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Applied and Environmental Microbiology, October 1999, p. 4490-4496, Vol. 65, No. 10
Fakultät für Biologie,
Mikrobielle Ökologie, Universität Konstanz, 78457 Konstanz,
Germany
Received 4 June 1999/Accepted 4 August 1999
It has been shown that the coexistence of methanogenesis and
reductive acetogenesis in the hindgut of the wood-feeding termite Reticulitermes flavipes is based largely on the radial
distribution of the respective microbial populations and relatively
high hydrogen partial pressures in the gut lumen. Using Clark-type
microelectrodes, we showed that the situation in Cubitermes
orthognathus and other soil-feeding members of the subfamily
Termitinae is different and much more complex. All major compartments
of agarose-embedded hindguts were anoxic at the gut center, and high
H2 partial pressures (1 to 10 kPa) in the alkaline anterior
region rendered the mixed segment and the third proctodeal segment (P3)
significant sources of H2. Posterior to the P3 segment,
however, H2 concentrations were generally below the
detection limit (<100 Pa). All hindgut compartments turned into
efficient hydrogen sinks when external H2 was supplied, but
methane was formed mainly in the P3/4a and P4b compartments, and in the
latter only when H2 or formate was added. Addition of
H2 to the gas headspace stimulated CH4 emission of living termites, indicating that endogenous H2
production limits methanogenesis also in vivo. At the low
H2 partial pressures in the posterior hindgut, methanogens
would most likely outcompete homoacetogens for this electron donor.
This might explain the apparent predominance of
methanogenesis over reductive acetogenesis in the hindgut of
soil-feeding termites, although the presence of homoacetogens in the
anterior, highly alkaline region cannot yet be excluded. In addition,
the direct contact of anterior and posterior hindgut compartments in
situ permits a cross-epithelial transfer of H2 or
formate, which would not only fuel methanogenesis in these
compartments, but would also create favorable microniches for
reductive acetogenesis. In situ rates and spatial distribution of
H2-dependent acetogenic activities are addressed in a
companion paper (A. Tholen and A. Brune, Appl. Environ. Microbiol.
65:4497-4505, 1999).
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Hydrogen Profiles and Localization of Methanogenic
Activities in the Highly Compartmentalized Hindgut of Soil-Feeding
Higher Termites (Cubitermes spp.)
*
Corresponding author. Mailing address: Fakultät
für Biologie, 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.
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