Defining Conserved Epibiotic Bacterial Genomes in the Trichodesmium Holobiont Using New Isolate Genomes and Field ‘Omic Techniques

The colony-forming, nitrogen fixing cyanobacteriaTrichodesmium spp. are important sources of ‘new’ nitrogen in the oligotrophic ocean. Despite this central biogeochemical role, many questions about their evolution, physiology and trophic interactions remain unanswered. Leveraging the high depth and low cost of next generation sequencing and the USC Trichodesmium culture collection (USCTCC), we describe six new Trichodesmium enrichment genomes and 12 hand-picked colony metagenomes from the 2018 Trans-Atlantic TriCoLim R/V Atlantis cruise. Mapping enrichment genomes to metagenome reads demonstrated remarkable conservation between two closely related Trichodesmium thiebautii isolate genomes (one from the Pacific and the other from the Atlantic Oceans) and field Trichodesmium (recruiting up to 50% of the environmental sequences). These data indicate that USCTCC isolates are ‘representative’ of colonial Trichodesmium in situ. Furthermore, de novo assembly of associated bacterial genomes from metagenomes and enrichment cultures, show that epibionts are conserved both in laboratory and natural samples and are phylogenetically different from free-living bacteria. The biogeochemical implications and genomic evolution of these potential mutualisms will be discussed.