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

Aquatic Microbial Ecology

J. P. Zehr¹  M. Robert Hamersley¹  T. Gunderson¹  K. A. Turk¹  A. Leinweber¹  D. G. Capone¹  N. Gruber¹ 

关键词: Nitrogen fixation;    Diazotrophy;    Southern California Bight;    Hypoxic basin;   

DOI  :  10.3354/ame01494

学科分类：生物科学（综合）

来源: Inter-Research

【 摘 要 】

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.

【 授权许可】

Unknown