Microalgal Production for Biomass and High-Value Products

Stephen P. Slocombe, John R. Benemann, Stephen P. Slocombe, John R. Benemann

• Publisher: CRC Press
• Year: 2017
• Print ISBN: 9781482219708
• eBook ISBN: 9781315356235
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Microalgae are a particularly interesting source of products that range from currently marketed human nutritionals and food ingredients, to potential sources of biofuels and animal feeds. Rapid advances in technology and commercial development are taking place worldwide. Importantly, algal cultivation does not compete with agriculture for land, water, and in some cases, fertilizer resources.

Microalgal Production for Biomass and High-Value Products covers the field from a variety of perspectives with 14 chapters contributed by recognized academic experts and industrial practitioners. The book presents the latest technologies and innovations in algal biomass production, from cultivation in open ponds and photobioreactors, to strain selection, synthetic biology, pest control, harvesting, and processing. It explores novel algal products and addresses key issues, including markets, supply chains, business strategies, legal issues, current products, and future prospects.

This book brings together the latest advances of interest to those already working in the field while providing an introduction to those beginning to learn about the promise of microalgae as a sustainable source of both specialty and commodity products. It gives stimulating overviews from many different perspectives that describe how laboratory and applied research are creating advances in commercial microalgae production. It also addresses the still many open questions and challenges in this field.

1 Algal Photosynthesis and Physiology

John A. Raven and John Beardall

CONTENTS

1.1 Introduction

1.2 Photon Harvesting and Transfer to Reaction Centers

1.3 Photochemistry and Downstream Reactions in the Thylakoid Membrane Leading to the Production of NADPH, ATP, and Oxygen

1.4 CO₂ Assimilation Using the Benson–Calvin Cycle: Apparent Inefficiencies and the Means of Countering Them

1.5 Assimilation of Other Resources Needed for Photolithotrophic Growth

1.6 Loss of Reduced Carbon Related to Growth and Maintenance

1.7 Conclusions

Acknowledgments

References

1.1 Introduction

Algae, herein referring to microalgae including cyanobacteria, occur in almost all environments, exhibiting a wide range of specific growth rates and productivities. The diversity of algae includes genotypes that can, through evolution, grow over large ranges of mean and extreme values of flux in photosynthetically active radiation (PAR) of both low and high concentration of dissolved nutrients and of wide ranges in temperature (Raven and Geider 2003). They can also deal with the influences of both abiotic factors, such as salinity and mixing in the water column, and biotic factors, such as grazers and parasites/parasitoids, including viruses (Falkowski and Raven 2007). Biotechnological applications require algae adapt to the light, nutrient, and temperature conditions in the cultivation system, allowing relatively high growth rates of individual cells and overall high culture productivity. In addition, such biotechnological processes must yield the desired outputs, from oils for use as fuels to nutritional supplements, such as carotenoids, polysaccharides, and other cell wall components and even the whole organisms as biomass for use in feed and food. The need for high culture productivities as well as the desired product composition and quality imposes significant limitations on the algal species and strains to be cultivated (Borowitzka 2013; Raven and Ralph 2015).

Within the range of genetically determined attributes of microalgae, there is considerable phylogenetic variation, for example, in light-harvesting and photoprotective pigments and in biomass composition. The genetic potential extends also to determining the extent of phenotypic acclimation to variations in environmental conditions on timescales similar to, or less than, the relatively short generation time of these organisms, from a few hours to days. Organisms isolated from a natural environment and placed in a production system will respond both genotypically, through selection of new more competitive variant strains (adaptation), and phenotypically, by physiological acclimation to such novel environments for optimal utilization of limiting resources (Rosen 1967). However, such selection and acclimation might not be appropriate for biotechnological applications, where the goal is to maximize production either of biomass or of selected cell constituents, such as oils, carotenoids, and polysaccharides. Isolation and screening for particular strains from nature, followed by genetic selection under the cultivation process and by genetic modification, could help to achieve these biotechnological goals to produce the desired products in large amounts and at high productivity. In addition to the cultivation process, the algal biomass must also be harvested, dried, or processed to extract the products of interest and the culture media and unused nutrients recycled, for such a process to be as self-contained as possible (Borowitzka 2013; Raven and Ralph 2015).

To produce specific products, such as oils from microalgae, maximizing the production rate of oil involves minimizing the biomass allocated to all other functions while maintaining or increasing carbon assimilation rates for biomass increase, especially oil production. This requirement underpins the following discussion of algal physiology and photosynthesis, bearing in mind that pathways that achieve a high overall conversion of substrate to product frequently also exhibit lower energetic efficiencies (Odum and Pinkerton 1955; Raven 1984a,b; Bar-Even et al. 2010, 2011, 2012; Fabris et al. 2012; Beardall and Raven 2013; Flamholz et al. 2013; Raven and Ralph 2015). To achieve a “minimum allocation” of energy and other resources to the production of biomass other than the desired product involves a greater energy input in operating the pathways that produce the desired product (e.g., lipids, etc.). Balancing the requirements for catalytic biomass and product formation is a key issue addressed herein, and photon energy harvesting and transformation is the starting point in such a discussion.

