Interactive effects of cell size, temperature and nutrient supply on resource allocation, metabolic rates and growth of marine phytoplankton
Phytoplankton metabolism and growth is highly variable and responsive to environmental conditions. Nutrient supply and temperature are two critical environmental drivers that control phytoplankton growth rates and resource allocation into subcellular components. Cell size is a key functional trait that affects virtually every aspect of phytoplankton biology, and phytoplankton size structure largely controls the trophic organisation of planktonic communities and their ability to export biogenic carbon toward the ocean interior. My PhD thesis project aims to study how the interactions among these three key factors, temperature, nutrient supply and cell size, affect phytoplankton physiology and metabolic activity. The approach consisted of a combination of database analysis and laboratory experiments. The results showed that the unimodal relationship between cell size and maximum growth rate is a robust pattern, as it is maintained irrespective of temperature and the growth rate metric employed. However, growth rates estimates based on fluorescence and chlorophyll tend to exceed those based on carbon and nitrogen, which reflects consistent patterns in the nutrient-dependent variability of cellular composition. Using chemostat and batch cultures of the cyanobacterium Synechococcus, growing under different combinations of temperature and nutrient availability, we found that the effect of temperature on metabolic rates and resource allocation becomes weaker when nitrogen limitation increases. These findings willallow us to advance in the mechanistic understanding of photosynthetic energy allocation in phytoplankton experiencing contrasting environmental conditions, which has implications for ocean productivity and biogeochemical cycling.