Microbial communities in the world ocean are affected strongly by oceanic circulation, creating characteristic marine biomes. The high connectivity of most of the ocean makes it difficult to disentangle selective retention of colonizing genotypes (with traits suited to biome specific conditions) from evolutionary selection, which would act on founder genotypes over time. The Arctic Ocean is exceptional with limited exchange with other oceans and ice covered since the last ice age. To test whether Arctic microalgal lineages evolved apart from algae in the global ocean, we sequenced four lineages of microalgae isolated from Arctic waters and sea ice. Here we show convergent evolution and highlight geographically limited HGT as an ecological adaptive force in the form of PFAM complements and horizontal acquisition of key adaptive genes. Notably, ice-binding proteins were acquired and horizontally transferred among Arctic strains. A comparison with Tara Oceans metagenomes and metatranscriptomes confirmed mostly Arctic distributions of these IBPs. The phylogeny of Arctic-specific genes indicated that these events were independent of bacterial-sourced HGTs in Antarctic Southern Ocean microalgae.

Architecture et Fonction des Macromolécules Biologiques CNRS Aix Marseille Université Marseille France

CNRS Research Federation for the Study of Global Ocean Systems Ecology and Evolution FR2022 Tara Oceans GOSEE Paris France

Département de Biologie Institut de Biologie Intégrative des Systèmes Université Laval Quebec Canada

Department of Biological Sciences King Abdulaziz University Jeddah Saudi Arabia

Department of Parasitology BIOCEV Faculty of Science Charles University Prague Czech Republic

Department of Plant and Microbial Biology University of California Berkeley Berkeley CA USA

Division of Infectious Diseases Department of Medicine University of Alberta and Department of Biological Sciences and University of Alberta Edmonton Canada

Génomique Métabolique Genoscope Institut de Biologie François Jacob Commissariat à l'Énergie Atomique CNRS Université Évry Université Paris Saclay Évry France

HudsonAlpha Institute for Biotechnology Huntsville AL USA

Institut de Biologie de l'ENS Département de Biologie École Normale Supérieure CNRS INSERM Université PSL Paris France

Sorbonne Université CNRS Station Biologique de Roscoff AD2M UMR 7144 Roscoff France

US Department of Energy Joint Genome Institute Lawrence Berkeley National Laboratory Berkeley CA USA

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