Richter...might you be thinking of the frequent discussions we have about anaerobic bacteria in live rock and sandbeds that gobble up our tank's nitrogenous wastes? If so...well, I SHOULD remember this much more clearly than I do...
...but if I DO recall correctly, our bacteria are definitely not your bacteria. Ours take nitrogen from nitrogenous compounts, especially nitrate, and through the various miracles of anaerobic biochemistry (yup, I forget precisely how...boy would my microbio professor thump me!) break that nitrogen free and release it as diatomic nitrogen (N2) gas, which ultimately diffuses/bubbles free of the tank.
Your nitrogen-FIXING bacteria are notable in that they break apart the atmosphere's abundant N2 gas, combine it with other molecules, and release it as a nitrogenous compound ("fixing" it, as in tying it down in a useable form) basically the reverse of what our bugs do. It's an impressive achievement on their part--nitrogen is not terribly reactive. I completely forget why they do this. But it makes plants happy!
rusty
