Life on a Stoichiometric Knife Edge: Biogeochemical Interactions and Trophic Interactions in Stromatolites in Rio Mesquites

Abstract

One of the unique features of CCB is its diverse and abundant microbialites (“stromatolites”). For example, in Rio Mesquites there are abundant spheroid or “oncoid” microbialites (akin to fossil stromatolites). These microbial communities form layers of CaCO₃ when photosynthesis raises pH and precipitates CaCO₃. We hypothesized that CaCO₃ precipitation accentuates P limitation in CCB as carbonate deposition can trap phosphate in inorganic and organic forms in the growing matrix. A series of field experiments tested these ideas. In the first, investigators determined the balance of calcification on oncoid stromatolites with and without snails. In the next series of experiments, oncoid stromatolites were fertilized with P, and responses of biomass C:N:P ratios, microbial communities, and performance of snails were assessed. Stromatolite C:P and N:P ratios were high but decreased under P fertilization. Surprisingly, P fertilization stimulated snail performance when C:P was reduced moderately but inhibited growth and increased mortality when C:P was strongly reduced, leading us to propose a “stoichiometric knife edge” for snails, with both high and low C:P being detrimental. Finally, studies in Rio Mesquites explored the process of calcification and supported the concept of an active “competition” for P between calcification and biotic P uptake. We emerge with a view that CCB’s intense P limitation is at least in part a result of widespread CaCO₃ precipitation and that this P limitation imposes a strong stoichiometric limitation on herbivore growth and thus is likely a strong selective factor for consumers.