AME 81:125-136 (2018)  -  DOI: https://doi.org/10.3354/ame01867

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