Phytoplankton responses to changing temperature and nutrient availability are consistent across the tropical and subtropical Atlantic.

Temperature and nutrient supply interactively control phytoplankton growth and productivity, yet the role of these drivers together still has not been determined experimentally over large spatial scales in the oligotrophic ocean. We conducted four microcosm experiments in the tropical and subtropical Atlantic (29°N-27°S) in which surface plankton assemblages were exposed to all combinations of three temperatures (in situ, 3 °C warming and 3 °C cooling) and two nutrient treatments (unamended and enrichment with nitrogen and phosphorus). We found that chlorophyll a concentration and the biomass of picophytoplankton consistently increase in response to nutrient addition, whereas changes in temperature have a smaller and more variable effect. Nutrient enrichment leads to increased picoeukaryote abundance, depressed Prochlorococcus abundance, and increased contribution of small nanophytoplankton to total biomass. Warming and nutrient addition synergistically stimulate light-harvesting capacity, and accordingly the largest biomass response is observed in the warmed, nutrient-enriched treatment at the warmest and least oligotrophic location (12.7°N). While moderate nutrient increases have a much larger impact than varying temperature upon the growth and community structure of tropical phytoplankton, ocean warming may increase their ability to exploit events of enhanced nutrient availability.

Effect of temperature on the unimodal size scaling of phytoplankton growth.

Contrary to predictions by the allometric theory, there is evidence that phytoplankton growth rates peak at intermediate cell sizes. However, it is still unknown if this pattern may result from the effect of experimental temperature. Here we test whether temperature affects the unimodal size scaling pattern of phytoplankton growth by (1) growing Synechococcus sp., Ostreococcus tauri, Micromonas commoda and Pavlova lutheri at 18 °C and 25 °C, and (2) using thermal response curves available in the literature to estimate the growth rate at 25 °C as well as the maximum growth rate at optimal temperature for 22 species assayed previously at 18 °C. We also assess the sensitivity of growth rate estimates to the metric employed for measuring standing stocks, by calculating growth rates based on in vivo fluorescence, chlorophyll a concentration, cell abundance and biomass (particulate organic carbon and nitrogen content). Our results show that the unimodal size scaling pattern of phytoplankton growth, with a peak at intermediate cell sizes, is observed at 18 °C, 25 °C and at the optimal temperature for growth, and that it prevails irrespective of the standing-stock metric used. The unimodal size scaling pattern of phytoplankton growth is supported by two independent field observations reported in the literature: (i) a positive relationship between cell size and metabolic rate in the picophytoplankton size range and (ii) the dominance of intermediate-size cells in nutrient-rich waters during blooms.

Effects of Temperature and Nutrient Supply on Resource Allocation, Photosynthetic Strategy, and Metabolic Rates of Synechococcus sp.

Temperature and nutrient supply are key factors that control phytoplankton ecophysiology, but their role is commonly investigated in isolation. Their combined effect on resource allocation, photosynthetic strategy, and metabolism remains poorly understood. To characterize the photosynthetic strategy and resource allocation under different conditions, we analyzed the responses of a marine cyanobacterium (Synechococcus PCC 7002) to multiple combinations of temperature and nutrient supply. We measured the abundance of proteins involved in the dark (RuBisCO, rbcL) and light (Photosystem II, psbA) photosynthetic reactions, the content of chlorophyll a, carbon and nitrogen, and the rates of photosynthesis, respiration, and growth. We found that rbcL and psbA abundance increased with nutrient supply, whereas a temperature-induced increase in psbA occurred only in nutrient-replete treatments. Low temperature and abundant nutrients caused increased RuBisCO abundance, a pattern we observed also in natural phytoplankton assemblages across a wide latitudinal range. Photosynthesis and respiration increased with temperature only under nutrient-sufficient conditions. These results suggest that nutrient supply exerts a stronger effect than temperature upon both photosynthetic protein abundance and metabolic rates in Synechococcus sp. and that the temperature effect on photosynthetic physiology and metabolism is nutrient dependent. The preferential resource allocation into the light instead of the dark reactions of photosynthesis as temperature rises is likely related to the different temperature dependence of dark-reaction enzymatic rates versus photochemistry. These findings contribute to our understanding of the strategies for photosynthetic energy allocation in phytoplankton inhabiting contrasting environments.