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Applied and Environmental Microbiology, February 2003, p. 779-786, Vol. 69, No. 2
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.2.779-786.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Hydrogen-Dependent Oxygen Reduction by Homoacetogenic Bacteria Isolated from Termite Guts

Hamadi I. Boga{dagger} and Andreas Brune*

Fachbereich Biologie, Mikrobielle Ökologie, Universität Konstanz, D-78457 Konstanz, Germany

Received 18 June 2002/ Accepted 13 November 2002

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 H2-CO2. The position of the bands coincided with the oxic-anoxic interface and depended on the O2 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 O2 and H2 electrodes, revealed a large capacity for H2-dependent oxygen reduction in Sporomusa termitida and Sporomusa sp. strain TmAO3 (149 and 826 nmol min-1 mg of protein-1, respectively). Both strains also reduced O2 with endogenous reductants, albeit at lower rates. Only in Acetonema longum did the basal rates exceed the H2-dependent rates considerably (181 versus 28 nmol min-1 mg of protein)-1). Addition of organic substrates did not stimulate O2 consumption in any of the strains. Nevertheless, reductive acetogenesis by cell suspensions of strain TmAO3 was inhibited even at the lowest O2 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 O2 at low partial pressures (up to 1.5 kPa in the case of strain TmAO3) and because of their large capacity for O2 reduction, homoacetogens can reestablish conditions favorable for growth by actively removing oxygen from their environment.

* Corresponding author. Mailing address: Fachbereich Biologie, LS Mikrobielle Ökologie, Universität Konstanz, Fach M 654, 78457 Konstanz, Germany. Phone: 49-7531-883282. Fax: 49-7531-882966. E-mail: andreas.brune{at}uni-konstanz.de.

{dagger} Present address: Botany Department, Jomo Kenyatta University of Agriculture and Technology, Nairobi, Kenya.

Applied and Environmental Microbiology, February 2003, p. 779-786, Vol. 69, No. 2
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.2.779-786.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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