Changes in functional composition and gene expression in eukaryotic plankton at the Atlantic-Arctic Polar front

In the Arctic Ocean, phytoplankton communities, dominated by eukaryotes, sustain the marine food web and influence biogeochemical cycles. While many are endemic, some are transported by currents from the Atlantic Ocean along an environmental gradient that steepens at the polar front. Despite these distinct biogeographies, the functional characteristics of these communities remain poorly characterized. Here, we analyzed twenty metatranscriptomes from a North Atlantic-Arctic environmental gradient considering ocean currents transport times. Functions related to the regulation of gene expression and cold acclimation were more abundant in Arctic metatranscriptomes. Using the PHATE dimensionality reduction algorithm, reconstructed transcriptomes of Bacillariophyta, Mamiellales, Pelagophyceae, and Phaeocystales revealed different gene expression patterns, including photosynthesis regulation. All groups showed upregulation of numerous key temperature-associated functions and a subset of shared functions, supporting a convergence in transcriptomic changes under polar conditions. This study, based on a new resource of reconstructed transcriptomes, addressed the functional differentiation of eukaryotic phytoplankton across the North Atlantic and Arctic Ocean. It evidenced both differential and convergent changes in gene expression across the polar front in widespread phytoplankton types. These results advance our understanding of the functional dynamics of phytoplankton along environmental gradients.