AME 63:193-205 (2011)  -  DOI: https://doi.org/10.3354/ame01494

ABSTRACT: We measured pelagic dinitrogen (N₂) fixation by incorporation of ¹⁵N₂ during regular cruises over 4 yr to deep hypoxic basins in the Southern California Bight, USA. N₂ fixation in the photic zone was dominated (80%) by nanoplankton (<10 µm). N₂ fixation rates in surface waters were near the upper range measured for nanoplankton in tropical waters, averaging 5.8 µmol m^–3 d^–1 at the San Pedro Ocean Time Series (SPOTS) station, and 2.4 µmol m^–3 d^–1 at the Santa Monica Bay Observatory (SMBO) station, with a maximum at SPOTS of 35 µmol m^–3 d^–1. Quantitative polymerase chain reaction (qPCR) assays and nested PCR-based clone libraries targeting the nitrogenase gene nifH indicate that the uncultivated unicellular cyanobacterial group A (UCYN-A) is an abundant diazotroph in the photic zone. Although N₂ fixation rates were highest at the surface, mean N₂ fixation averaged 0.07 µmol m^–3 d^–1 at depths of 500 and 885 m within hypoxic basin waters (<10% O₂ saturation). When integrated over the aphotic water column, this deep N₂ fixation may account for as much as one-third of the total areal N₂ fixation, estimated at 150 µmol N m^–2 d^–1. These deep hypoxic N₂ fixers were an assemblage of heterotrophic bacteria, including Alpha- and Gammaproteobacteria and putative sulfate-reducing bacteria. Our results suggest that N₂ fixation could play a role in other hypoxic, high-nitrate waters.

KEY WORDS: Nitrogen fixation · Diazotrophy · Southern California Bight · Hypoxic basin

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Cite this article as: Hamersley MR, Turk KA, Leinweber A, Gruber N, Zehr JP, Gunderson T, Capone DG (2011) Nitrogen fixation within the water column associated with two hypoxic basins in the Southern California Bight. Aquat Microb Ecol 63:193-205. https://doi.org/10.3354/ame01494 Export citation Share:    Facebook - - linkedIn