a, b) and Field emission scanning...
Chapter 10
Figure 10.1 Schematic representation of conventional and milking approach in d...
Figure 10.2 Schematic representation of the milking concept.
Figure 10.3 Schematic illustration of proposed cycle for sustainable extractio...
Chapter 11
Figure 11.1 Schematic representation of lipid biosynthesis pathway in diatoms:...
Figure 11.2 Pathway for lipid enhancement through gene modification.
Chapter 13
Figure 13.1 Laboratory-scale biofilm reactor setup for diatom cultivation.
Figure 13.2 Study sites of bioreactor installation.
Figure 13.3 Mean areal cell density during the growth period.
Figure 13.4 Embedded bright field and fluorescent images of bacterial strains:...
Figure 13.5 Phylogenetic tree revealing species homology of the bacterial stra...
Figure 13.6 Phylogenetic tree revealing species homology of the bacterial stra...
Figure 13.7 PCA plot showing correlation among station’s physicochemical param...
Figure 13.8 Response surface plots of hexane-methanol optimization.
Figure 13.9 Response surface plots of methanol-H
2
SO
4
optimization.
Figure 13.10 Response surface plots of reaction time and temperature optimizat...
Figure 13.11 FAME composition analysis of field harvested biofilm biomass at d...
Figure 13.12 Material balance of microalgal biorefinery.
Chapter 14
Figure 14.1 Diatom demonstrates a number of structural properties, including a...
Figure 14.2 Third-generation biodiesel generation from diatoms and algae.
Figure 14.3 Transesterification reaction and biodiesel production from TAG.
Figure 14.4 A generalized method and procedure of lipids extraction from diato...
Guide
Cover Page
Table of Contents
Series Page
Title Page
Copyright Page
Preface
Acknowledgements
Begin Reading
Index
Also of Interest
WILEY END USER LICENSE AGREEMENT
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Diatoms: Biology and Applications
Founding Editors: Richard Gordon and Joseph SeckbachSeries Editor: Janice L. Pappas ([email protected])
There are about 100,000 species and as many papers written about them since their discovery three hundred years ago. The literature on diatoms shows about 60,000 papers during the period 2006-2023. In this context, it is timely to review the progress to date, highlight cutting-edge discoveries, and discuss exciting future perspectives. To fulfill this objective, this Diatom Series provides a comprehensive and reliable source of information on diatom biology and applications as well as to enhance interdisciplinary collaborations required to advance knowledge and applications of diatoms.
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Diatom Cultivation for Biofuel, Food and High-Value Products
Edited by
Vandana Vinayak
School of Applied Sciences, Dr. Hari Singh Gour Vishwavidhyalaya (A Central University), Sagar, Madhya Pradesh, India
and
Richard Gordon
Gulf Specimen Marine Lab & Aquarium, Florida, USA
This edition first published 2025 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2025 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-394-17448-5
Cover image provided by Dr Vandana Vinayak and Urvashi SoniCover design by Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
Preface
Diatoms are emerging as pivotal players in addressing some of the most pressing environmental and energy challenges of our time. This book delves into the multifaceted world of diatoms, exploring their cultivation, harvesting, and diverse applications. By harnessing the potential of diatoms, we can develop sustainable solutions that benefit both industry and the environment, contributing to a healthier planet for future generations.
The cultivation and harvesting of diatoms present unique challenges and opportunities. This book provides a comprehensive overview of the technological limitations and needs in this field, highlighting innovative methods such as the use of chitosan as a natural flocculant. These advancements are crucial for optimizing the efficiency and effectiveness of microalgae harvesting processes, its advantages over synthetic flocculants, and its potential for large-scale applications.
Diatoms, with their intricate cell wall structures and biochemical compositions, offer immense potential for various applications, including bio-fuel production, bioremediation, and even space exploration. This book examines the technoeconomic prospects of diatom cultivation, the design and implementation of algal reactors, and the potential of diatoms as a source of biofuel and other value-added products. Diatoms are unique among microalgae due to their silica-based cell walls, known as frustules, which exhibit intricate nanostructures. These frustules not only provide mechanical protection but also have potential applications in nanotechnology and materials science.
One of the key features of diatoms is their ability to produce valuable biochemical compounds. This book provides an in-depth analysis of the pigments and proteins found in Phaeodactylum tricornutum, exploring their potential applications in various industries. Diatoms are known for their diverse and unique biochemical compositions, which include pigments such as chlorophylls, carotenoids, and fucoxanthin. These pigments have potential applications in the food, pharmaceutical, and cosmetic industries due to their antioxidant and anti-inflammatory properties.
