Effects of temperature on microbial parameters associated with decaying plant litter in a stream microcosm experiment

Increases in mean surface air temperature under climate-change predictions are expected to affect microbial activity, as well as carbon dynamics in aquatic ecosystems. I explored responses of litter-associated decomposers along temperature ranges typical of temperate streams in laboratory microcosms simulating stream conditions. The objectives of this study were to determine (1) whether the effects of temperature on stream leaf-associated microorganisms can be explained by the Metabolic Theory of Ecology (MTE); (2) if the effects of temperature are the same across temperatures commonly found in temperate streams; and (3) if there are differences in the magnitude of responses to temperature among various microbial parameters. The experiment measured physiological responses to temperature (5 levels, 4-20°C) of natural microbial assemblages colonizing Liriodendron tulipifera leaf litter in a stream at the Coweeta Hydrologic Laboratory, NC during peak litter fall. In microcosm experiments, leaf litter decomposition rates, fungal biomass (ergosterol), fungal and bacterial production (radiolabeled tracers), spore production by aquatic fungi, and microbial respiration rates were followed. I found that responses of aquatic litter-associated microorganisms to increases in temperature were more complex than predicted by the MTE, with more pronounced responses (higher apparent activation energy, E) at lower temperatures. For some parameters, estimates of E at lower temperatures were higher than values often reported for respiration (~0.65 eV), suggesting that microbial carbon processing in streams could be especially sensitive to temperature increases during the key winter period of high activity. These trends may have important implications for stream ecosystems under climate-change scenarios, since bulk leaf litter inputs and peak microbial activity coincide with the coldest season (autumn-winter) in temperate streams