The cyanobacterial nitrogen fixation paradox in natural waters

Nitrogen fixation, the enzymatic conversion of atmospheric N (N ₂) to ammonia (NH ₃), is a microbially mediated process by which "new" N is supplied to N-deficient water bodies. Certain bloom-forming cyanobacterial species are capable of conducting N ₂ fixation; hence, they are able to circumvent N limitation in these waters. However, this anaerobic process is highly sensitive to oxygen, and since cyanobacteria produce oxygen in photosynthesis, they are faced with a paradoxical situation, where one critically important (for supporting growth) biochemical process is inhibited by another. N ₂-fixing cyanobacterial taxa have developed an array of biochemical, morphological, and ecological adaptations to minimize the "oxygen problem"; however, none of these allows N ₂ fixation to function at a high enough efficiency so that it can supply N needs at the ecosystem scale, where N losses via denitrification, burial, and advection often exceed the inputs of "new" N by N ₂ fixation. As a result, most marine and freshwater ecosystems exhibit chronic N limitation of primary production. Under conditions of perpetual N limitation, external inputs of N from human sources (agricultural, urban, and industrial) play a central role in determining ecosystem fertility and, in the case of N overenrichment, excessive primary production or eutrophication. This points to the importance of controlling external N inputs (in addition to traditional phosphorus controls) as a means of ensuring acceptable water quality and safe water supplies. Nitrogen fixation, the enzymatic conversion of atmospheric N ₂ to ammonia (NH ₃) is a  microbially-mediated process by which "new" nitrogen is supplied to N-deficient water bodies.  Certain bloom-forming cyanobacterial species are capable of conducting N ₂ fixation; hence they are able to circumvent nitrogen limitation in these waters. However, this anaerobic process is highly sensitive to oxygen, and since cyanobacteria produce oxygen in photosynthesis, they are faced with a paradoxical situation, where one critically-important (for supporting growth) biochemical process is inhibited by another. Diazotrophic cyanobacterial taxa have developed an array of biochemical, morphological and ecological adaptations to minimize the "oxygen problem"; however, none of these allows N ₂ fixation to function at a high enough efficiency so that it can supply N needs at the ecosystem scale, where N losses via denitrification, burial and advection often exceed the inputs of "new" N by N ₂ fixation.  As a result, most marine and freshwater ecosystems exhibit chronic N-limitation of primary production.  Under conditions of perpetual N limitation, external inputs of N from human sources (agricultural, urban, industrial) play a central role in determining ecosystem fertility and in the case of N-overenrichment, excessive primary production, or eutrophication. This points to the importance of controlling external N inputs (in addition to traditional phosphorus controls) as a means of ensuring acceptable water quality and safe water supplies.