Biofuel production from diatoms is a promising avenue for sustainable energy. This book explores the methods for lipid extraction from diatoms, the economic feasibility of biofuel production, and the industrial aspects of this process. Diatoms are considered one of the most promising sources of biofuels due to their high lipid content and rapid growth rates.
The potential of diatoms extends beyond Earth, with applications in long-term space missions and life support systems. This book discusses the testing of diatom growth under microgravity conditions and the potential use of diatoms in space applications. Diatoms have been proposed as a potential component of life support systems for long-term space missions due to their ability to produce oxygen and remove carbon dioxide.
Genetic engineering plays a crucial role in enhancing the lipid production of diatoms. This book delves into the genetic modifications and stress conditions that can increase oil production in diatoms, providing insights into the future prospects of this field. Genetic engineering techniques, such as CRISPR-Cas9 and RNA interference, have been used to modify the metabolic pathways of diatoms to enhance lipid production.
The book discusses the potential of these techniques for improving the efficiency and yield of diatom-based biofuels, as well as the challenges and ethical considerations associated with genetic engineering. Further, the work highlighted in this wide-ranging book is divided into three parts:
Part I: Culture Methods. This section introduces the foundational aspects of microalgae and diatom cultivation. Chapter 1 discusses the adaptation of chitosan-based harvesting methods for microalgae flocculation, addressing the challenges and technological limitations in microalgae cultivation and harvesting. The chapter delves into the various methods of microalgae harvesting, including centrifugation, filtration, and flocculation, with a particular focus on the use of chitosan as a natural flocculant. It explores the mechanisms of chitosan flocculation, its advantages over synthetic flocculants, and its potential for large-scale applications. Chapter 2 explores the trends, scope, and technoeconomic prospects of diatom cultivation, including the design and implementation of algal reactors and the potential of diatoms as a source of biofuel and other value-added products. The chapter discusses the cultivation of diatoms in photobioreactors and open ponds, highlighting the advantages and limitations of each system. It also examines the potential of diatoms in the biofuels industry, focusing on their high lipid content and rapid growth rates. The economic feasibility of diatom-based biofuel production is analysed, considering factors such as feedstock availability, production costs, and market demand. Chapter 3 provides an in-depth analysis of the pigments and proteins found in Phaeodactylum tricornutum, exploring their potential applications in various industries. It discusses the diverse and unique biochemical compositions of diatoms, including pigments such as chlorophylls, carotenoids, and fucoxanthin, and their potential applications in the food, pharmaceutical, and cosmetic industries. The chapter also examines the protein content of diatoms, including enzymes and bioactive peptides, and their potential uses in nutraceuticals and functional foods.
Part II: High-Value Products. This section covers advanced applications and innovative techniques in the field of diatoms and microalgae. Chapters 4 and 5 examine the nature and applications of diatom cell walls, including their purification processes and industrial uses. The chapters discuss the silica frustules of diatoms, which exhibit intricate nanostructures and have potential applications in nanotechnology and materials science. They explore the methods for extracting and purifying diatom frustules, as well as their potential for creating high-value products. Chapter 6 discusses the biochemical engineering of diatoms for health and biorefinery concepts, highlighting the potential of diatoms in producing biofuels and other high-value products. The chapter examines the genetic modifications and stress conditions that can enhance lipid production in diatoms, providing insights into the future prospects of this field. Chapter 7 focuses on the metabolic and transcriptomic stress and engineering of diatoms to enhance lipid production, exploring the stress conditions that can increase oil yield. The chapter delves into the genetic engineering techniques, such as CRISPRCas9 and RNA interference, that have been used to modify the metabolic pathways of diatoms to enhance lipid production. It discusses the potential of these techniques for improving the efficiency and yield of diatom-based biofuels, as well as the challenges and ethical considerations associated with genetic engineering. Chapter 8 examines the potential of diatoms in space applications, including long-term space missions and life support systems. The chapter discusses the challenges of cultivating diatoms in space, including the effects of microgravity on growth and metabolism, and explores the potential benefits of using diatoms for cultivation within bio-regenerative life support system (BLSS). Chapter 9 discusses diatom as prospective green anode material.
