Global distribution of cultivated Trichodesmium subclades revealed by multi-omics

Trichodesmium is an important oceanic N2 fixing cyanobacterial genus that has been shown to provide up to 50% of new N to oligotrophic regimes. Despite its importance, we know relatively little about the genomic potential and functional diversity of the two major clades of Trichodesmium (Clade I and Clade III, hereafter Thieb and Tery, respectively). With the expanded collection of Trichodesmium isolates in the USCTCC (University of Southern California Trichodesmium Culture Collection), we sequenced genomes from seven cultivated strains to further characterize the genomic diversity within the genus. For example, sequencing the genome of the "gigantic", red Trichodesmium contortum surprisingly shows that they are closely related to the smallest Trichodesmium clade, TeryA. The high genomic identity between TeryA and T. contortum (>98.5% ANI) and lack of implicated auxiliary genes suggests their large biovolume differences might be transcriptional or epigenetic in origin. Furthermore, these data show that using Tery-subclades are a more accurate designation than the classical Trichodesmium species delineation. Finally, we placed the analysis of these genomes in an ecological context via read mapping with globally distributed RNA and DNA datasets. Our data show that Tery clades (A&B) are both lower in relative abundance compared to Thieb in global oceans, generally co-occur when detected in the field, and are highly linked to decreased salinity and increased temperatures, especially for sampling locations in the Bay of Bengal. Lastly, even though TeryA members are undersaturated with respect to current CO2 concentrations, our phenotypic and biogeography data suggests that salinity/ocean color could limit their predicted global impact as climate changes.