A day in the life of Prochlorococcus: Diel ecological oscillations of cyanobacteria, viruses and grazers in the North Pacific Subtropical Gyre

Stephen Beckett1, David Jean Robert Demory1, Ashley Coenen2, John Casey3, Christopher L Follett4, Mathilde Dugenne5, Paige Elizabeth Connell6, Michael Carlson7, Sarah K Hu8, Samuel T Wilson9, Daniel Muratore10, Angela Boysen11, Matthew Harke12, Elaine Luo13, Rogelio Rodriguez1, Shengyun Peng14, Kevin Becker8, Sacha Coesel11, Daniel Richard Mende15, Anitra E Ingalls16, Benjamin AS Van Mooy8, Sonya Dyhrman17, Jonathan P Zehr18, E. Virginia Armbrust11, Edward DeLong9, David M Karl19, David A Caron6, Debbie Lindell7, Michael J Follows3, Angelicque E White20, Francois Ribalet21 and Joshua S Weitz22, (1)Georgia Institute of Technology, School of Biological Sciences, Atlanta, GA, United States, (2)Georgia Institute of Technology, School of Physics, United States, (3)Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge, United States, (4)University of Liverpool, Earth, Ocean, and Ecological Sciences, Liverpool, United Kingdom, (5)University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA, Oceanography, Honolulu, HI, United States, (6)University of Southern California, Biological Sciences, Los Angeles, CA, United States, (7)Technion Israel Institute of Technology, Faculty of Biology, Haifa, Israel, (8)Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, Woods Hole, MA, United States, (9)Daniel K. Inouye Center for Microbial Oceanography: Research and Education, University of Hawaii at Manoa, Honolulu, HI, United States, (10)Santa Fe Institute, United States, (11)University of Washington, School of Oceanography, Seattle, United States, (12)Columbia University of New York, Lamont-Doherty Earth Observatory, Palisades, NY, United States, (13)University of Hawaii, C-MORE, Honolulu, HI, United States, (14)Georgia Institute of Technology, School of Biological Sciences, Atlanta, United States, (15)University of Hawaii at Manoa, C-MORE, Honolulu, HI, United States, (16)University of Washington Seattle Campus, School of Oceanography, Seattle, United States, (17)Columbia University, Lamont-Doherty Earth Observatory, Palisades, NY, United States, (18)University of California Santa Cruz, Ocean Sciences, Santa Cruz, United States, (19)University of Hawaii at Manoa, Department of Oceanography, Honolulu, United States, (20)University of Hawaii at Manoa, Department of Oceanography, Honolulu, HI, United States, (21)University of Washington, School of Oceanography, Seattle, WA, United States, (22)University of Maryland, United States
Abstract:
Prochlorococcus is the most abundant photosynthetic organism on Earth and plays a key role in oligotrophic oceans such as the North Pacific Subtropical Gyre (NPSG). Theory suggests that light availabilty and size-dependent processes are core drivers of growth and mortality in oceanic ecosystem communities. A key question is how much viruses contribute to the mortality of phytoplankton relative to grazers. Motivated by timeseries measurements made via Lagrangian sampling in the NPSG we assess population dynamics of Prochlorococcus, micrograzers and viruses over daily timesecales. To understand this dataset we developed a size-structured dynamical model of Prochlorcoccus cells whose growth is light dependent and are subject to infection by viruses and to ingestion by grazers. Using our process-based model we are able to evaluate how in situ ecological rhythms compare with in silico ecological rhythms. In particular, we assess the strength of virus-induced mortality relative to grazing mortality in the NPSG and explore the sensitivity of this balance with respect to model parameterization. Our study provides a methodology for bridging community-level data from a variety of sources with dynamical modelling to explain ecosystem processes on short-term daily timescales. Preliminary results show that our model is capable of recapitulating ecosystem dynamics observed in the NPSG. Within this system, our model suggests that Prochlorococcus mortality peaks during the night and that grazing mortality is much greater than viral-induced mortality. Whilst phytoplankton growth is light-driven, we suggest that NPSG data is best fit by a model in which grazing and infection processes are also driven in a diel manner. More generally, we investigate potential drivers of the differential importance of viral lysis relative to grazer mortality and assess potential variations of population dynamics on daily timescales. Our results help drive our understanding of the dynamics, processes and the functioning of oceanic ecological communities across daily timescales.
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