Abstract

The cyanobacterium Trichodesmium is important in ocean nitrogen (N) and carbon (C) biogeochemistry through fixation of N2 gas and CO2 into NH3, amino acids, and carbohydrates. These essential compounds then fuel primary productivity, bacterial and grazing communities, and upper trophic levels. In the oligotrophic surface oceans, they are one of few taxa capable of utilizing both atmospheric C and N. Trichodesmium fall into 2 diazotrophic phylogenomic clades named "Thieb" and "Tery" after the most well characterized botanical species, Trichodesmium thiebautii and Trichodesmium erythraeum. Thieb is commonly the most abundant clade in all major ocean regions and paradoxically, is also the least characterized in terms of morphology, genomics, and ecophysiology due to underrepresentation in culture collections. In this study, 25 novel strains of Thieb were enriched in culture from the Sargasso Sea, phenotypically characterized, and whole genome sequenced. Cultures were found to belong to four well-supported subclades (i.e., ThiebA, ThiebB, ThiebC, and ThiebD). Trichodesmium subclades were found to have environmentally relevant differences in colony shape, gene content, cell dimensions, and inferred temperature range. Specifically, ThiebD and ThiebC had the largest biovolumes whereas ThiebA and ThiebB had the smallest. Importantly ThiebB also became relatively more abundant only in bulk seawater metagenomes collected at high temperature (>28{degrees}C). This finding indicates that as ocean warming continues, there may be shifts in Trichodesmium community structure towards smaller cells, which would affect overall C+N standing stocks.