Hydrogen production by photoheterotrophic bacteria is principally sim­ilar to that of blue-green algae, capable of fixing nitrogen and produc­ing hydrogen. The microbes are capable of converting large varieties of organic compounds to carbon dioxide and hydrogen up to 50 kg/(m2 • yr). Practical applications of these bacteria are more of an engineering problem than one of scientific “know-how.” The scope of newer research exists on the noncyclic hydrogen production by these microbes unin­hibited by nitrogen. Dilute wastes can be utilized by the photosynthetic bacteria, which is an added advantage over those of the methane fer — mentors. The conventional fermentation of organic substrates to methane or hydrogen is theoretically limited to 80% and 20%, and prac­tically to 65% and 15%. The difference is accounted for by the synthesis of ATP and cell biomass. ATP is produced in presence of light and reac­tions are driven at its expense, if hydrogen is produced by nitrogenase.