1.2 Photon Harvesting and Transfer to Reaction Centers

The absorption of photons in photosynthesis involves pigments (chromophores) bound to specific apoproteins in light-harvesting pigment complexes. These apoproteins alter the absorption properties of the chromophores compared to their free state and facilitate the very high efficiency of absorbed photon excitation energy (exciton) transfer to, ultimately, a reaction center pigment complex, where the transformation of exciton energy into chemical energy takes place. Note that the reaction centers absorb photons at somewhat longer wavelengths than the light-harvesting pigment complexes; this is necessary for the efficient transfer of exciton energy from light harvesting to reaction center complexes (Falkowski and Raven 2007; Raven et al. 2013, 2014; Raven and Ralph 2015). In all oxygenic photosynthetic organisms (except the cyanobacterium Acaryochloris and its relatives, which use chlorophyll d), reaction centers use the red absorption peak of chlorophyll a at 680 nm in photosystem II (PSII) and at 700 nm in photosystem I (PSI).

The range of light-harvesting pigments (Table 1.1) covers the entire visible spectrum. Table 1.1 also shows that there is variation in the specific absorption coefficients of the pigments, which indicates the rate of absorption of photons at a given incident photon flux density at the relevant absorption maximum. Integrating the specific absorption coefficient over the entire absorption spectrum of the pigment allows calculation of its effectiveness in absorbing photons and the photons required for synthesizing the pigment–apoprotein complex. Such calculations show that a dominant factor in variations in resource costs (i.e., energy, nitrogen in protein and, through the need for RNA in protein synthesis, phosphorus) of photon absorption is the quantity of apoprotein associated with each molecule of the pigment (Table 1.1) (Raven 1984a,b, 2013a,b; Raven et al. 2013). The apoproteins of light-harvesting pigment–protein complexes can be the most abundant proteins in photosynthetic cells acclimated and/or adapted to low PAR levels (Table 1.1) (Raven 1984a,b; Raven et al. 2013; Raven and Ralph 2015).

A diversity of light-harvesting pigments allows complete coverage of the 400–700 nm wavelength range, and altering their relative quantities would adjust the absorption spectrum of the organism to that of the incident light quality. The argument as to the utility of a great diversity of pigments applies most clearly to very small cells where there is a minimal self-shading in individual algal cells (the package effect) and the absorption properties of pigment–apoproteins are distinctly shown (Raven 1984a,b, 1994, 1996). Larger cells have more pigments in the optical path through the cell, so there is a larger package effect, resulting in reduced impact of individual pigment species on the overall absorption spectrum than in smaller cells. This argument applies to individual cells and thus is amenable to natural selection for best adaptation to different light quality.

TABLE 1.1

Range of Specific Absorption Coefficients of the Chromophores and the Mass of Apoproteins per mol Photosynthetic Chromophore, of Oxygenic Photosynthetic Organisms and the Range of Photosynthetic Chromophore Apoproteins as a Fraction of Total Cell Protein of Microalgae

Metric	Value and Units	References
Mean (400–700 nm) specific absorption coefficient of photosynthetic chromophores	1212–3367 m^2 mol^−1 chromophore	Raven (1984a)
Mass of apoprotein per mol photosynthetic chromophore	2.0–15.7 kg protein per mol chromophore	Raven (1984a)
Mass of apoprotein of photosynthetic ch...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Preface
• Acknowledgments
• Editors
• Contributors
• Introduction
• Chapter 1 Algal Photosynthesis and Physiology
• Chapter 2 Algal Growth Kinetics and Productivity
• Chapter 3 Microalgae Strain Isolation, Screening, and Identification for Biof uels and High-Value Products
• Chapter 4 Screening and Improvement of Marine Microalgae for Oil Production
• Chapter 5 Estimating the Maximum Achievable Productivity in Outdoor Ponds
• Chapter 6 Genetic Engineering of Microalgae
• Chapter 7 Crop Protection in Open Ponds
• Chapter 8 Molecular Diagnostic Solutions in Algal Cultivation Systems
• Chapter 9 Terrestrial Agriculture and Aquaculture Waste Treatment
• Chapter 10 Supply of CO to Closed and Open Photobioreactors
• Chapter 11 Harvesting and Dewatering of High-Productivity Bulk Microalgae Systems
• Chapter 12 Cultivation of for Astaxanthin Production
• Chapter 13 ALGAFARM
• Chapter 14 Heterotrophic Culturing of Microalgae
• Index