Nitrogen fixation by the UCYN-A/haptophyte symbiosis in coastal waters of the southern California Current System

The availability of fixed nitrogen (N) is an important factor limiting biological productivity in the oceans. N₂fixation is a source of N in oligotrophic marine environments, but is typically considered unimportant in coastal waters, where low temperatures and high dissolved inorganic N (DIN) concentrations are thought to inhibit N₂fixation. However, recent reports of N₂fixation and the presence of diverse groups of N₂-fixing Bacteria and Archaea in coastal regions challenge this paradigm. A potentially important N₂-fixing taxa in coastal regions is the UCYN-A/haptophyte symbiosis, since several sublineages have been reported in DIN-replete, polar, and upwelling-influenced coastal waters. The present study investigates the significance of UCYN-A/haptophyte symbioses to nearshore N budgets by measuring whole community N₂fixation rates, single-cell UCYN-A N₂fixation rates and UCYN-A abundance and diversity during two cruises to the southern California Current System (CCS). We found that N₂-fixing assemblages were diverse, but dominated by UCYN-A during the spring upwelling (May) and fall oceanic (October) seasons across the entire region. In the spring, peak UCYN-A1 abundance was measured in warm coastal waters above a shoaling nitracline, while UCYN-A2 persisted in deeper waters where abundances correlated with the nitracline. In the fall, UCYN-A abundances were high (up to 10⁷nifHcopies L^-1) and evenly distributed across the region. Both sublineages were fixing N₂, and rates of cell-specific N₂fixation in the larger UCYN-A2/haptophyte symbiosis were higher (10-37 fmol N cell^-1d^-1) than in the UCYN-A1/haptophyte symbiosis (4-8 fmol N cell^-1d^-1). We measured whole community N₂fixation rates throughout this region during both cruises, but these rates did not always correlate with UCYN-A abundances. This is the first report of widespread N₂fixation in the CCS, and our findings suggest that UCYN-A/haptophyte symbioses are important coastal N₂-fixers. These findings will enable improved representation of N₂-fixer growth and activity in coastal systems, and aid the integration of coastal N₂fixation into current biogeochemical and ecosystem models.