In photosynthesis, solar energy is used by photosystem II to split water and release oxygen, electrons and protons. Photosystem I uses solar energy to produce the reducing power required to fix carbon dioxide, and the electrons are passed along the electron transfer system, eventually generating ATP. In the direct use of solar energy to convert water into hydrogen the electrons are transferred along the electron transfer chai until the penultimate step catalysed by ferredoxin, where the electrons are transferred to a hydrogenase, converting protons into hydrogen (Fig. 5.12). These reactions are carried out by green microalgae and blue-green Cyanobacteria sp.

However, hydrogenases are inhibited by oxygen so that the concentration of oxygen needs to be kept below 0.1%. In the case of C. reinhardtii oxygen is removed

Glucose

I

I

Fig. 5.11. The pathway involved in the production of acetone and butanol by Clostridium acetobutylicum.

H2O

by respiration but because substrate is consumed the efficiency is low. In some cyano­bacteria such as Anabena cylindrica photosynthesis is split between two types of cells. Photosystem II functions in the vegetative cells whereas the heterocysts contain the carbon-fixing portion and a hydrogenase enzyme which is protected from the inhibi­tory effects of oxygen by a thick cell wall.