In this case, growing in the light photosynthesis is used for growth and to store car­bohydrates. When the organism is switched to aerobic dark conditions the stored carbohydrates or cell material is metabolized in the same way as in Fig. 5.12, yielding hydrogen. This type of two-stage process has been observed in Cyanobacteria sp.

Photo-fermentation

Some photoheterotrophic bacteria (purple non-sulfur bacteria) such as Rhodobacter sp. and Rhodospirillum sp. convert organic acids in the light into carbon dioxide and hydrogen (Fig. 5.13). The key enzyme in these organisms is nitrogenase which requires ATP to produce hydrogen. The nitrogenase is inhibited by oxygen, ammonia and high nitrogen to carbon ratios so that oxygen-free conditions are required.

Carbon monoxide metabolism (water-shift reaction)

Some photoheterotrophic bacteria, for example Rhodospirillum rubrum, can metab­olize carbon monoxide in the dark in a reaction similar to the water-shift reaction:

CO + H2O = CO2 + H2 (5.11)