Phosphorus resource heterogeneity affects the structure and function of microbial food webs

It is well established that phosphorus (P) concentrations tend to structure communities and limit the productivity of both freshwater and marine ecosystems. However, the heterogeneity of P may also affect aquatic foods webs. For example, P exists in inorganic and organic forms of varying sizes and complexity. To determine how this heterogeneity influences microbial food webs, we tested the effects of four different P sources (PO4, phytic acid, 2-aminoethylphosphonic acid, and ATP) separately and as a mixture using a set of experimental mesocosms. Based on multiple model comparisons, we determined that many response variables (nutrient levels, bacterial production, respiration, and NPP) were most sensitive to the supply of inorganic P alone (i.e., PO4); however, in some cases (BGE and chlorophyll) the communities responded similarly to organic sources of P common in biological reactions (i.e., ATP). Because the effects of P on aquatic systems are mediated by the base of the food web, we quantified the effects of our resource manipulations on the composition and diversity of heterotrophic and phototrophic microbial communities using 16S rRNA gene sequencing. P source had strong effects on the microbial communities, consistent with knowledge that microorganisms vary in the ecological strategies used for acquiring different forms of P. In sum, our results suggest that resource diversity provides insight into the phosphorus-driven assembly and functioning of aquatic ecosystems.