18Phytoplankton communities significantly contribute to global biogeochemical cycles of elements 19 and underpin marine food webs. Although their uncultured genetic diversity has been estimated by 20 planetary-scale metagenome sequencing and subsequent reconstruction of metagenome-assembled 21 genomes (MAGs), this approach has yet to be applied for eukaryote-enriched polar and non-polar 22 phytoplankton communities. Here, we have assembled draft prokaryotic and eukaryotic MAGs 23 from environmental DNA extracted from chlorophyll a maximum layers in the surface ocean across 24 the Arctic Circle in the Atlantic. From 679 Gbp and estimated 50 million genes in total, we 25 recovered 140 MAGs of medium to high quality. Although there was a strict demarcation between 26 polar and non-polar MAGs, adjacent sampling stations in each environment on either side of the 27 Arctic Circle had MAGs in common. Furthermore, phylogenetic placement revealed eukaryotic 28MAGs to be more diverse in the Arctic whereas prokaryotic MAGs were more diverse in the 29 Atlantic south of the Arctic Circle. Approximately 60% of protein families were shared between 30 polar and non-polar MAGs for both prokaryotes and eukaryotes. However, eukaryotic MAGs had 31 more protein families unique to the Arctic whereas prokaryotic MAGs had more families unique to 32 south of the Arctic circle. Thus, our study enabled us to place differences in functional plankton 33 diversity in a genomic context to reveal that the evolution of these MAGs likely was driven by 34 significant differences in the seascape on either side of an ecosystem boundary that separates polar 35 from non-polar surface ocean waters in the North Atlantic. 36 37 carbon cycle and marine food webs [3]. Furthermore, the significance of organism interactions and 48 specifically symbiosis for cycling of energy and matter was revealed, with viral-host dynamics as 49 the most impactful form of these biotic interactions [4,5]. 50 51Linking functional microbial diversity, estimated by metagenomics and metatranscriptomics, with 52 microbial activity as part of physico-chemical ecosystem properties shed light on how different 53 microbial groups contribute to biogeochemical cycling of elements [1, 6, 7]. These results built the 54 foundation for estimating how changing oceans due to global warming might impact the diversity 55 and activity of ocean microbes [7, 8]. However, to fully explore the role of microbes and their 56 interactions in changing environmental conditions, we must understand their metabolic capabilities 57 in an evolutionary context [9, 10]. As the majority of marine microbes are unculturable and because 58 genomic information is required to reconstruct their metabolic evolution, metagenome-assembled 59 genomes (MAGs) offer a solution [11,12]. Although most MAGs are not at the level of quality 60 achieved through sequencing cultures of isolated strains, they provide genome-level insights into 61 the microbial diversity of natural ecosystems. Due to their small size and structural s...

Background Phytoplankton communities significantly contribute to global biogeochemical cycles of elements and underpin marine food webs. Although their uncultured genomic diversity has been estimated by planetary-scale metagenome sequencing and subsequent reconstruction of metagenome-assembled genomes (MAGs), this approach has yet to be applied for complex phytoplankton microbiomes from polar and non-polar oceans consisting of microbial eukaryotes and their associated prokaryotes. Results Here, we have assembled MAGs from chlorophyll a maximum layers in the surface of the Arctic and Atlantic Oceans enriched for species associations (microbiomes) with a focus on pico- and nanophytoplankton and their associated heterotrophic prokaryotes. From 679 Gbp and estimated 50 million genes in total, we recovered 143 MAGs of medium to high quality. Although there was a strict demarcation between Arctic and Atlantic MAGs, adjacent sampling stations in each ocean had 51–88% MAGs in common with most species associations between Prasinophytes and Proteobacteria. Phylogenetic placement revealed eukaryotic MAGs to be more diverse in the Arctic whereas prokaryotic MAGs were more diverse in the Atlantic Ocean. Approximately 70% of protein families were shared between Arctic and Atlantic MAGs for both prokaryotes and eukaryotes. However, eukaryotic MAGs had more protein families unique to the Arctic whereas prokaryotic MAGs had more families unique to the Atlantic. Conclusion Our study provides a genomic context to complex phytoplankton microbiomes to reveal that their community structure was likely driven by significant differences in environmental conditions between the polar Arctic and warm surface waters of the tropical and subtropical Atlantic Ocean.

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