plant surface microbiology.pdf

3 Methaogenic Microbial Communities Asociated with Aquatic Plants 43
Another implication of the observations concerns the structure of the
methanogenic microbial community on the roots, which seem to be very simple,
consisting only of H 2-producing fermenting and H 2-consuming methanogenic
microorganisms. However, experiments with excised rice roots have
demonstrated a more complex community of fermenting bacteria including
vigorous fermentative production of acetate, propionate and butyrate (Conrad
and Klose 1999, 2000). Interestingly, a significant percentage (up to 60 %)
of these fatty acids was produced by reduction of CO 2. The stable isotope signature
of the produced acetate was consistent with the production by CO 2
reduction (Conrad et al. 2002). Acetate production from CO 2 indicates that
homoactogenic bacteria were active, a likely conclusion, since homoacetogenic
Sporomusa are members of the rice root microflora (Rosencrantz et al.
1999). Homoactogens have also been found on the roots of sea grass (Küsel et
al. 1999, 2001). Approximately 30 % of the root epidermal cells of sea grass
were colonized with microorganisms that hybridized with an archaeal probe
suggesting the presence of methanogens (Küsel et al. 1999). Presently, little is
known about the fate of the produced fatty acids. Propionate and butyrate can
potentially be further converted to acetate, CO 2 and H 2 by syntrophic bacteria,
which are present in the anoxic rice soil, followed by H 2/CO 2-dependent
methanogenesis (Krylova et al. 1997). Syntrophic oxidation of acetate, however,
is unlikely since [2- 14 C]acetate was hardly turned over in root preparations
(Lehmann-Richter et al. 1999). The most likely fate of the acetate produced
by the root microflora is its escape into the bulk soil where it is
methanogenically decomposed (Fig. 5). Alternatively, acetate may be a substrate
for anaerobic bacteria using nitrate, ferric iron or sulfate as electron
Fig. 5. Conceptual model of the
localization of methanogenic
archaea (MA), homoacetogenic
bacteria (HAB), methane-oxidizing
bacteria (MOB) and aerobic
bacteria (AB) in vicinity of rice
roots and to each other, and the
flow of organic carbon. The insertion
of lateral roots (and root
tips) are the most likely sites
where O 2 and organic substrates
(e.g., sugars) are released into the
soil