12 new marine Synechococcus isolate genome assemblies. Marine Synechococcus genome assemblies

Synechococcus is a genus of unicellular cyanobacteria found throughout the global surface ocean and a large driver of the Earth’s carbon cycle. Here we introduce 12 new, high-quality draft genomes of marine Synechococcus isolates spanning several clades. We integrate them phylogenomically and pangenomically with previously available reference genomes and utilize ~100 environmental metagenomes largely sourced from the TARA Oceans project to assess the global distributions of these currently accessible genomic lineages. We show that the newly provided clade II isolates are by far the most representative (abundant) of the recovered in situ populations, and that these genomic lineages possess the smallest genomes yet recovered of the genus (2.14 ± 0.05Mbps; mean ± 1SD) while concurrently hosting some of the highest GC contents (60.67 ± 0.16%). This is in direct opposition to what Synechococcus’s nearest relative, Prochlorococcus, demonstrates – wherein decreasing genome size via genomic streamlining consistently coincides with a strong decrease in GC content – suggesting this new sub-clade of Synechococcus appears to have convergently undergone genomic streamlining, but aloft a fundamentally different evolutionary trajectory. Based on GC content, nucleotide and amino acid usage frequencies, and correlations with environmental data, we suggest that nitrogen availability may have been one of the key contributing factors in the divergence of Synechococcus and Prochlorococcus.