Out of Antarctica: new insights into Antarctic Subcluster 5.2 picocyanobacteria based on high-quality genomes

2. Abstract Synechococcus-like cyanobacteria are cosmopolitan unicellular picocyanobacteria that have colonized diverse aquatic and terrestrial habitats. The so-called ‘subcluster 5.2’ represents a particularly diversified subgroup, including marine and freshwater organisms adapted to extreme conditions, notably polar environments. We increased the genomic representation of polar taxa in this subcluster by reconstructing new high-quality genomes from five Antarctic lacustrine strains and one Arctic freshwater isolate using a combination of small Illumina and long Nanopore reads. A maximum likelihood (ML) phylogenomic analysis of these new assemblies combined with all publicly available good quality assemblies of the subcluster 5.2 suggests evidence of a dispersal process from Antarctica. Indeed, the topology of the phylogenomic tree indicates one basal Antarctic lineage followed by the emergence of two lineages, one Antarctic and one non-Antarctic (Spain). This finding is further supported by a 16S rRNA ML phylogenetic and a pangenomic analysis. Although secondary colonization of Antarctica by cyanobacteria following the cooling of the continent 34 million years ago has been reported, this study is the first to support an ‘Out-of-Antarctica’ scenario inside subcluster 5.2. Impact statement This study provides novel insights into the evolutionary history and biogeography of Synechococcus-like picocyanobacteria. By expanding the genomic sampling of polar subcluster 5.2, we reveal that Antarctic taxa are not monophyletic and are intermixed with non-Antarctic lineages, suggesting an unprecedented Out-of-Antarctica dispersal scenario. Our results, supported by 16S rRNA, phylogenomic and pangenomic analyses, highlight the role of polar regions as both a refuge and a source of microbial diversity. This work advances our understanding of cyanobacterial adaptation, radiation and genome evolution in extreme environments. Data summary Raw sequencing reads and genome assemblies from this study have been deposited in the European Nucleotide Archive (ENA) under Project accession number PRJEB103944. Raw sequencing reads are available under accession numbers ERR15933635 to ERR15933638, ERR15933641 to ERR15933644, ERR15903357, ERR15903358, ERR15905256 and ERR15905257. The assemblies can be accessed under accession numbers GCA_977927515, GCA_977927525, GCA_977927535, GCA_977927545, GCA_977927555 and GCA_977929215. The scripts and code as well as large supplementary files generated during this study are available on Figshare and can be downloaded from https://doi.org/10.6084/m9.figshare.30772781 [1]. Supplementary data files are also provided and contains supplementary figures (Fig. S1-S4) and tables (Table S1-S6). Competing Interest Statement The authors have declared no competing interest.