UCYN-A/haptophyte symbioses dominate N2 fixation in the Southern California Current System

• Published in: ISME Communications Vol. 1; no. 1; p. 42
• Main Authors: Turk-Kubo, Kendra A, Mills, Matthew M, Arrigo, Kevin R, van Dijken, Gert, Henke, Britt A, Stewart, Brittany, Wilson, Samuel T, Zehr, Jonathan P
• Format: Journal Article
• Language: English
• Published: London Springer Nature B.V 26.08.2021; Nature Publishing Group UK
• Subjects: Chlorophyll; Coastal waters; Community composition; Cyanobacteria; Ecotypes; Metabolism; Nitrates; Nitrogen; Nitrogen fixation; Nitrogenation; Oceans; Seawater; Symbiosis; Upwelling; Southern California; Mexico; Baja California Mexico; California; United States > US
• ISSN: 2730-6151; 2730-6151
• DOI: 10.1038/s43705-021-00039-7

Abstract The availability of fixed nitrogen (N) is an important factor limiting biological productivity in the oceans. In coastal waters, high dissolved inorganic N concentrations were historically thought to inhibit dinitrogen (N2) fixation, however, recent N2 fixation measurements and the presence of the N2-fixing UCYN-A/haptophyte symbiosis in nearshore waters challenge this paradigm. We characterized the contribution of UCYN-A symbioses to nearshore N2 fixation in the Southern California Current System (SCCS) by measuring bulk community and single-cell N2 fixation rates, as well as diazotroph community composition and abundance. UCYN-A1 and UCYN-A2 symbioses dominated diazotroph communities throughout the region during upwelling and oceanic seasons. Bulk N2 fixation was detected in most surface samples, with rates up to 23.0 ± 3.8 nmol N l−1 d−1, and was often detected at the deep chlorophyll maximum in the presence of nitrate (>1 µM). UCYN-A2 symbiosis N2 fixation rates were higher (151.1 ± 112.7 fmol N cell−1 d−1) than the UCYN-A1 symbiosis (6.6 ± 8.8 fmol N cell−1 d−1). N2 fixation by the UCYN-A1 symbiosis accounted for a majority of the measured bulk rates at two offshore stations, while the UCYN-A2 symbiosis was an important contributor in three nearshore stations. This report of active UCYN-A symbioses and broad mesoscale distribution patterns establishes UCYN-A symbioses as the dominant diazotrophs in the SCCS, where heterocyst-forming and unicellular cyanobacteria are less prevalent, and provides evidence that the two dominant UCYN-A sublineages are separate ecotypes.