Dark fermentation reactions do not require light energy.  These are capable of constantly producing hydrogen from organic compounds throughout the day and night. Typically these reactions are coupled to the formation of carbon dioxide or formate.  Important reactions that result in hydrogen production start with glucose, which is converted to acetic acid:
:CHO  +  2 HO   →  2 CHCOH  +  2 CO  +  4 H
A related reaction gives formate instead of carbon dioxide:
:CHO  +  2 HO   →  2 CHCOH  +  2 HCOH  +  2 H
These reactions are exergonic by 216 and 209 kcal/mol, respectively.
Using synthetic biology, bacteria can be genetically altered to enhance this reaction.
Photofermentation differs from dark fermentation, because it only proceeds in the presence of light. Electrohydrogenesis is used in microbial fuel cells.