# An Explicit Test of Kill the Winner: Protistan Grazing and Phage Lysis Differentially Impact Fast‐Growing Bacterial Taxa in the Coastal Antarctic

## TL;DR

This study examines how protists and bacteriophages affect fast-growing bacteria in the coastal Antarctic, finding that they selectively target some of the most abundant bacteria.

## Contribution

The study introduces the concept of 'kill select winners' to describe selective top-down control of bacterial populations by protists and phages.

## Key findings

- Protists and phages show strong intra-seasonal changes in their impact on bacterial mortality.
- Not all fast-growing bacteria are equally targeted, indicating selective predation and lysis.
- The study provides new insights into top-down control mechanisms in marine bacterial communities.

## Abstract

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.

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. The difference between these two treatments, calculated by fitting a linear model to bacterial abundance data, indicates strong intra‐seasonal change of bacteriophage and protistan contributions to mortality.