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 lifestyles is unclear. Here single-cell measurements reveal that Prochlorococcus at the base of the photic zone in the Eastern Mediterranean Sea obtain only ~20% of carbon required for growth by photosynthesis. This is supported by laboratory-calibrated calculations based on photo-physiology parameters and compared with in situ growth rates. Agent-based simulations show that mixotrophic cells could grow tens of metres deeper than obligate photo-autotrophs, deepening the nutricline by ~20 m. Time series from the North Atlantic and North Pacific indicate that, during thermal stratification, on average 8–10% of the Prochlorococcus cells live without enough light to sustain obligate photo-autotrophic populations. Together, these results suggest that mixotrophy underpins the ecological success of a large fraction of the global Prochlorococcus population and its collective genetic diversity.
Bibliographical noteFunding Information:
We thank the captain and crew of the R/V Mediterranean Explorer and T. Reich, for help during the work at sea, M. Krom and A. Tsemel for the nutrient analyses, M. Ofek-Lalzar for assistance with the bioinformatics analysis, A. Grüttmüller for NanoSIMS routine operation, I. Tsakalakis for help with hourly PAR estimation, and J. Casey for the discussion about C uptakes rates. This study was supported by grant RGP0020/2016 from the Human Frontiers Science Program (to M.V., H.-P.G. and D.S.), by grant 1786/20 from the Israel Science Foundation (to D.S.) and by grant number 1635070/2016532 from the NSF-BSF program in Oceanography (NSFOCE-BSF, to D.S.). The NanoSIMS at the Leibnitz-Institute for Baltic Sea research in Warnemuende (IOW) was funded by the German Federal Ministry of Education and Research (BMBF), grant identifier 03F0626A. M.J.F. and W.Z. are grateful for support from the Simons Foundation through the Simons Collaboration on Ocean Processes and Ecology (SCOPE 329108 to M.J.F.) and the Simons Collaboration for Computational BIOgeochemical Modeling of marine EcosystemS (CBIOMES 549931 to M.J.F.).
© 2022, The Author(s).
ASJC Scopus subject areas
- Applied Microbiology and Biotechnology
- Microbiology (medical)
- Cell Biology