Drivers of metabolic density-dependence: how resource availability and conspecific cues affect phytoplankton

Metabolism is density-dependent from unicellular to multicellular organisms. Understanding what drives metabolic suppression is important to explain population growth given the link between metabolism and biomass production. In the simplest scenario, metabolic suppression is caused by a reduction in resource availability with increasing population density. But both theory and experiments suggest that organisms can actively downregulate metabolism in crowded conditions. We experimentally disentangle the importance of resource competition and conspecific cues that signal crowding on the metabolism of three phytoplankton species of varying cell sizes at different growth phases. All species downregulated some aspects of their metabolism in response to cues; this response varied in strength but was not explained by differences in species size. The addition of nutrients weakened and, in some cases, completely removed metabolic suppression, indicating that resources mediate responses to cues and that some species are more sensitive to nutrients than intraspecific signalling. Overall, respiration rates were more responsive to cues than photosynthesis, showing a differential regulation of energy intake and expenditure driven by the interplay of resource availability and conspecific cues. These factors also led to rapid plastic changes in cell size possibly related to cell division and growth. Altogether, changes in size and metabolism indicate that cues can trigger self-regulatory adjustments that limit growth, but these effects are modulated by nutrient availability and species traits not related to size. These results suggest that growth predictions solely based on resource availability might overestimate the rates at which organisms and populations grow, with important implications for how we describe species and community dynamics.