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

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.

Symbiotic diazotrophic UCYN-A strains co-occurred with El Niño, relaxed upwelling, and varied eukaryotes over 10 years off Southern California Bight

“…[2,10,20]). UCYN-A has been found previously in the Southern California Current System and Monterey Bay [21,22]. Notably, UCYN-A has been found at our study site, the San Pedro Ocean Time-series (SPOT) as deep as 890m [23], and at a nearby daily time series off the coast of Catalina Island [24].…”

“…Similarly, UCYN-A and hosts regularly appeared in Monterey Bay, North of our study site, in August-November, after regional upwelling had ceased [22]. Previous studies have also related UCYN-A1 abundances to temperature: although UCYN-A distribution may not be directly influenced by temperature [9,19], warmer water conditions are favorable to increased UCYN-A1 relative abundance [21,22].…”

“…In tandem, these observations indicate that the "open ocean" ecotype of the UCYN-A symbiosis is advected into our study system by warm, tropical waters during El Niño and periods of relaxed upwelling, which foster lower biomass and coincide with higher temperatures, providing ideal conditions for the open-ocean ecotype to proliferate. UCYN-A1 relative abundances and nitrogen fixation rates were observed to increase days after upwelling in nearshore samples from the Scripps Pier and the Alaskan Beaufort shelf [21,55].…”

“…Future populations of UCYN-A1 may adhere to this trend, such that the symbiosis reaches greater abundances following future El Niño events. Because UCYN-A symbioses comprise up to 95% of the nitrogen fixing population in Southern California [21], this could substantially increase inputs of fixed nitrogen to this region.…”

Stable isotopes of nitrate record effects of the 2015–2016 El Niño and diatom iron limitation on nitrogen cycling in the eastern North Pacific Ocean

“…N 2 fixation rates have been reported for this region (Mills et al 2020). Just south of the CalCOFI domain, N 2 fixation was shown to account for 4%-5% of total production and between 8% and 83% of new production (Turk-Kubo et al 2021). As such, we cannot discount this potential influence on δ 15 N NO3 .…”