Hydrogen-dependent oxygen reduction by homoacetogenic bacteria isolated from termite guts

Although homoacetogenic bacteria are generally considered to be obligate anaerobes, they colonize the intestinal tracts of termites and other environments that are not entirely anoxic in space or time. In this study, we investigated how homoacetogenic bacteria isolated from the hindguts of various termites respond to the presence of molecular oxygen. All strains investigated formed growth bands in oxygen gradient agar tubes under a headspace of H-2-CO2. The position of the bands coincided with the oxic-anoxic interface and depended on the O-2 partial pressure in the headspace; the position of the bands relative to the meniscus remained stable for more than 1 month. Experiments with dense cell suspensions, performed with Clark-type O-2 and H-2 electrodes, revealed a large capacity for H-2-dependent oxygen reduction in Sporomusa termitida and Sporomusa sp. strain TmAO(3) (149 and 826 nmol min(-1) mg of protein(-1), respectively). Both strains also reduced O-2 with endogenous reductants, albeit at lower rates. Only in Acetonema longum did the basal rates exceed the H-2-dependent rates considerably (181 versus 28 nmol min(-1) mg of protein)(-1)). Addition of organic substrates did not stimulate O-2 consumption in any of the strains. Nevertheless, reductive acetogenesis by cell suspensions of strain TniAO3 was inhibited even at the lowest O-2 fluxes, and growth in nonreduced medium occurred only after the bacteria had rendered the medium anoxic. Similar results were obtained with Acetobacterium woodii, suggesting that the results are not unique to the strains isolated from termites. We concluded that because of their tolerance to temporary exposure to O-2 at low partial pressures (up to 1.5 kPa in the case of strain TmAO(3)) and because of their large capacity for O-2 reduction, homoacetogens can reestablish conditions favorable for growth by actively removing oxygen from their environment.