Okay, I'm back,
As to reverse fluidized bed filtration, I have not researched this and I doubt any research in this area exists. I would guess that a single FBF would not have enough volume to filter out O2, allowing anaerobic conditions and still provide a large surface area for the bacterial colonization. However, I would guess that it may be possible to string multiple filters together and achieve this goal... Of course, I can't claim it would last 37 years
As to nitrogen fixation, it looks like the only problem is that nitrogenase is inhibited by O2. As a consequence, nitrogen fixing organisms tend to have fairly low tolerance for O2. Therefore, it's unclear to me if these organisms would be active under highly aerobic conditions such as a trickle filter... however, this being said, I did find some possible leads.
As reviewed by Gruber N. (2005) there doesn't appear to be a mass balance of nitrogen in the upper ocean either and Gruben suggests nitrogen fixation by Trichodesmium, may be the source of the extra nitrogen. Trichodesmium is photosynthetic and therefore must be protected somehow from produced O2. Interestingly, as it turns out, although Trichodesmium single celled cyanobacterium, they are colonial in nature and form trichomes (filaments hair like structures). Berman-Frank et al. (2001) showed that within these structures, individual cells could modulate oxygen production and consumption, throughout the photo- period and therefore this mechanism may provide temporal and spatial protection against O2 production (Berman-Frank 2001). Unfortunately, it is unclear whether or not they may also be protected from O2 diffusion from high concentrations of extracellular O2.
However, there are other possibilities as well, Ribbe et al. (1997) examined the bacterium S. thermoautotrophicus, which links N2 fixation with the oxidation of superoxide. They concluded that, as a result of the unique mechanisms, S. thermoautotrophicus is "absolutely O2-insensitive". So although, nitrogenase, is inhibited by O2, there is certainly evidence for a number of protective mechanisms and WaterKeeper's hypothesis is certainly feasible.
So, WaterKeeper, I guess you can keep your job at least for another week..
Berman-Frank I, Lundgren P, Chen YB, Küpper H, Kolber Z, Bergman B, Falkowski P.
Segregation of nitrogen fixation and oxygenic photosynthesis in the marine cyanobacterium Trichodesmium.
Science. 2001 Nov 16;294(5546):1534-7.
Gruber N.
Oceanography: a bigger nitrogen fix.
Nature. 2005 Aug 11;436(7052):786-7.
Ribbe M, Gadkari D, Meyer O. N2 fixation by Streptomyces thermoautotrophicus involves a molybdenum-dinitrogenase and a manganese-superoxide oxidoreductase that couple N2 reduction to the oxidation of superoxide produced from O2 by a molybdenum-CO dehydrogenase. J Biol Chem. 1997 Oct 17;272(42):26627-33.