Marine phytoplankton are responsible for about half of the photosynthesis on Earth. Many are mixotrophs, combining photosynthesis with heterotrophic assimilation of organic carbon but the relative contribution of these two carbon sources is not well quantified. Here, single-cell measurements reveal that Prochlorococcus at the base of the photic zone in the Eastern Mediterranean Sea are obtaining only ~20% of carbon required for growth by photosynthesis. Consistently, laboratory-calibrated evaluations of Prochlorococcus photosynthesis indicate that carbon fixation is systematically too low to support published in situ growth rates in the deep photic layer of the Pacific Ocean. Furthermore, agent-based model simulations show that mixotrophic cells maintain realistic growth rates and populations 10s of meters deeper than obligate photo-autotrophs, deepening the nutricline and Deep Chlorophyll Maximum by ~20 m. Time-series of Prochlorococcus ecotype-abundance from the subtropical North Atlantic and North Pacific suggest that up to 30% of the Prochlorococcus cells live where light intensity is not enough to sustain obligate photo-autotrophic populations during warm, stratified periods. Together, these data and models suggest that mixotrophy underpins the ecological success of a large fraction of the global Prochlorococcus population and its collective genetic diversity.

Primary Presenter: Daniel Sher, University of Haifa (dsher@univ.haifa.ac.il)

Authors:

Zhen Wu, Massachuusetts Institute of Technology  ()

Dikla Aharonovich, Univerfsitry of Haifa  ()

Dalit Roth-Rosenberg, University of Haifa  ()

Osnat Weissberg, University of Haifa  ()

Tal Luzzatto-Knaan, University of Haifa  ()

Angela Vogts, Leibnitz Institute for Baltic Sea Research  ()

Luca Zoccarato, Leibniz-Institute of Freshwater Ecology and Inland Fisheries  ()

Falk Eigemann, Leibniz-Institute for Baltic Sea Research  ()

Hans-Peter Grossart, Leibniz-Institute of Freshwater Ecology and Inland Fisheries  ()

Maren Voss, Leibniz-Institute for Baltic Sea Research  ()

Michael Follows, Massachusetts Institute of Technology  ()