Corg-cycle is coupled with Norg-cycle. Nitrogen cycle begins by nitrogen fixation. Nitrogenase enzyme is present in prokaryotes exclusively and distributed among different groups. N2-fixation is an energetically expensive process. It occurs only on severe limitation on the bound nitrogen. It is facilitated in a reductive environment. The main groups of nitrogen-fixing bacteria are cyanobacteria and anaerobic bacteria. Aerobic nitrogen-fixing bacteria are either plant symbionts or organisms within the community supplied in excess by the nitrogen-free organic matter. Corg:Norg ratio over 20 stimulates N2-fixation. Fixed N2 is included into the biomass. Then the regenerative cycle begins. In the sea, its zone is just below the photic zone. Nitrogen is liberated in the proteolytic pathway with ammonification as summation of the process. NH4 + is either re-assimilated, or is metabolized by the two-step conversion into nitrate by chemosynthetic nitri-fiers in the presence of O2. Nitrate is assimilated by phytoplankton or plants. Part of it escapes from the productive zone. In the anoxic zone, in the presence of available organic matter, various denitrifiers use nitrate and nitrite as oxidants and reduce it to N2 closing the cycle. As a variant, nitrate reduction to ammonia occurs but this is a less important pathway. Denitrification is the closing step in the nitrogen cycle. Nitrate makes a reservoir of bound nitrogen in the deep cold ocean of ^20Eg. It is noticeable how important denitrification is in the marine environment for decomposition of organic compounds including hydrocarbons. Limitation of the availability of bound nitrogen is the major problem for plant productivity. In the sea, seasonal exhaust of nitrates definitely determines algal development as it was demonstrated for the Northern Sea. For the Pacific, a peculiar change in plankton composition indicates an N- and P-cycle interrelation: in nitrate-limited conditions cyano-bacteria dominate when there is enough phosphate, whereas in phosphate-limited conditions algae are the main group in phytoplankton.
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