Acetate Synthesis from H 2 plus CO 2 by Termite Gut Microbes

Abstract

Gut microbiota from Reticulitermes flavipes termites catalyzed an H 2 -dependent total synthesis of acetate from CO 2 . Rates of H 2 -CO 2 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 H 2 + CO 2 , as were small amounts of propionate, butyrate, and lactate-succinate. However, H 2 -CO 2 formicogenesis seemed largely unrelated to acetogenesis and was believed not to be a significant reaction in situ. Little or no CH 4 was formed from H 2 + CO 2 or from acetate. H 2 -CO 2 acetogenesis was inhibited by O 2 , KCN, CHCl 3 , 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 H 2 -CO 2 acetogenesis, suggesting that bacteria were the acetogenic agents in the gut. H 2 -CO 2 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 H 2 -CO 2 -acetogenic bacteria were isolated from N. nigriceps , and a preliminary account of their morphological and physiological properties is presented. Results indicate that in termites, CO 2 reduction to acetate, rather than to CH 4 , 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.