Microaerobic dark fermentative hydrogen production by the photosynthetic bacterium, Rhodobacter capsulatus JP91

The photosynthetic bacterium Rhodobacter capsulatus produces hydrogen under nitrogen-limited, anaerobic, photosynthetic conditions. The present study examined whether R. capsulatus can produce hydrogen under microaerobic conditions in the dark with limiting amounts of O-2 and fixed nitrogen. The relationship between hydrogen production, different O-2 concentrations and carbon sources as well as two different N sources, glutamate and ammonium, were studied in batch culture using a Hup strain of R. capsulatus. The effect of different O-2 concentrations, ranging from 0.5 to 20%, on hydrogen production was examined in dark batch cultures of R. capsulatus grown on RCV medium. Different carbon sources, e.g. glucose, succinate, lactate, acetate and malate, were used at various concentrations (20-40 mM). Similarly, different concentrations of glutamate and ammonium (2-9 mM) were examined for optimum microaerobic dark hydrogen production. Maximum hydrogen production was observed at an O-2 concentration of 4-8%. There was a highly positive correlation between O-2 and growth (r(2) = 0.67), whereas O-2 concentration and hydrogen productivity were negatively correlated (r(2) = 20.3). Succinate (25 mM) together with glutamate (3.5 mM) gave the highest specific hydrogen productivity [5.61 mmol hydrogen/(mg cell dry weight/ml)]. The maximum average hydrogen yield was 0.6 mol hydrogen/mol malate followed by 0.41 mol hydrogen/mol lactate, 0.36 mol hydrogen/mol succinate, whereas minimum amounts of hydrogen were produced from glucose and acetate (0.16 mol hydrogen/mol and 0.07 mol hydrogen/mol, respectively). The implications for developing a system capable of improved hydrogen production are discussed.