Fate of Immediate Methane Precursors in Low-Sulfate, Hot-Spring Algal-Bacterial Mats

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The fates of acetate and carbon dioxide were examined in several experiments designed to indicate their relative contributions to methane production at various temperatures in two low-sulfate, hot-spring algal-bacterial mats. [2-14C]acetate was predominantly incorporated into cell material, although some 14CH4 and 14CO2 was produced. Acetate incorporation was reduced by dark incubation in short-term experiments and severely depressed by a 2-day preincubation in darkness. Autoradiograms showed that acetate was incorporated by long filaments resembling phototrophic microorganisms of the mat communities. [3H]acetate was not converted to C3H4 in samples from Octopus Spring collected at the optimum temperature for methanogenesis. NaH14CO3 was readily converted to 14CH4 at temperatures at which methanogenesis was active in both mats. Comparisons of the specific activities of methane and carbon dioxide suggested that of the methane produced, 80 ± 6% in Octopus Spring and 71 ± 21% in Wiegert Channel were derived from carbon dioxide. Addition of acetate to 1 mM did not reduce the relative importance of carbon dioxide as a methane precursor in samples from Octopus Spring. Experiments with pure cultures of Methanobacterium thermoautotrophicum suggested that the measured ratio of specific activities might underestimate the true contribution of carbon dioxide in methanogenesis.

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