Part III: Low-Value Products. This section highlights the environmental and industrial applications of diatoms for low value products. Chapter 10 discusses diatom cell disruption and milking via a nanobiorefinery for bio-fuel production, utilizing techniques like pulsed electric fields, high pressure homogenisation, ultrasonication, etc. Chapter 11 discusses genetic engineering and metabolic engineering in diatoms for oil production. Chapter 12 explores the use of diatoms for heavy metal bioremediation, discussing the mechanisms and challenges involved. The chapter examines the high capacity of diatoms for accumulating heavy metals, making them suitable for bioremediation applications. It explores the mechanisms of heavy metal uptake by diatoms, including biosorption and bioaccumulation, and discusses the challenges and limitations of using diatoms for bioremediation. Chapter 13 discusses diatoms for biofuel in biofilm reactors, it discusses transesterification of diatom oil and parameters for optimisation and Chapter 14 discusses diatom harvesting for lipid production like bubble wrap (Bubble Farming), oozing, mechanical pressure, pulse electric field etc.
We are pleased to present this book, which documents the latest advancements in the cultivation, harvesting value added products from diatoms, and applications of microalgae and diatoms. By fostering an interdisciplinary exchange of ideas and actions, we hope to contribute to the innovative design of high-performing systems that provide practical solutions to global challenges. We trust that this book will be a valuable resource for researchers, industry professionals, and anyone interested in the potential of microalgae and diatoms.
There are four general methods for agricultural scale growth of diatoms and other microalgae: Runways; Photobioreactors; Bubble Farming; Biofilms. These vary widely in ease to contaminate; requirement for flat land; gas barrier; water retention; nutrients requirement; scalability; running energy, and cost.
The editors and Clifford Merz have been developing Bubble Farming at a laboratory scale. It needs outdoor, agricultural testing to see if it needs adjustments for variable weather conditions, such as bubble size, UV screening, wind and bird resistance, humans and animals popping bubbles by stepping on it, and when to harvest and reusability. Crops that could grow under air pillows need exploring. Here we have discussed Bubble Farming in Chapters 1 and 14. A combination of Bubble Farming and Biofilm cultivation can be imagined. For example, a large surface area could be provided by 0.3mm diameter steel ball bearings, contained in flat air pillows with a thin layer of water with nutrients. Using directed light to guide motile diatoms and concentrate them is also worth consideration, as this could require no energy.
Vandana Vinayak
Richard Gordon
January, 2025
Acknowledgements
Vandana Vinayak is extremely thankful to enthusiastic team members and students of Diatom Nanoengineering and Metabolism Laboratory and Indo French Project CEFIPRA grant no PPMB 7133/2020 and Dr Hari Singh Gour Vishwavidyalaya (A Central University), Sagar, Madhya Pradesh, India. She is thankful to Professor Richard Gordon and Scrivener Publishers for this opportunity.
Last but not the least her parents, family, and brother Shaheed Major Kapil Vinayak for kept her joyful in this journey.
Richard Gordon is grateful to the authors for contributing this basic material for all who endeavor to grow diatoms.
Part ICULTURE METHODS
1Adaptation of Chitosan-Based Harvesting Methods for Flocculation of Microalgae
Mainavi Patel1, Hirak Parikh2 and Gayatri Dave1*
1P D Patel Institute of Applied Sciences, Charotar University of Science and Technology, Anand, Gujarat, India
2Department of Earth and Environmental Systems, Indiana State University, Terre Haute, Indiana, USA
Abstract
This chapter addresses the potential of microalgae, focusing on diatoms because of their distinctive features and high nutritional value. Single-celled photosynthetic microalgae have been around for three billion years. They have also been used as a food source for humans over 2000 years, with Spirulina gaining global popularity as a dietary supplement. Diatoms’ high lipid content makes them an excellent source for biofuel production, offering a renewable and environmentally friendly alternative to conventional fossil fuels.
The cultivation of microalgae poses challenges due to their small size, low settling rate, and susceptibility to damage during harvesting. Various harvesting methods, such as physical (mechanical), chemical, and biological techniques, have been explored to address these challenges. Flocculation, a widely used method, involves the formation of aggregates (flocs) to separate microalgae from the growth medium or water. The natural polysaccharide chitosan has gained attention as an eco-friendly and effective flocculant for microalgae harvesting. Studies have shown the potential benefits of combining chitosan with other techniques, such as electroflotation, electrolysis, and magnetic nanoparticles, to enhance harvesting efficiency and reduce processing costs. The chapter emphasizes the significance of interdisciplinary research to enhance microalgae cultivation and harvesting methods, unlocking their full potential for sustainable applications in various industries.
Keywords: Chitosan, harvesting methods, flocculation, microalgae
1.1 Microalgae
Microalgae are single-celled photosynthetic organisms residing in aquatic habitats [1.1]. Microalgae have been around for three billion years, and their exceptional potential in a variety of industries, including food, feed, fuel, and pharmaceuticals, has drawn the attention of both researchers and businesses [1.2]. Since microalgae cells include carbohydrates, proteins, lipids, vitamins, antioxidants, and necessary trace elements, they have great nutritional significance for people and animals [1.3] [1.4].
