Suppression of methanogenesis by dissimilatory Fe(III)-reducing bacteria in tropical rain forest soils: implications for ecosystem methane flux

Tropical forests are an important source of atmospheric methane (CH(4)), and recent work suggests that CH(4) fluxes from humid tropical environments are driven by variations in CH(4) production, rather than by bacterial CH4 oxidation. Competition for acetate between methanogenic archaea and Fe(III)-reducing bacteria is one of the principal controls on CH(4) flux in many Fe-rich anoxic environments. Upland humid tropical forests are also abundant in Fe and are characterized by high organic matter inputs, steep soil oxygen (02) gradients, and fluctuating redox conditions, yielding concomitant methanogenesis and bacterial Fe(III) reduction. However, whether Fe(III)-reducing bacteria coexist with methanogens or competitively suppress methanogenic acetate use in wet tropical soils is uncertain. To address this question, we conducted a process-based laboratory experiment to determine if competition for acetate between methanogens and Fe(III)-reducing bacteria influenced CH(4) production and C isotope composition in humid tropical forest soils. We collected soils from a poor to moderately drained upland rain forest and incubated them with combinations of (13)C-bicarbonate, (13)C-methyl labeled acetate ((13)CH(3)COO(-)), poorly crystalline Fe(III), or fluoroacetate. CH(4) production showed a greater proportional increase than Fe(2+) production after competition for acetate was alleviated, suggesting that Fe(III)-reducing bacteria were suppressing methanogenesis. Methanogenesis increased by approximately 67 times while Fe(2+) production only doubled after the addition of (13)CH(3)COO(-). Large increases in both CH(4) and Fe(2+) production also indicate that the two process were acetate limited, suggesting that acetate may be a key substrate for anoxic carbon (C) metabolism in humid tropical forest soils. C isotope analysis suggests that competition for acetate was not the only factor driving CH(4) production, as (13)C partitioning did not vary significantly between (13)CH(3)COO(-) and (13)CH(3)COO(-) + Fe(III) treatments. This suggests that dissimilatory Fe(III)-reduction suppressed both hydrogenotrophic and aceticlastic methanogenesis. These findings have implications for understanding the CH(4) biogeochemistry of highly weathered wet tropical soils, where CH(4) efflux is driven largely by CH(4) production.