A "google" revealed this. """ However, atmospheric nitrogen is unavailable for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. Although earth’s atmosphere is an abundant source of nitrogen, most is relatively unusable by plants[3]. Chemical processing, or natural fixation (through processes such as bacterial conversion--see rhizobium), are necessary to convert gaseous nitrogen into forms usable by living organisms. This makes nitrogen a crucial part of food production. The abundance or scarcity of this "fixed" form of nitrogen, (also known as reactive nitrogen), dictates how much food can be grown on a piece of land.""" from:
http://en.wikipedia.org/wiki/Nitrogen_cycleI do know that some bluegreen algae (Cyanobacteria) can fix nitrogen from the atmosphere. As far as I know most N (available nitrogen) that is available in natural unpolluted waters is from animal wastes byproductes (ammonia, decompostion, death, decay processes). Animals being bacteria to predators.
From:
http://www.visionlearning.com/library/module_viewer.php?mid=98""In fact, in order for plants and animals to be able to use nitrogen, N2 gas must first be converted to more a chemically available form such as ammonium (NH4+), nitrate (NO3-), or organic nitrogen (e.g. urea - (NH3)2CO). The inert nature of N2 means that biologically available nitrogen is often in short supply in natural ecosystems, limiting plant growth and biomass accumulation.""
Research has revealed some other items on this topic:
Nitrogen fixing bacteria associated with duck weed:
http://aem.asm.org/cgi/reprint/43/4/823.pdfAnother N fixing source:
http://www.sciencedirect.com/science?_ob...0fbef26560ff35b