Molecular hydrogen production by a thermotolerant Rubrivivax gelatinosus using raw cassava starch as an electron donor

Abstract

Thirteen strains of phototrophic purple non-sulfur bacteria selected from 226 isolates showed the ability to digest raw cassava starch at elevated temperature, 40°C, under illuminated anaerobic conditions. A selected strain, designated as SB24, produced more amylolytic enzyme activities toward raw and cooked cassava starch, when grown in cooked starch than when grown in raw starch. SB24 showed photoproduction of molecular hydrogen using raw cassava starch as an electron donor, when incubated with illumination under anaerobic conditions at 40°C. In small scale (23 mL) culture, SB24 produced hydrogen from raw cassava starch after 20-24 h of cultivation, with a 3-fold higher rate of H2 production and a 3-fold higher total accumulation of hydrogen at 72 h, as compared to when it used malate as an electron donor. In the larger scale reactor containing a 5.5 liter culture of SB24, hydrogen was produced at an earlier time using raw cassava starch than using malate. In addition, the highest rate of H2 production (38.79 mL H2 liter culture-1 h-1) by SB24 with raw cassava starch was 7 times higher than that with malate, while the total volume of H2 accumulated with raw cassava starch (4.61 liters of H2 at 90 h) was almost two-fold higher than with malate. Raw starch from rice, sticky rice, corn and mungbean, could also be used as electron donors for H2 production by SB24 at 40°C. From morphological and biochemical characteristics, comparison of 16S rDNA sequence, and comparative studies of the bacterial characteristics, SB24 was found to be a thermotolerant anoxygenic phototrophic purple non-sulfur bacterium, Rubrivivax gelatinosus.