Cell Death Helps to Stabilize Populations of <i>Prochlorococcus</i>

Abstract Prochlorococcus is one of the dominant phytoplankton species in tropical and subtropical pelagic marine ecosystems. To maintain its population equilibrium in oligotrophic waters, the dominant species Prochlorococcus requires high loss rates to offset its high abundance and growth rates. While theoretical frameworks recognize multiple loss processes for Prochlorococcus (viral lysis, environmental stress, programmed cell death), prevailing population studies attribute main mortality to grazing. This oversight stems from in situ data gaps of non‐grazing mortality processes. In this study, we firstly formulated simple differential equations that took account of population losses due to both grazing and cell death. We then used ship‐based flow cytometry to assess the abundance of both non‐reproductive and live cells of two Prochlorococcus ecotypes in the South China Sea, and we calculated cell loss rates due to grazing and cell death. We found that: (a) vertical profiles of the abundance and cell death rate of Prochlorococcus were consistent, and they differed between high‐light and low‐light adapted ecotypes; (b) the high‐light adapted ecotype dominated at depths shallower than 50 m; the low‐light adapted ecotype dominated in the deep chlorophyll maximum layer (75 m); (c) during daylight hours, the cell death rate was significantly greater than the loss rate due to grazing, with nighttime reversal. The field results confirmed that natural cell mortality helped maintain population equilibrium.