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.
Highlights
Biological dinitrogen (N2) fixation is an important source of new N in N-limited ocean gyres [1]
This study evaluated N2 fixation along nearshore to offshore transects in the Southern California Current System (SCCS), which has been largely ignored as a region with respect to N2 fixation, despite net primary production being generally N-limited at the regional scale [38]
N2 fixation in the SCCS Marine N cycle measurements have not been focused on N2 fixation in coastal SCCS waters; our study demonstrates that it is widespread along the continental shelf between the Southern California Bight and Sebastián Vizcaíno Bay, and in adjacent offshore waters
Summary
Biological dinitrogen (N2) fixation is an important source of new N in N-limited ocean gyres [1]. N2 fixation, the energetically expensive process that converts N2 into biologically available ammonia, is carried out by diverse Bacteria and Archaea called diazotrophs. N2 fixation accounts for up to 70% of new N in the oligotrophic gyres [2], making diazotrophs critical components of open ocean biogeochemical cycles. There have been numerous recent reports of diazotrophs and N2 fixation in various temperate [4,5,6,7,8,9,10,11,12,13,14] and polar [15,16,17] coastal environments. In one of the most wellstudied temperate coastal regions, the Western North Atlantic continental shelf, N2 fixation can support up to 50% of net community production [6, 7]
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