The influence of light/dark cycle at low light frequency on the desulfurization by a photosynthetic microorganism

H2S dissolved in water can be converted to elementary sulfur or sulfate by the photosynthetic bacterium Chlorobium thiosulfatophilum. The effects of t he light/dark cycle on cell growth and the rate of sulfide removal were inv estigated to develop an appropriate fermentation strategy. Dark fermentatio n was also studied without addition of H2S and CO2 as electron and carbon s ources. Average specific growth rates of bacterial cultures with a continuo us supply of H2S and CO2 both in light and dark conditions were occurred in the range of 0.008 to 0.009 h(-1), indicating little dependence on the lig ht/dark cycle, but about 25% of the growth rate that was occurred only in t he presence of light. Average H2S removal capacities for cultures grown und er the Light/dark cycles of 14/10, 12/12, and 9/15 h, respectively, with a continuous supply of feed gases, were 0.08, 0.07, and 0.04 mu mol H2S.min(- 1)/mg protein.l(-1) in the dark, and was slightly less than those in the li ght. H2S removal capacity with variation of the light/dark cycle was about 30-60% of that obtained in the continuously illuminated cultures. ATP conce ntration in the dark decreased from 0.43 to 0.37 mg ATP.mg protein(-1) as t he daily dark duration decreased from 15 to 10 h. The production rate for l actic acid from a culture grown without a supply of mixtures of H2S and CO2 gases was 0.218 g Lactic acid.l(-1).h(-1), much more than that grown with a supply of feed gas mixtures. Time-averaged concentrations of lactic acid produced overall during the light and dark periods were 13.7 g lactic acid. l(-1) during the light/dark cycle of 14/10 h without a supply of feed gas, and 3.1 and 2.4 g lactic acid.l(-1) during the cycles of 9/15 and 14/10 h, respectively, with a supply of feed gas.