Fermentative hydrogen production by a new chemoheterotrophic bacterium Rhodopseudomonas palustris P4

A newly isolated Rhodopseudomonas palustris P4 for CO-dependent H₂ production was studied for its capability of fermentative H₂ production in batch cultivations. Important parameters investigated include pH, temperature, concentrations of phosphate and glucose, intermittent purging of culture broth by argon gas, and kind of sugars. The pH of the culture medium significantly decreased as fermentation proceeded due to the accumulation of various organic acids, and this inhibited the H₂ production seriously. The use of fortified phosphate at 60-300 mM could alleviate this inhibition. The increase of glucose concentration (1-20 g/l) resulted in higher H₂ production, but the yield of H₂ production (mmol H₂/mmol glucose) gradually decreased with increasing glucose concentration. Intermittent purging of the culture broth by argon gas improved H₂ production. Carbon mass balance showed that, in addition to cell mass, ethanol, acetate and CO₂ were the major fermentation products that comprised more than 70% of carbon consumed. R. palustris P4 could utilize various monosaccharides (glucose, galactose, fructose), disaccharides (lactose, sucrose) and starch to produce H₂. However, the H₂ production rate with disaccharides and starch was much slower than that with monosaccharides. The maximal H₂ yield and H₂ production rate were estimated to be 2.76 mmol H₂/mmol glucose and 29.9 mmol H₂/g cell h, respectively. These results indicate that, although isolated for CO-dependent H₂ production, R. palustris P4 has a high potential as a fermentative H₂ producer.