Role of temperature and nutrients as drivers of resource allocation, elemental stoichiometry, metabolism and growth in marine phytoplankton

We have conducted experiments with both laboratory cultures and natural plankton assemblages, together with basin-scale observations across the Atlantic Ocean, to investigate the interactive effects of temperature and nutrient supply on phytoplankton across multiple levels of biological organization, including molecules, cells, populations and communities. Laboratory data indicate that nutrient supply has a stronger effect than temperature on photosynthetic protein abundance, C:N stoichiometry, photosynthesis and growth. Due to changing resource allocation into photosynthetic machinery, the chlorophyll a (chl a) content of cells is strongly dependent on both temperature and nutrient availability, which has implications for the use of chl a concentration as a proxy for phytoplankton biomass in the ocean. Across the tropical and subtropical Atlantic, experimental nutrient enrichment consistently causes an increase in chl a concentration, picoeukaryote abundance and the contribution of small nanophytoplankton to total biomass, all of which take place irrespective of temperature. Light-harvesting capacity is synergistically stimulated by warming and nutrient addition in both picocyanobacteria and picoeukaryotes. The latitudinal variability in elemental composition of different phytoplankton groups, determined on single cells with X-ray microanalysis across the temperate, subtropical and tropical Atlantic, reveals the effect of changing temperature and nutrient supply on C:N:P stoichiometry. Our experimental and observational results suggest that while changes in nutrient supply have a stronger effect than temperature on growth, metabolic rates, community structure and elemental stoichiometry, the warming of the surface ocean may increase the ability of tropical phytoplankton assemblages to exploit events of enhanced nutrient availability. Across multiple levels of biological organization, nutrient limitation tends to reduce the effects of temperature on phytoplankton ecophysiology.