Microbial productivity in variable resource environments

The rate, timing, and quality of resource supply exert strong controls on a wide range of ecological processes. In this study, we used field experiments and simulation models to explore how aquatic heterotrophic bacteria respond to variation in quality (low versus high) and schedule (Press versus Pulse) of terrestrial-derived resource subsidies. Field experiments revealed that one-time Pulse additions of resources in the form of dissolved organic carbon (DOC) caused short-lived (<48 hr) peaks in bacterial productivity (BP), which translated into large differences across treatments: cumulative BP was twice as high in the Pulse versus Press treatment under low resource quality, and five times as high under high resource quality. Importantly, bacteria were much less productive when DOC was supplied more regularly, suggesting resource quality is conditional upon resource schedule. Modeling results suggest the mobilization rate of refractory to labile carbon, an index of resource quality, was critical in determining cumulative differences in BP between Pulse and Press resource environments (BP_Pulse:Press ratios). Moreover, BP_Pulse:Press ratios were substantially larger when our model allowed for realistic changes in bacterial growth efficiency (BGE) as a function of bacterial consumption (BC). Together, our study suggests that resource schedule is important in determining the flow of material and energy from microbes to higher trophic levels in aquatic food webs. An improved understanding of the effects of resource variability on microorganisms is therefore critical for predicting potential changes in ecosystem functioning in response to environmental change, such as altered DOC fluxes from terrestrial to aquatic ecosystems.