Microbial communities living in Sphagnum are known to constitute early indicators of ecosystem disturbances, but little is known about their response ( including their trophic relationships) to climate change. A microcosm experiment was designed to test the effects of a temperature gradient ( 15, 20, and 25 degrees C) on microbial communities including different trophic groups ( primary producers, decomposers, and unicellular predators) in Sphagnum segments (0-3 cm and 3-6 cm of the capitulum). Relationships between microbial communities and abiotic factors ( pH, conductivity, temperature, and polyphenols) were also studied. The density and the biomass of testate amoebae in Sphagnum upper segments increased and their community structure changed in heated treatments. The biomass of testate amoebae was linked to the biomass of bacteria and to the total biomass of other groups added and, thus, suggests that indirect effects on the food web structure occurred. Redundancy analysis revealed that microbial assemblages differed strongly in Sphagnum upper segments along a temperature gradient in relation to abiotic factors. The sensitivity of these assemblages made them interesting indicators of climate change. Phenolic compounds represented an important explicative factor in microbial assemblages and outlined the potential direct and ( or) indirect effects of phenolics on microbial communities.