Resource Mismatch in Phototrophic Subaerial Biofilms and Resulting Phototroph/Heterotroph Ratios

Photosynthesis-driven microbial communities are frequently observed on air-exposed stone such as, for example, marble surfaces. These environments are stark, subject to varying water, temperature, UV and other stresses. As a consequence, resident microbes likely have significant maintenance requirements. Those microbes however, have access to plentiful energy when conditions are favorable for photosynthesis. High rates of photosynthesis for purposes of energy generation may also generate excess organic carbon which can be released as extracellular overflow, becoming available for heterotrophic organisms. Here we propose a simple model to explore possible consequences of this hypothesized resource mismatch. The model predicts that heterotrophs can invade purely phototrophic communities, with the resulting mixed community eventually approaching a phototroph/heterotroph ratio at steady state as a consequence of stoichiometric balance. Results from field observations at three geographically separated sites (Lincoln Memorial, Washington D.C., Merchants' Exchange, Philadelphia PA, and the Federal Hall National Memorial, New York NY), analyzed using two independent methods, are seen to match model predictions, demonstrating both cyanobacterial dominated communities as well as, mostly, mixed communities with a heterotrophs-to-cyanobacteria biovolume ratio of approximately 0.4.