Picoeukaryotes have their map: Global abundance in present and future climates

Diversity, abundance and distribution of phytoplankton are three key aspects to understand how environmental changes will affect nutrient and carbon cycles. Picophytoplankton is a diverse group and includes two Cyanobacteria lineages, Prochlorococcus and Synechococcus, and the genetically diverse picoeukaryotes. In contrast to Cyanobacteria, the global distribution of picoeukaryotes is poorly understood. Our objective was to develop a niche model for picoeukaryotes to quantify how present and future climates shape the distribution of picophytoplankton. Niche models are powerful tools to understand environmental factors that govern phytoplankton distributions. We formulated a niche model based on 16.475 field observations using artificial neural networks. Our model included temperature, light and nitrate concentration and explained 46% of total variation in abundance. Global picoeukaryotes abundance was 1.4x10²⁶ cells unevenly distributed with minimums in extreme polar regions and tropical gyres and maximums at cold and nutrient rich waters. The cell abundance increased exponentially with light availability from 10E2.5 to 10E4 cells/ml. The abundance was lowest at the extreme low temperature range but increased to a maximum abundance at 8.5°C. Then, the abundance declined by an order of magnitude and reached a minimum at ~21°C. At high temperatures, we saw an increase in cell numbers above 21°C leading to intermediate concentrations in tropical waters. Nitrate had a smaller but significant effect whereby we observed elevated cell concentrations at intermediate nitrate levels. Our analysis showed a clear global niche partitioning among picophytoplankton whereby picoeukaryotes dominate colder waters, Synechococcus is most common in waters between 10 and 15°C and Prochlorococcus dominates tropical waters. In moderate climate change scenario (CMIP5 RCP 4.5) for 2100, projections of picoeukaryotes showed larger changes in abundance than in expansion in distribution. Picophytoplankton is a major component of global photosynthetic biomass, is largely subjected to environmental change, and thus is a key group that will shape global biogeochemical cycles under future climates.