Oxygenic Photosynthesis

Cyanobacteria and green algae are photosynthetic microorganisms widespread in nature that survive even in extreme climatic conditions. They are able to harness solar energy and convert it into chemical energy by simul­taneous splitting of water to molecular oxygen and protons with following fixation of CO2, according to the general equation of photosynthesis:

6CO2 + 6H2O / 6(CH2O)+6O2 (21.1)

The photosynthetic electron transfer reactions are usually divided into two stages—the "light reactions", where light energy is converted into the chemical energy of strong reductants and the "dark reactions", where CO2 is reduced into organic compounds by using chemical energy obtained from the light reactions. The simplified scheme of photosynthetic light reactions is presented in Figure 21.1. Photosynthetic light reactions involve electron flow through three major protein com­plexes: photosystem II (PSII), Cytochrome b6f (Cyt bef), and photosystem I (PSI) embedded into the thylakoid membrane. The light reactions start with capture of pho­tons by the pigment molecules in the antenna complexes and subsequent transfer of light energy to PSI and PSII reaction centers, where primary charge separation occurs and photosynthetic electron transport reactions are initiated.

The two reaction centers, PSII and PSI, function simul­taneously, but in series. PSII is the only known biocatalyst that can oxidize water, which is energetically a poor elec­tron donor. The oxidation—reduction midpoint potential of water is +0.82 V at pH 7. In PSII the photolysis of water is driven by the oxidized reaction center, P680+ (the midpoint potential of P680/P680+ is +1.2 V at pH 7).

The electrons extracted from water on the lumenal side of PSII are transferred via the PSII reaction center, plasto — quinone (PQ), the Cyt bf complex and plastocyanin to PSI, which after excitation directs electrons to ferredoxin (Fd), ferredoxin-NADP+ reductase (FNR) and, finally, to generate reduced nicotinamide adenine dinucleotide phosphate (NADPH). This process is known as the linear electron transport (LET). Concomitantly with electron transfer reactions, protons are transferred inside of the thylakoid lumen creating a proton gradient across the thylakoid membrane, which in turn drives adenosine triphosphate (ATP) production via the ATP synthase complex. Sometimes, the electrons are recycled from NADPH or Fd to PQ in the process known as the cyclic electron transport, whereby DpH is generated without production of NADPH. NADPH produced by LET is further used by carbon metabolism, and many other metabolic pathways. The excess of reduced carbon is stored in cells as carbohydrates or lipids. An unique feature of photosynthetic microorganisms is that under specific conditions, most of them are able to redirect the flow of electrons originated from water splitting to the enzymes that mediate H2 production (Figure 21.1).

Biophotolysis of water by microalgae has been under investigation for over 70 years. H2 production by the anaerobically adapted and CO2-depleted suspension of Scenedesmus obliquus in light was reported for the first time by Gaffron and Rubin (1942). Three decades later, it was revealed that filamentous cyanobacteria, Anabaena cylindrica is also able to evolve H2 and O2 simulta­neously under Ar atmosphere (Benemann and Weare, 1974). Despite intensive research on the structure and function of photosynthetic protein complexes, we are still lacking a fundamental understanding of the molec­ular factors regulating the entire electron transfer chain from water to H2 in oxygenic photosynthetic organisms.

In this chapter, we mainly focus on H2 production by oxygenic photosynthetic microorganisms via the

light-dependent direct and indirect biophotolysis pathways. During direct biophotolysis H2 is derived from the electrons originated from water splitting at PSII, whereas for indirect biophotolysis electrons are mainly supplied by degradation of intracellular carbon compounds produced in photosynthetic carbon reduc­tion reactions.