The Chinese first used microalgae (Nostoc) as a food source over 2,000 years ago. Eventually, Spirulina gained global popularity as a food supplement due to its exceptional nutritional properties. Spirulina is composed of 62% amino acids and is a source of vitamins A, B1, B2, B12, and phytopigments like carotenoids and xanthophylls [1.5]. Additionally, it contains essential fatty acids, including linolenic acid, which the human body cannot synthesize.
Microalgae biomass is exceptionally rich in lipids; under certain growth conditions, some strains can accumulate 20–50% triacylglycerols, while others can produce up to 80% lipids [1.6]. Microalgae are an excellent source for producing biofuel due to their high lipid content, higher growth rates, adaptability to environmental conditions and capacity to grow on non-arable ground [1.7] [1.8]. Microalgae-based biofuels also benefit from being renewable and emitting fewer greenhouse gases than traditional fossil fuels.
Diatoms belonging to the Bacillariophyta group have unique siliceous cell walls with intricate patterns and shapes known as frustules [1.9]. They are found in aquatic and semi-aquatic habitats, including freshwater and marine water. Diatoms’ remarkable diversity, ecological significance and biotechnological potential are fascinating [1.10].
1.2 Microalgae Cultivation and Challenges
Microalgae exhibit rapid growth rates; their high photosynthesis rate and ability to store significant byproducts make them suitable for industrial feedstock [1.11]. Furthermore, microalgae cultivation does not require fertile soil, extensive freshwater supplies or herbicides, distinguishing them from other crops [1.12]. Open and closed culture systems have been exploited for maximum cultivation of microalgae. An open cultivation system involves growing microalgae in outdoor environments exposed to natural sunlight and air. These include raceway ponds, shallow ponds and natural open ponds. However, these systems are more susceptible to contamination, temperature fluctuations and other environmental variables.
On the other hand, closed systems such as enclosed bioreactors or photobioreactors entail growing microalgae in a controlled environment. These systems precisely control light intensity, temperature, nutrient concentrations and carbon dioxide levels. As a result, closed systems tend to produce higher biomass and have a lesser risk of contamination (Figure 1.1) [1.13] [1.14].
In specialized microalgae groups, the diatoms encompass diverse species with varying growth requirements. Each species may have specific temperature, light, nutrient, and pH preferences [1.15] [1.16]. Identifying the optimal conditions for diatom growth and understanding the particular needs of target species is crucial for successful cultivation. Also, silica supplementation in cultivating vessels is essential for optimum diatom growth [1.17]. Usually, diatoms are cultivated for two purposes:
Laboratory-level research studies
Aquaculture and bio-fuel production.
Xenic culture cultivates diatoms with other microorganisms [1.18]. This method allows the study of diatoms in a more natural environment, as they often coexist with other natural microbes. Xenic cultures can be established by collecting water samples from natural habitats, such as freshwater or marine environments, and inoculating diatoms and the associated microbial community in culture media.
Figure 1.1 Methods for microalgae cultivation.
On the other hand, Axenic cultures are grown without any other metabolizing cells and can be used to study the pure physiology and metabolism of the diatom itself. This can be particularly useful for studying the biochemical pathways and genetic regulation of diatom growth and metabolism [1.19].
Obtaining biomass is easy if you have the proper seed culture, a suitable cultivation medium, and a light source. However, there are two significant difficulties at two different stages of growth, such as seeding and harvesting:
The environmental sampling often contains diatom cells closely surrounded by grazers. Also, grazers are of similar size to diatoms and hence cannot be separated by filtration, limiting the representation of all of taxas during the enrichment and thereby further cultivation. For this reason, a method for pooling diatom fractions from heterogeneous natural water samples is required.
After cultivation, the biomass must be collected from diluted water for further application.
However, despite the immense potential of microalgae, there are still significant challenges associated with their large-scale biomass production. These challenges include optimizing cultivation techniques, such as selecting suitable strains, optimizing growth conditions, and enhancing cultivation efficiency to achieve higher yields [1.20]. Scaling up production processes and establishing cost-effective harvesting and extraction methods are crucial focus areas. Furthermore, ensuring the sustainability of microalgae cultivation, such as minimizing water and nutrient requirements, preventing contamination, and managing waste byproducts, remains an ongoing challenge.
