FERMENTATION IN CYANOBACTERIA

Although cyanobacteria are oxygenic phototrophic organisms, they often thrive in environments that become periodically anoxic. This is parti cularly the case in the dark when photosynthetic oxygen evolution does not take place. Whereas cyanobacteria generally utilize endogenous st orage carbohydrate by aerobic respiration, they must use alternative w ays for energy generation under dark anoxic conditions. This aspect of metabolism of cyanobacteria has received little attention but neverth eless in recent years a steadily increasing number of publications hav e reported the capacity of fermentation in cyanobacteria. This review summarizes these reports and gives a critical consideration of the ene rgetics of dark fermentation in a number of species. There are a varie ty of different fermentation pathways in cyanobacteria. These include home-and heterolactic acid fermentation, mixed acid fermentation and h omoacetate fermentation, Products of fermentation include CO2, H-2, fo rmate, acetate, lactate and ethanol. In all species investigated, ferm entation is constitutive. All enzymes of the fermentative pathways are present in photoautotrophically grown cells. Many cyanobacteria are a lso capable of using elemental sulfur as electron acceptor. In most ca ses it seems unlikely that sulfur respiration occurs, The main advanta ge of sulfur reduction seems to be the higher yield of ATP which can b e achieved during fermentation. Besides oxygen and elemental sulfur no other electron accepters for chemotrophic metabolism are known so far in cyanobacteria. Calculations show that the yield of ATP during ferm entation, although it is low relative to aerobic respiration, exceeds the amount that is likely to be. required for maintenance, which appea rs to be very low in these cyanobacteria. The possibility of a limited amount oi biosynthesis during anaerobic dark metabolism is discussed.