Coastal upwelling enhances abundance of a symbiotic diazotroph (UCYN-A) and its haptophyte host in the Arctic Ocean

Abstract

The apparently obligate symbiosis between the diazotroph Candidatus Atelocyanobacterium thalassa (UCYN-A) and its haptophyte host, Braarudosphaera bigelowii, has recently been found to fix dinitrogen (N2) in polar waters at rates (per cell) comparable to those observed in the tropical/subtropical oligotrophic ocean basins. This study presents the novel observation that this symbiosis increased in abundance during a wind-driven upwelling event along the Alaskan Beaufort shelfbreak. As upwelling relaxed, the relative abundance of B. bigelowii among eukaryotic phytoplankton increased most significantly in waters over the upper slope. As the host’s nitrogen demands are believed to be supplied primarily by UCYN-A, this response suggests that upwelling may enhance N2 fixation as displaced coastal waters are advected offshore, potentially extending the duration of upwelling-induced phytoplankton blooms. Given that such events are projected to increase in intensity and number with ocean warming, upwelling-driven N2 fixation as a feedback on climate merits investigation.

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

“…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]. The daily sampling resolution of these cruises or their proximity to shore may explain discrepancies with our study.…”

“…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].…”

Nitrogen Fixation at the Mid‐Atlantic Bight Shelfbreak and Transport of Newly Fixed Nitrogen to the Slope Sea

“…Frontal dynamics are thought to drive upwelling of nutrient‐rich slope waters with commensurate increases in productivity associated with fast‐growing phytoplankton (e.g., diatoms; Allen et al., 2005; Mangolte et al., 2022; Zhang et al., 2013). We posit that certain endosymbiotic diazotrophs and their hosts may be well‐poised to respond rapidly to nutrient injections, as observed for coastal UCYN‐A strains in the other upwelling systems (e.g., Mills et al., 2020; Selden et al., 2022). Moreover, if upwelling is driven by water mass convergence in the bottom boundary layer, as has been previously hypothesized (Barth et al., 2004; Houghton & Vizbeck, 1998; Linder et al., 2004), then upwelled waters may carry iron (derived from shelf sediments) into the euphotic zone, potentially relieving iron‐limitation of N 2 fixation (Moore et al., 2009).…”

“…Samples were stored at 80°C until analysis. DNA was extracted using the AllPrep MiniKit (Qiagen) following manufacturer's protocols but with the addition of a bead-beater step as described in Selden et al (2022).…”

“…Subsequent development and broad application of both molecular and microscopic approaches led to the identification of numerous other diazotrophs which differ in their biogeography (Zehr & Capone, 2020), including several symbiotic diazotrophic clades residing within diatom (Carpenter et al., 1999; Schvarcz et al., 2022) and haptophyte (Thompson et al., 2012) hosts. These groups are thought to be less sensitive to ambient dissolved inorganic N (Zehr & Capone, 2020) and, in some cases, appear to thrive in coastal upwelling systems (e.g., Mills et al., 2020; Selden et al., 2022; Voss et al., 2006), potentially extending the life of phytoplankton blooms where ambient N is drawn down in advance of other nutrients. Realization that these organisms significantly augment ocean fixed N inputs, and that community N 2 fixation in many systems is less sensitive to ambient N than previously thought (Knapp, 2012; Mulholland & Capone, 2000; Mulholland et al., 2001) has led to increased interest in coastal N 2 fixation.…”