1.3 Microalgae Harvesting: Technological Limitations and Needs
Harvesting microalgae from reaction vessels such as closed bioreactors to open ponds is another mammoth task. Several factors make harvesting difficult, which are summarized below. Small cell size and low settling rate: Microalgae cells are typically tiny, ranging from 1–10 micrometer in diameter. Their small size and low settling rate make separating them from the growth medium or water difficult. Traditional gravitational force-mediated sedimentation methods could be more effective, requiring developing innovative harvesting techniques [1.21].
The second key reason is that biomass is often found in dilute concentrations scattered in large volumes of water, making the harvesting process even more challenging. Efficient methods are needed to concentrate the biomass before harvesting to reduce energy and cost requirements [1.22].
Cell fragility and sensitivity: Microalgae cells are delicate and can easily rupture or damage during harvesting. Mechanical methods like centrifugation or filtration can cause cell breakage, biomass loss and reduced product quality. Gentle and non-invasive harvesting techniques must be developed to maintain cell integrity and preserve the valuable components of microalgae. Furthermore, the grazers, particularly protozoa and dinoflagellates, negatively impact the biomass yield. Effective separation and purification techniques are necessary to ensure a clean and uncontaminated final product [1.23].
Many microalgae harvesting methods are effective at small scales but struggle to scale up. Commercial production requires scaling up the harvesting process while maintaining efficiency and cost-effectiveness. Additionally, a harvesting method choice can affect the environment. Energy and chemicals may be consumed in some methods, posing environmental concerns. Developing sustainable and eco-friendly harvesting techniques that minimize the ecological footprint is essential.
Addressing these challenges requires interdisciplinary research and development efforts, encompassing engineering, biology, chemistry, and environmental science. By overcoming these obstacles, the efficient and sustainable harvesting of microalgae biomass can be achieved, unlocking the full potential of microalgae for various applications, including bio-fuels, food, feed, and pharmaceuticals [1.24].
Despite these challenges, ongoing research and technological advancements continue to address these limitations and unlock the full potential of microalgae for biomass production. With further exploration and innovation, microalgae show promise as a sustainable and valuable resource for multiple industries, contributing to a more sustainable and greener future.
1.4 Harvesting Methods
Dewatering of microalgal biomass is a crucial step for producing feed, fuel and other high-value products. It has been reported that 20–30% of microalgal biomass cost is due to harvesting costs [1.25]. Although several methods are available, appropriate harvesting methods should be selected based on the end products and properties of microalgal strains. Present harvesting methods include physical (mechanical), chemical, and biological methods. Sedimentation, centrifugation and filtration are the standard physical techniques, along with a few electrical practices, such as flotation and electrolysis. Due to the small size and colloidal stability of microalgal cells in suspension, sedimentation is not viable [1.23]. Centrifugation, commonly applied for high-value products, is costly and cannot be employed for large volumes. It has been reported that separating microalgae by settling and centrifugation consumes harvesting energy of 1MJ/kg of dry biomass, which is more than that required for wood (0.7–0.9 MH/ kg) [1.26]. Filtration can harvest larger species, such as Spirulina; however, smaller species, such as Chlorella, may fail to get purified.
Flocculation is another prominent technique for dewatering microalgal biomass [1.22] [1.27]. This method involves the formation of aggregates (flocs) by separating the solute particles from the solution occurring through one or combined mechanisms: (i) charge neutralization, where the charged microalgal cell surface is neutralized by incorporating oppositely charged ions, polymers or colloids, (ii) electrostatic patch, where binding of opposite charges results in patches, (iii) bridging, where a bridge is formed between two microalgal cell surfaces by the use of polymers and (iv) sweeping, where the smaller particles get entrapped into large flocs as they settle, aiding their quick removal from the liquid [1.28].
Chemical flocculating or coagulating agents have been studied extensively for biomass dewatering. The majority of the two chemical flocculants observed were inorganic and organic. Frequently utilized inorganic flocculants are ferric chloride, aluminum sulfate, ammonia and ferric sulfate [1.29] [1.30]. Uduman et al. reported three inorganic flocculants’ optimum dosage and pH: lime, alum and ferric sulfate. The report suggested that alum and ferric sulfate gave better flocculating ability than lime. Unlike alum and ferric sulfate, the optimum dose of lime was almost twice the quantity. However, lime flocculation would be favorable if the suspension included magnesium or calcium ions [1.31]. Zhu et al. showed that Al2(SO4)3 efficiently increased flocculating Chlorella vulgaris cells with the achieved concentration of 2.5g/L. The flocculating efficiency, as well as the optimum dosage required, varies from cell to cell. Although inorganic flocculants provide good harvesting efficiency, the high cost of chemicals, high dosage and metal ion contamination in biomass residues limits their application for human consumption [1.32].
