To reduce the carbon dioxide in the air to organic compounds, cells
need a source of electrons, which, as a rule, are carried by hydrogen atoms.
The source of these electrons varies with the organism. In green sulfur bacteria (chlorobia of B-8), the electrons come from hydrogen sulfide (H
2
S),
although they may also come from hydrogen gas (H2
). The purple sulfur
bacteria (B-3) also use H2
S as the hydrogen donor. In purple nonsulfur
bacteria, such as Rhodospirillumand Rhodopseudomonas(B-3), the hydrogen donor is a small organic molecule such as lactate, pyruvate, or ethanol.
Thus, the general phototrophic equation can be written as