In situ cell division and mortality rates of SAR11, SAR86, Bacteroidetes , and Aurantivirga during phytoplankton blooms reveal differences in population controls.

Net growth of microbial populations, i.e., changes in abundances over time, can be studied using 16S rRNA fluorescence hybridization (FISH). However, this approach does not differentiate between mortality and cell division rates. We used FISH-based image cytometry in combination with dilution culture experiments to study net growth, cell division, and mortality rates of four bacterial taxa over two distinct phytoplankton blooms: the oligotrophs SAR11 and SAR86, the copiotrophic phylum , and its genus . Cell volumes, ribosome content, and frequency of dividing cells (FDC) co-varied over time. Among the three, FDC was the most suitable predictor to calculate the cell division rates for the selected taxa. The FDC-derived cell division rates for SAR86 of up to 0.8 d and of up to 1.9 d differed, as expected for oligotrophs and copiotrophs. Surprisingly, SAR11 also reached high cell division rates of up to 1.9 d, even before the onset of phytoplankton blooms. For all four taxonomic groups, the abundance-derived net growth (-0.6 to 0.5 d) was about an order of magnitude lower than the cell division rates. Consequently, mortality rates were comparably high to cell division rates, indicating that about 90% of bacterial production is recycled without apparent time lag within one day. Our study shows that determining taxon-specific cell division rates complements omics-based tools and provides unprecedented clues on individual bacterial growth strategies including bottom-up and top-down controls.