Using energy flows to characterize microbial assemblages across landscapes

Background/Question/Methods   Microbially mediated litter decomposition is a critical step in the cycling of carbon and nutrients through ecosystems. The application of energy principles to describe the metabolic activities of microbial assemblages under varying environmental contexts may provide insights into the relationship between microbial organization and carbon dynamics at the landscape scale. From the viewpoint that microbial communities behave “rationally” in their resource use to acquire carbon and energy, basic questions arise as to how microbial communities differ in their acquisition of energy from a given substrate. In this preliminary investigation I asked (1) How do conventional measures of decomposition (rate of material loss) relate to energy loss from substrate? (2) Do energy loss patterns from remaining substrates follow climatic sequences or other environmental gradients?

Results/Conclusions   Archived material from an historic study in which the leaf litter of a single tree species, Metrosideros polymorpha, was set to decay in 10 different sites on the island of Hawai'i. Archived residues of leaf litter collected at 0, 3, 12, and 24 months were scanned using Fourier transform infrared spectroscopy to provide a chemical signature of material at each time interval. I investigated the “energy release curve” of a given substrate—the order in which molecular bonds are attacked and the resulting calorie content of remaining residues over time. Preliminary results graphically illustrate the distinction between carbon transformation (energy utilization) and gross measures of mass loss. Comparisons of the resource-utilization trajectories from decomposition sites with identical decay rates of a common litter showed that equal rates of mass loss do not equal identical patterns of resource consumption. Thermal analysis confirmed differences in the energy content of residual materials, suggesting differences in energy flows through the detrital food web in sites that exhibited equal decay rates. No patterns in residual energy content were observed along precipitation or nutrient gradients. While rudimentary, these simple observations recommend the application of thermal (energy) analysis coupled with fine-scaled measures of matter transformations to compare the functional capacities of microbial communities. Future work will combine measures of microbial biomass and activity with those of substrate transformation to test the notion of functional redundancy of soil microbial composition in resource use.