Oxygenic phototrophs. metabolism

Equation (25) describes a process of electron transfer from donor molecules to CO2 (for both oxygenic and anoxygenic photosynthesis). Unlike the anaerobic processes described in Section 3, in which the electron transfer is frequently extracellular (between cells), the electron transfer in photosynthesis is intracellular. As such, it can occur completely via biological electron carriers, and without need for free electron carriers such as H2. However, the presence of hydrogenase enzymes in representatives from both classes of phototrophs [2,24,25] makes H2 metabolism possible in both accessory and primary roles. 

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