W . and Bonachela , Juan A . and Levin , Simon A . and Martiny , Adam C . ( 2014 ) Impact of ocean phytoplankton diversity on phosphate uptake

25 We have a limited understanding of the consequences of variations in microbial biodiversity on ocean ecosystem functioning and global biogeochemical cycles. A core process is macronutrient uptake by microorganisms, as the uptake of nutrients controls ocean CO2 fixation rates in many regions. Here, we ask if variations in ocean phytoplankton biodiversity lead to novel functional relationships between environmental 30 variability and phosphate (Pi) uptake. We analyzed Pi uptake capabilities and cellular allocations among phytoplankton groups and the whole community throughout the extremely Pi-depleted Western North Atlantic Ocean. Pi uptake capabilities of individual populations were well described by a classic uptake function but displayed adaptive differences in uptake capabilities that depend on cell size and nutrient availability. Using 35 an eco-evolutionary model as well as observations of in situ uptake across the region, we confirmed that differences among populations lead to previously uncharacterized relationships between ambient Pi concentrations and uptake. Supported by novel theory, this work provides a fundamentally new empirical basis for describing and understanding assimilation of limiting nutrients in the oceans. Thus, it demonstrates that microbial 40 biodiversity, beyond cell size, is important for understanding the global cycling of nutrients. Significance statement Nutrient uptake is a central property of ocean biogeochemistry; but our 45 understanding of this process is based on lab cultures or bulk environmental studies. Thus, mathematical descriptions of nutrient uptake, at the heart of most biogeochemical models (including ones used by the Intergovernmental Panel on Climate Change), must rely on this limited information. Hence we have little knowledge of how natural phytoplankton populations vary in their abilities to take up 50 key nutrients. Using new analytical techniques, this study provides the first comprehensive in situ quantification of nutrient uptake capabilities among dominant phytoplankton groups. Supported by a model that considers plastic ecological responses in an evolutionary context, this work further provides a fundamentally new framework for the integration of microbial diversity to describe and understand the 55 controls of ocean nutrient assimilation.