AME 63:193-205 (2011)  -  DOI:

Nitrogen fixation within the water column associated with two hypoxic basins in the Southern California Bight

M. Robert Hamersley1,*, K. A. Turk2, A. Leinweber3, N. Gruber3,4, J. P. Zehr2, T. Gunderson5, D. G. Capone5

1Soka University of America, 1 University Dr., Aliso Viejo, California 92656, USA
2Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA
3Institute of Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles, California 90965, USA
4Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, 8092 Zürich, Switzerland
5Wrigley Institute for Environmental Studies, University of Southern California, Los Angeles, California 90089, USA

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.

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