Photosynthesis

Biotechnological Applications with Microalgae

Matthias Rögner, Matthias Rögner

• Éditeur: De Gruyter
• Année: 2021
• ISBN:
  9783110717044
• Édition: 1
• Sujet: Biological Sciences
• Sous-sujet: Biotechnology

This book assembles state-of-the-art approaches for harnessing light energy as a model to develop natural systems such as biofuels. After the basics and potential of photosynthesis of microalgae it discusses topics from engineering micro-algae towards increased photosynthetic efficiency till the optimization of photobioreactor techniques for enhanced biotechnological applications such as cyanobacteria.

1 Parameters of photosynthesis relevant for a biotechnological application

Hitesh Medipally

Marc M. Nowaczyk

Matthias Rögner

Abstract

This chapter introduces natural photosynthesis and the potential of its redesign in living organisms. Due to their high application potential in biotechnology, microalgae are the main focus of this chapter. We do not cover the potential of semiartificial and artificial photosynthesis as this was recently laid out in a detailed report of Acatech [1]. Instead, we emphasize the significance of such systems for the development and improvement of modified natural systems and show their potential and limitations. Besides the design of such self-reproducing organisms and the state-of-the-art methods involved, we also focus on the design of the environment, especially on the optimization of photobioreactors and new developments. Overall, this chapter serves as an introduction for the more specialized in-depth analyses presented in the following chapters of this book.

1.1 Photosynthesis in microalgae – evolution and limitations

1.1.1 What are “microalgae”?

Microalgae comprise both pro- and eukaryotic single cell organisms living either in seawater or freshwater (Fig. 1). Their sizes range between about 1 and 50 μm with some colonies consisting of cell clusters. As phototrophs, they are capable to survive through photosynthesis, synthesizing high-energy organic compounds from inorganic materials using light as an energy source. Notably, microalgae are primary producers in aquatic environments with tens of thousands of species growing far more quickly than land plants (Fig. 2). As they can easily be (mass) cultivated, they have a great potential to be used in bio-industry, provided they are purified and isolated. Under stable conditions, their energy production efficiency is reportedly around ten times higher than for land plants [2, 3].

Fig. 1: Three microalgae species representing various levels of complexity: evolutionarily Gloeobacter represents one of the most simple structured cells (left, lacking an internal membrane system), followed by Synechocystis PCC 6803 (middle, with internal membrane system consisting of various layers). In contrast to these two bacterial species, Chlamydomonas reinhardtii (right) belongs to the eukaryotes and provides a chloroplast as organelle. Common to all three species is the presence of thylakoid membranes (green), which harbor the components of the photosynthetic electron transport chain (Fig. acc. to ref. 4).

Fig. 2: (A) Evolution of water-splitting photosynthesis goes in line with a transformation of the atmosphere. (B) Distribution of photosynthetic organisms worldwide according to chlorophyll measurements from space (figures from ref. 5, 6).

1.1.2 Which premises are relevant for picking a suitable model organism?

For biotechnological applications – depending on the desired product – it may be mandatory that the organism is genetically transformable, has a low doubling time, is genetically stable and is robust. Additionally, the large range of microalgae existing in nature (most still being uncharacterized) also allows to choose species adapted to extreme environmental conditions such as psychrophilic algae (<15 °C) (tolerating low temperatures) and thermophiles (45–70 °C) which may be advantageous for various biotechnological applications. In this context, it may be useful to mention that the upper limit of microalgae performing oxygenic photosynthesis may be around 70–75 °C, while in case of thermophiles individual components like isolated photosystems (i.e., PS1) can tolerate much higher temperatures. With respect to future mass cultivation and also to the final economic evaluation of the process costs it may be important whether a freshwater or a seawater/marine organism is chosen: In case of a location close to the sea, choosing a marine organism may save considerable costs, especially as freshwater is becoming more and more limiting (current global percentages of seawater vs. freshwater being approx. 97.5%/2.5%).

Here we have chosen Synechocystis PCC 6803 (see above, hereafter Synechocystis) as a model organism to illustrate the internal composition of the photosynthetic machinery and the most decisive parameters for future biotechnological applications. However, in principle, most of these parameters are valid for all kinds of microalgae (and even – with some minor deviations – for higher plants).

1.1.3 Which are the rate-limiting aspects of photosynthetic efficiency?

Figure 3A shows the major electron transport (ET) routes in cyanobacterial membranes: while the outer cytoplasmic membrane provides the respiratory ET chain but lacks both photosystems, the focus of biotechnological engineering should be the thylakoid membrane (TM) which contains the complete photosynthetic ET chain, starting with the light-triggered water-splitting reaction at PS2 up to the acceptor side of PS1, which plays a central role in energy distribution, depending on physiological requirements. Figure 3B, based on structural investigations on the molecular level, reveals that all three intrinsic membrane protein complexes (PS2, Cyt. b₆f complex and PS1) form oligomeric complexes, which in case of PS1 and PS2 are also active as (isolated) monomers, whereas the b₆f complex requires a dimeric state to function. Within the TM, depending on the light conditions, the oligomeric complexes of PS2 and PS1 are in equilibrium with the respective monomers. This reflects the high adaptability of the cellular system to changing light conditions which is difficult/impossible to imitate in a semiartificial system.

Fig. 3: (A) Membrane systems in the cyanobacterial Synechocystis cell and major electron flows in cytoplasmic (CM) and thylakoid (TM) membrane. (B) Crystal structures of the three major photosynthetic electron transport complexes (PS2, b₆f and PS1 [7, 8, 9]) and schematic view of their electron transport routes.

Figure 4 shows the kinetics of electron flow through this light-powered ET chain. Apparently, PQ reduction at PS2 is the rate-limiting step for the light reaction [10]. In the absence of this step and other limitations, the light-triggered reactions of PS2 and PS1 could proceed with much higher speed, which is decisive for an evaluation of the capacity of photosynthetic systems and their potential for biotechnological applications. This can be convincingly demonstrated with a semiartificial system, consisting of isolated photosystems which are immobilized on gold electrodes (Figs. 4 and 5, respectively) [11]. In this case, the function of the Cyt. b₆f complex is replaced by an artificial tailor-made redox polymer, that is, osmium polymers and/or phenothiazine, respectively, which enable to immobilize photosystems on the electrode surfaces [12]. They are not rate limiting and guarantee an optimized downhill electrode transport (Fig. 4B). It could be shown that in case of PS1 the ET rate under these conditions co...