Organic flocculating agents, such as polyacrylamide, polyacrylic acid, and polyamine, are commonly utilized. Although these water-soluble polymers can be highly effective in low dosages, their toxic monomeric residues lead to questionable consumption due to health and environmental consequences [1.33].
Scientists have recently tried replacing conventional chemical flocculants with nontoxic, eco-friendly, biodegradable, or natural polymers. Biopolymers such as chitosan, tannins, gums, cellulose, and alginates have taken hold.
1.5 Chitosan as Natural Flocculant
Chitosan is a naturally appearing linear polysaccharide made of ß-linked D-glucosamine and N-acetyl D-glucosamine [1.34]. Commercially, chitosan is derived from the waste product of shrimp, chitin (Figure 1.2). Further deacetylation of the acetamido group converts chitin into chitosan [1.35]. The product obtained is a positively charged primary amine group at low pH. Industrially, chitosan is highly resourceful. It is used in biomedical engineering, pharmaceuticals, dentistry, biotechnology, and the food industry [1.36]. Apart from these applications, it is also widely applied in water and wastewater treatment. The biodegradability, nontoxicity and reusability of chitosan make it an ideal flocculant [1.37] [1.38]. In a low pH solution, the positively charged amine group interacts with the negatively charged microalgae surface, which induces flocculation utilizing charge neutralization or bridging mechanisms [1.39].
Figure 1.2 Commercial production of chitosan from shrimp and crab waste.
Usually, chitosan adsorbing on a diatom may attach to another diatom particle’s bare surface, forming bridges through patching and charge neutralization mechanisms. When flocculation and adsorption processes are correlated, tuning the process to obtain the desired floc characteristics is possible. Adsorption and flocculation must coincide with functioning rather than as an independent mechanism.
Furthermore, the molecular dynamic simulation study for chitosan and diatom cell surface protein silaffin by Patel et al. elaborated that the chitosan-silaffin interaction mainly involves the silaffin’s PRO276, VAL278, THR280, SER281, ALA283, SER285, GLN287, GLN290, GLU296, SER310, SER311, ARG312, PRO313, ASP315, and ARG346, the majority of which are the polar residues with an average of 40% contact across the simulation time (Figure 1.3) [1.40]. Here, the -NH2 and -OH functional groups of the chitosan play a crucial role in forming the specific H-bond with the protein residues. The interaction is non-stabilizing, suggesting that chitosan is an ideal diatom flocculant. Also, unlike the other microalgae, Patel et al. applied the chitosan flakes instead of a chitosan solution and observed a pH-independent flocculation of diatoms. They are projecting the lone chitosan flakes for diatom flocculation.
Yang et al. reported the harvesting efficiency of four flocculants (Tanfloc, Cationic starch, Chitosan, Moringa oleifera) for Chlorella vulgaris and Scenedesmus obliquus [1.41]. In their observation, they stated that the pH of culture media affected the harvesting efficiency of flocculants. As chitosan is insoluble in water under alkaline conditions, maximum harvesting efficiency was noted only up to pH 7.0 (90%). Biomass concentration also affected the optimum dosage of flocculants. When biomass was 0.3 g/L, optimum dosage of chitosan was 3 mg/L for C. vulgaris and 1 mg/L for S. obliquus. Even if the biomass concentration was increased to 1.5 g/L, of all the four flocculants, chitosan had the highest harvesting efficiency in less dosage. Furthermore, they have noted that, although chitosan has low dosage requirements, due to the narrow activity pH range, the applicability of chitosan is limited for the selected microalgae.
Hadiyanto et al. performed flocculation of Chlorella sp. using chitosan as a bio-flocculant. They utilized response surface methodology (RSM) with a central composite design (CCD) to improve the flocculation efficiency and biomass recovery parameters. In their study, 400 ml of culture with a 12 mg/ml concentration was tested with varying pH, chitosan concentration, cationic inducer and stirring speed. According to their study, chitosan at a concentration of 100 mg/ml was required to promote flocculation for 15 minutes, followed by adding 50 mg/ml ZnCl2 (cationic inducer) at a pH of 8 and stirring at 240 rpm. A 96.12% flocculation efficiency and 312.5 mg biomass recovery were achieved, which served as a better alternative to synthetic flocculants [1.42].
Figure 1.3 Chemical interaction of chitosan and diatom’s cell surface protein silaffin (purple dashed line indicates H-bond).
A recent study by Islam et al. explored chitosan-mediated microalgae harvesting, followed by producing high-quality bio-crude from harvested biomass. A co-culture of Scenedesmus sp. and Chlorella sp. cultivated in domestic wastewater was utilized for the experiment. The optimization of biomass harvesting was achieved using response surface methodology (RSM) to find the interrelation between independent and response variables. The study concluded that chitosan-harvested microalgae co-culture showed >95% harvesting efficiency at a 17–26 mg/L dosing rate, more than 11 minutes of mixing and >75 rpm mixing speed [1.43].
