Phosphorus resource heterogeneity in microbial food webs

Food webs are often regulated by the bottom-up effects of resource supply rate. However, heterogeneity within a resource pool may also affect the structure and function of communities. To test this hypothesis, we measured the responses of aquatic microbial food webs in experimental mesocosms to the addition of 4 different phosphorus (P) sources: orthophosphate (PO43-), 2-aminoethylphosphonate (AEP), adenosine triphosphate (ATP), and phytic acid (PA). Based on 16S rRNA gene sequencing, we found that P resource heterogeneity altered community assembly for bacteria and eukaryotic algae, suggesting that these microbial functional groups may be comprised of P-specialists. In contrast, cyanobacteria were relatively unaffected by our treatments, suggesting that these microorganisms may adopt a more generalist strategy for P-acquisition. Furthermore, our results revealed that P resource heterogeneity affected food web and ecosystem attributes such as nutrient concentrations, bacterial productivity, algal biomass, and ecosystem respiration. Lastly, we found no evidence for non-additive effects of resource heterogeneity based on a treatment where a set of mesocosms received all 4 sources of P. Instead, our results support the view that there may be non-substitutable classes of P in aquatic ecosystems. Specifically, microbial food webs were more sensitive to P-containing biomolecules (PO43- and ATP) than P-containing structural or storage molecules (AEP and PA). Our results demonstrate that not all P resources are the same; although historically overlooked, P resource heterogeneity may have important implications for understanding and predicting the structure and function of aquatic communities.