UC Santa Cruz

Crocosphaera watsonii is an ecologically important marine unicellular diazotrophic cyanobacterium. It is often abundant in oligotrophic ocean regions where it provides fixed nitrogen to nutrient-limited phytoplankton communities. Previous genetic studies have observed genetic rearrangements but very little sequence variation among natural populations or cultivated strains of Crocosphaera. Those strains exhibit two phenotypes (large- and small-cell) with characteristics that suggest different ecological roles and niches. Prior to this work, the genetic basis for the phenotypic differences was unknown, and molecular methods for enumerating natural C. watsonii could not differentiate between phenotypes. To address those unanswered scientific questions, these studies compared the genomes of six C. watsonii strains, three of each phenotype, which were isolated over large spatial and temporal distances. A large portion of those genome sequences were shared among all strains with nearly 100% nucleotide identity. However, there were also genes that were specific to each strain, and others were specific to each phenotype, including some which could explain phenotypic differences (e.g. EPS biosynthesis). Relative to small-cell strains, large-cell strains had larger genomes and additional genetic capabilities, including possibly increased adaptations to iron and phosphorus limitation. Clustering based on genome sequences and content showed that strains with a common phenotype were evolutionarily most closely related, regardless of their time and location of isolation. Surprisingly, the genome of the C. watsonii type-strain, WH8501, was quite unusual, even compared to those with the same phenotype, suggesting it may not be appropriately representative of the species. To investigate distributions of Crocosphaera types in the marine environment, molecular assays were developed, based on phenotype-specific genes, and applied to samples from the North and South Pacific. In those samples, small-cells dominated in the upper 75 m where abundance of both types was much greater, while large-cells dominated in samples with lower counts between 100 m and 175 m. There was also more evidence that large-cells form aggregates in the N. Pacific. Future studies will be important to determine which of the initial C. watsonii patterns described here can be generalized, both in genomes, and in natural distributions of the two types.