Due to the limitations of chitosan’s reactivity and processability, researchers have made several modifications worldwide. The efficiency of chitosan can be increased by physical or chemical alteration. Methods such as electroflotation, electrolysis and using magnetic nanoparticles in conjunction with chitosan have been derived to enhance recovery efficiency. Apart from these, the synergistic effects of chitosan and inorganic flocculants have been studied by Vu et al. [1.44].
1.6 Chitosan in Conjunction with Other Physicochemical Methods
1.6.1 Electroflotation and Mechanical Stirring
Electroflotation relies on generating gas bubbles at an electrode surface, typically through water electrolysis. The gas bubbles attach to the targeted algae cells, causing them to rise to the surface and form a floating layer, which can then be easily separated from the liquid medium. Once the algal cells have accumulated at the liquid surface, the foam layer is collected using various methods. The foam can be mechanically skimmed off or siphoned, and the harvested algal biomass can then undergo further processing, such as dewatering or drying, for subsequent applications.
Flocculation combined with electroflotation for varying pH and cell concentrations for Chlorella sp. was studied by Zhou et al., who noted a rise of 5% in the flocculation efficiency of chitosan. According to the report, chitosan had >90% flocculation efficiency at pH slightly acidic to neutral at a concentration range of 8–12 mg/L. On the other hand, with an increase in microalgal concentration, the dosage of chitosan needed to be increased. For 0.63 g/L biomass, >85% flocculating efficiency was observed at 12 mg/L chitosan concentration. Although chitosan alone gave appropriate flocculating efficiency, stirring enhances the formation of flocs. Thus, optimum stirring parameters were determined for 0.5 g/L biomass density with 10 mg/L chitosan concentration. According to that study, a high stirring rate reduces the mixing time and damages the cells; hence, flocculation efficiency can be improved at 90–120 rpm with a mixing time of 30 s [1.45].
1.6.2 Electroflocculation
Electrocoagulation is an electrochemical process that uses metal electrodes to destabilize and aggregate algal cells in a liquid medium. The metal electrodes typically release metal ions (e.g., aluminum or iron). These ions neutralize the negative charge on the algal cell surfaces, causing them to coagulate and form larger flocs. These flocs are then separated by adding chitosan or other flocculants.
Zhu et al. applied an electric current to the system to enhance the charge neutralization capacity of chitosan. According to their report, lone Chitosan (4 mg/L) had only 16.9% efficiency, whereas when an electric current of 0.6A was supplied, flocculation efficiency was raised to approximately 65%. Due to the imposed voltage, microalgae cell surface charges escalated, which was observed by the shift in zeta potential [1.46].
In a study by Fayad et al., electro-coagulation-flocculation (ECF) in batch mode was investigated for microalgae Chlorella vulgaris. The recovery was studied for various stirring speeds, electrode material, current density, pH and inter-electrode distance. In their observations, they noted that aluminum electrodes proved to be more efficient than iron electrodes. After studying the zeta potential, the mechanism of harvesting was elucidated, which showed that at pH 4 and 6, the mechanism was charge neutralization, whereas it was sweeping at pH 8 [1.47].
In yet another report, the potential for ECF was investigated for the recovery of Scenedesmus almeriensis. Here, the authors studied both batch and continuous modes of harvesting. According to the reports, the continuous mode was found to be more realistic to minimize the required residence time. At a conductivity of 5 mS.cm-1, 12 V voltage and 23 mA.cm-2 current density, 98% biomass recovery was achieved. Residence times of 3 to 6 s were enough to recover 85–95% of the biomass, resulting in a reduction of specific energy consumption up to 0.2 kWh.kg-1 [1.48].
Hou et al. reported a novel technique integrating alkali-induced flocculation and electrolysis, named salt-bridge electroflocculation (SBEF). The method was developed to harvest Nannochloropsis oculata. The method provided a high harvesting efficiency of 90.4% under 300mA in 45 minutes. The energy consumption was also reduced to 1.50 Wh/g biomass. A 3.85-fold increase (2.75 to 10.59 µm) in mean floc size was also observed, which led to ease in filtration of flocs from SBEF. Also, SBEF did not bring any external flocculants to biomass, which may affect the algal metabolites [1.49].
