N-Fixation And Related O-2 Constraints On Model Marine Diazotroph Pseudomonas Stutzeri Bal361

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 N-2 gas into biomass via N-fixation, reaching N-fixation rates upwards of 0.046 fmol N cell(-1) h(-1). Planktonic BAL361 cells exhibited nitro genase activity at similar to 54 mu M O-2 or less-an O-2 threshold notably lower than that recently reported (similar to 160 mu M O-2) 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 (in cluding N-rich conditions) may be key to ultimately enable diazotrophs like BAL361 to overcome the 'O-2 problem' and perform N-fixation via microoxic zones within aerobic marine bulk waters.