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Aquatic Microbial Ecology

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AME 81:125-136 (2018)  -  DOI:

N-fixation and related O2 constraints on model marine diazotroph Pseudomonas stutzeri BAL361

Ryan W. Paerl*, Tobias N. G. Hansen, Nathalie N. S. E. Henriksen, Asmus K. Olesen, Lasse Riemann

Marine Biological Section, University of Copenhagen, 3000 Helsingør, Denmark
*Corresponding author:

ABSTRACT: Marine non-cyanobacterial diazotrophs are widespread in the ocean and can be the dominant nitrogen (N) fixers in certain regions. Lagging behind distribution and diversity data for these diazotrophs is a fundamental understanding of their physiologies—particularly in regards to dealing with oxygen, a potential inhibitor of N-fixation present in most of the ocean. To address this constraint, we conducted multiple experiments with Pseudomonas stutzeri BAL361, a model marine planktonic non-cyanobacterial diazotroph previously isolated from the Baltic Sea. Here, we confirm that BAL361 uses nitrogenase to convert N2 gas into biomass via N-fixation, reaching N-fixation rates upwards of 0.046 fmol N cell-1 h-1. Planktonic BAL361 cells exhibited nitrogenase activity at ~54 µM O2 or less—an O2 threshold notably lower than that recently reported (~160 µM O2) in experiments with BAL361 where large aggregates were observed. Provision of hydrophobic or hydrophilic particles or surfaces, used previously to stimulate N-fixation by aerobic natural communities, did not enhance N-fixation by aerobic BAL361 cultures. We empirically show that bulk N-fixation under aerobic conditions by BAL361 alone is possible by aggregation; however, it remains elusive how low numbers of solitary planktonic BAL361 cells in nature accomplish this same feat. Our findings draw new attention to the possibility that nutrient-rich conditions (including N-rich conditions) may be key to ultimately enable diazotrophs like BAL361 to overcome the ‘O2 problem’ and perform N-fixation via microoxic zones within aerobic marine bulk waters.

KEY WORDS: Diazotrophs · N-fixation · Pseudomonas · Oxygen · Marine particles

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Cite this article as: Paerl RW, Hansen TNG, Henriksen NNSE, Olesen AK, Riemann L (2018) N-fixation and related O2 constraints on model marine diazotroph Pseudomonas stutzeri BAL361. Aquat Microb Ecol 81:125-136.

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