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

Kendra A Turk-Kubo1, Matthew M Mills2, Kevin R Arrigo2, Gert van Dijken2, Britt Anderson Henke1, Brittany Stewart3, Samuel Wilson4 and Jonathan P Zehr5, (1)University of California Santa Cruz, Ocean Sciences, Santa Cruz, CA, United States, (2)Stanford University, Earth System Science, Stanford, CA, United States, (3)University of Southern California, Los Angeles, United States, (4)University of California, San Diego, United States, (5)University of California Santa Cruz, Ocean Sciences, Santa Cruz, United States
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.