Can Organisms Regulate Global Biogeochemical Cycles?

Global biogeochemical cycles are being profoundly affected by human activities; therefore, it is critical to understand the role played by organisms in their regulation. Autotrophic organisms can regulate nutrient abundance at local scales through resource consumption, but most resources are inaccessible to them at global scales, either because of physical barriers or because of the presence of non-assimilable chemical forms of nutrients. Here we present a generic model of resource access limitation and apply it to the oceanic cycles of iron, phosphorus, and silicon to examine whether phytoplankton can regulate the concentrations of these key nutrients. Our model predicts that autotrophs cannot at the same time strongly impact accessible nutrients and exert perfect regulation on inaccessible nutrients. We show that the ability of organisms to regulate inaccessible nutrient pools strongly depends on passive physical and chemical flows, and on the fraction of the system that is accessible to organisms. Components of global climate change such as increasing water column stratification might result in a further decrease of the biotic regulation of inaccessible nutrients in freshwater and marine systems.