# Heterotrophic bacteria trigger transcriptome remodelling in the photosynthetic picoeukaryote Micromonas commoda

## TL;DR

This study shows how a tiny ocean alga, Micromonas commoda, changes its gene activity when exposed to different bacteria, affecting processes like photosynthesis and growth.

## Contribution

The study reveals both shared and unique algal transcriptomic responses to three distinct marine bacteria, highlighting the complexity of phytoplankton-bacteria interactions.

## Key findings

- Micromonas commoda altered 17% of its genome's gene expression when co-cultured with three marine bacteria.
- Shared responses included reduced expression of photosynthesis and carbon fixation pathways.
- Unique responses included changes in biotin biosynthesis and nitrogen assimilation triggered by one specific bacterium.

## Abstract

Marine biogeochemical cycles are built on interactions between surface ocean microbes, particularly those connecting phytoplankton primary producers to heterotrophic bacteria. Details of these associations are not well understood, especially in the case of direct influences of bacteria on phytoplankton physiology. Here we catalogue how the presence of three marine bacteria (Ruegeria pomeroyi DSS‐3, Stenotrophomonas sp. SKA14 and Polaribacter dokdonensis MED152) individually and uniquely impact gene expression of the picoeukaryotic alga Micromonas commoda RCC 299. We find a dramatic transcriptomic remodelling by M. commoda after 8 h in co‐culture, followed by an increase in cell numbers by 56 h compared with the axenic cultures. Some aspects of the algal transcriptomic response are conserved across all three bacterial co‐cultures, including an unexpected reduction in relative expression of photosynthesis and carbon fixation pathways. Expression differences restricted to a single bacterium are also observed, with the Flavobacteriia P. dokdonensis uniquely eliciting changes in relative expression of algal genes involved in biotin biosynthesis and the acquisition and assimilation of nitrogen. This study reveals that M. commoda has rapid and extensive responses to heterotrophic bacteria in ways that are generalizable, as well as in a taxon specific manner, with implications for the diversity of phytoplankton‐bacteria interactions ongoing in the surface ocean.

The small green alga Micromonas commoda altered relative expression of 17% of its genome in co‐cultures with each of three surface ocean bacterial taxa, compared with axenic growth. Transcriptional responses in 377 genes occurred regardless of the bacterium, dominated by changes in photosynthesis, carbon storage and cell division expression. Yet another 1324 responses were unique to one of the bacteria. The number and diversity of effects of marine bacteria on M. commoda physiology were unanticipated.