Powerful fermentative hydrogen evolution of photosynthate in the cyanobacteriuni Lyngbya aestuarii BL J mediated by a bidirectional hydrogenase

Cyanobacteria are considered good models for biohydrogen production because they are relatively simple organisms with a demonstrable ability to generate H-2 under certain physiological conditions. However, most produce only little H-2, revert readily to H-2 consumption, and suffer from hydrogenase sensitivity to O-2. Strains of the cyanobacteria Lyngbya aestuarii and Microcoleus chthonoplastes obtained from marine intertidal cyanobacterial mats were recently found to display much better H-2 production potential. Because of their ecological origin in environments that become quickly anoxic in the dark, we hypothesized that this differential ability may have evolved to serve a role in the fermentation of the photosynthate. Here we show that, when forced to ferment internal substrate, these cyanobacteria display desirable characteristics of physiological H-2 production. Among them, the strain L. aestuam BL J had the fastest specific rates and attained the highest H-2 concentrations during fermentation of photosynthate, which proceeded via a mixed acid fermentation pathway to yield acetate, ethanol, lactate, H-2, CO2, and pyruvate. Contrary to expectations, the H-2 yield per mole of glucose was only average compared to that of other cyanobacteria. Thermodynamic analyses point to the use of electron donors more electronegative than NAD(P)H in Lyngbya hydrogenases as the basis for its strong H-2 production ability. In any event, the high specific rates and H2 concentrations coupled with the lack of reversibility of the enzyme, at the expense of internal, photosynthetically generated reductants, makes L. aestuarii BL J and/or its enzymes, a potentially feasible platform for large-scale H-2 production.