Impact of metabolic strategy of phytoplankton on functions and structure of the associated bacterial community using metatranscriptome

Interactions between eukaryotes and bacterioplankton are at the core of the microbial loop and have been studied in various aquatic systems to understand ecosystem function and energy transfer. Shallow eutrophic waters, overgrown by vegetation, are a common habitat type in the Baltic proper but have been less studied compared to off-shore pelagic systems. A high-resolution characterization of the bacterial and eukaryotic communities in a coastal Baltic Sea habitat was performed to reveal associations and community composition shifts. Sampling was done at two time points during a declining late summer temperature peak (22.5°C to 18.7°C) in the Åland archipelago. 16S and 18S rRNA gene amplicon sequencing was performed as well as RNA-seq for functional studies of the metatranscriptome. This was followed by co-occurrence analyses based on count data and taxonomic as well as functional annotations. The community compositions at the two time points were significantly different from each other. Primarily there was a transition from at phototrophic to a heterotrophic metabolism among the eukaryotes. Due to this transition, a decrease in primary production and an increase in the predation pressure on the bacterial community and processes of decomposition would be expected. Among the bacteria there are different taxa dominating the free-living and the attached factions. The functional results show that Ammonia becomes a more important nutrient as the metabolic strategy of the community shifts from a phototrophic to a more heterotrophic one. Taken together, the metabolic strategy of the eukaryote community have different effects on the free-living and the attached bacterial fractions, and also have effects on the functions of both the eukaryote and the bacterial community. Thus, associations between eukaryote and bacteria are important to consider as a community structuring force also at non bloom conditions.