1.6.3 Synergistic Effects of Chitosan and Inorganic Flocculants
The maximum harvesting efficiency was observed at a high dosage when inorganic flocculants (ferric chloride and aluminum sulfate) were used as flocculants per liter of Chlorella vulgaris suspension by Vu et al. [1.44]. At 450 mg/L concentration, ferric chloride provided 85% efficiency, while aluminum sulfate provided 80% efficiency. Chitosan and synthetic cationic polymer FlopamTM were also tested. At 450 mg/L, chitosan had only 60% efficiency, whereas Flopam had more than 95% efficiency. Synergistic effects were investigated to enhance chitosan’s flocculation efficiency. An inorganic flocculant was paired with chitosan to double flocculate. A 57% increase was recorded in ferric chloride-chitosan dual flocculation, while a 24% rise was observed when chitosan was paired with the aluminum sulfate [1.44].
1.6.4 Integrated Flocculation
The use of nanoparticles has been widely accepted lately for various applications. In a report by Augustine et al., chemo-magnetic and integrated chitosan-magnetic nanoparticles were utilized to flocculate Parachlorella kessleri-I. Magnetic nanoparticles and chitosan-magnetic nanoparticles were synthesized by the chemical co-precipitation method. Initially, the effect of pH was studied for the flocculation of P. kessleri-I. Later, the impact of pH on different flocculants at a uniform concentration was tested, in which chitosan (75 mg/L) and chitosan-MNPs at pH 8.5 gave 85% and 80% harvesting efficiency, respectively. After identifying the optimum pH of chitosan and CS-MNP, the effect of concentration at a stagnant pH was studied. At pH 8.5, chitosan yielded 85% flocculating efficiency at a concentration of 50 mg/L and >90% at 100 mg/L. Chitosan-coated magnetic nanoparticles showed similar results, where at a concentration of 100–150 mg/L, 85% efficiency was noted. On the other hand, flocculants, such as FeCl3, ZnCl2, Al2(SO4)3 and ZnSO4, offered 70–75% efficiency at lower concentrations. Apart from efficiency, speed and cost, another significant advantage of using chitosan-magnetic nanoparticles was the reusability of the flocculant and the culture medium [1.50].
1.7 Comparison of Different Harvesting Methods
Different harvesting agents and methods have been summarized above. It can be stated that the flocculation efficiency of each compound and each technique depends on the culture organism, its biomass and the culture medium pH. As studied, the mechanism behind flocculation/coagulation is charge neutralization and bridging. Methods such as electroflotation, electrolysis, dual flocculation and chemo-magnetic nanoparticles were reported to shift harvesting efficiency. It was observed that electroflotation increased the efficiency to a certain extent, whereas the efficiency observed for electrolysis was only 65%. When the dual flocculation studies were carried out, it was observed that the synergistic effects of inorganic flocculants with chitosan improved the efficiency. Similarly, instead of using chitosan alone, magnetic nanoparticles coated with chitosan would be an effective alternative with high flocculation efficiency. Although the solitary effect of chitosan was sufficient for flocculation, synergistic effects and utilization of magnetic nanoparticles would be helpful as the dosage requirement would reduce and, simultaneously, the toxicity and energy for the processing would decrease.
In addition, diatoms’ flocculation can be challenged by their intricate silica cell walls, which may require special considerations to prevent damage to the frustules during flocculation.
1.8 Conclusion
As the need for harvesting microalgae increases, biopolymer-chitosan utilization becomes essential. Although the flocculating ability of chitosan is sufficient, new methods developed should be utilized in combination such that the dosage requirement of chitosan can be eased. The flocculant should be selected based on the properties of the microalgal culture to be harvested. Chitosan residues after biomass flocculation are biodegradable and nontoxic; hence, the medium can be reused, reducing nutrient requirements. Alternatives for chitosan can be the consumption of Tanfloc or FlopamTM, which have been reported to provide high harvesting efficiency at low dosages.
Most studies on title flocculants for microalgae demand specific chitosan pretreatments; however, recent reports have suggested that owing to direct interaction between silaffin and chitosan, the chitosan flakes can directly be applied for flocculation of diatoms from a raw water sample. The developed process is highly accurate and precise for diatoms, leaving microalgae and protozoa in the supernatant and giving flocs within 60 minutes.
Furthermore, most other methods are selective towards particular microalgal cells. The authors of this chapter see a need for two sets of flocculants. One group should be selective for specific algal cells and modified according to cell chemistry. At the same time, the other flocculants should have non-selective interaction and can flocculate the different types of algal cells. The chitosan solution at a specific pH could serve the purpose of selective flocculant. In contrast, chitosan flakes can be employed as a less particular flocculating agent for pooling more diatoms from environmental samples.
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