UC San Diego

Protists and bacteriophages exert top-down control on bacterial populations. Previous work in the coastal Antarctic demonstrates the potential for intra-seasonal variability of this top-down control driven by the extreme seasonal contrast in bacterial growth rates. We evaluated whether predators kill the winner wherein protists and phages preferentially impact the most abundant members of bacterial assemblages over an austral summer with weekly dilution experiments. Seawater from 10 m was divided into two serial dilutions with either 0.2 μm (to evaluate protist grazing) or 30 kDa (to evaluate protist grazing and lysis from bacteriophage) filtered water. We observed strong intra-seasonal change of bacteriophage and protistan contributions to mortality. A comparison of activity per ASV from amplicon sequencing over our dilution experiments to a predicted minimal doubling time indicates that kill the winner is occurring during the top-down control of only a few bacteria. As not all bacterial taxa with a predicted low mean doubling time demonstrated high activity in our dilution experiments, our results indicate protists and phage selectively target some fast-growing or abundant bacteria which we term kill select winners (KsW). Overall, our evaluation of bacterial abundance and community structure provides unprecedented knowledge of top-down control of marine bacteria.