Acetate Synthesis from H2 plus CO2 by Termite Gut Microbes †

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RESUMO

Gut microbiota from Reticulitermes flavipes termites catalyzed an H2-dependent total synthesis of acetate from CO2. Rates of H2-CO2 acetogenesis in vitro were 1.11 ± 0.37 μmol of acetate g (fresh weight)−1 h−1 (equivalent to 4.44 ± 1.47 nmol termite−1 h−1) and could account for approximately 1/3 of all the acetate produced during the hindgut fermentation. Formate was also produced from H2 + CO2, as were small amounts of propionate, butyrate, and lactate-succinate. However, H2-CO2 formicogenesis seemed largely unrelated to acetogenesis and was believed not to be a significant reaction in situ. Little or no CH4 was formed from H2 + CO2 or from acetate. H2-CO2 acetogenesis was inhibited by O2, KCN, CHCl3, and iodopropane and could be abolished by prefeeding R. flavipes with antibacterial drugs. By contrast, prefeeding R. flavipes with starch resulted in almost complete defaunation but had little effect on H2-CO2 acetogenesis, suggesting that bacteria were the acetogenic agents in the gut. H2-CO2 acetogenesis was also observed with gut microbiota from Prorhinotermes simplex, Zootermopsis angusticollis, Nasutitermes costalis, and N. nigriceps; from the wood-eating cockroach Cryptocercus punctulatus; and from the American cockroach Periplaneta americana. Pure cultures of H2-CO2-acetogenic bacteria were isolated from N. nigriceps, and a preliminary account of their morphological and physiological properties is presented. Results indicate that in termites, CO2 reduction to acetate, rather than to CH4, represents the main electron sink reaction of the hindgut fermentation and can provide the insects with a significant fraction (ca. 1/3) of their principal oxidizable energy source, acetate.

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