Nitrogen is an essential component of amino acids and proteins but, because the gas is very inert, its incorporation in the biosphere is largely dependent on nitrogen fixation by certain bacteria, including cyanobacteria. These are able to convert gaseous nitrogen into reactive ammonia by means of the enzyme nitrogenase:
nitrogenase N2 + 3H2 ^ 2NH3
This ammonia can then be incoroporated into proteins. The nitrifying bacteria are able to convert ammonium ions into nitrite while others can convert the latter into nitrate ions:
Nitrate is extremely important as a biolimiting nutrient for photoautotrophic primary production. Formation of gaseous nitrogen from nitrate is achieved by the anaerobic denitrifying bacteria which use the nitrate ion as a hydrogen receptor for the oxidation of sulphur:
Other bacteria can produce nitrites and later ammonia using the nitrate ion for reduction while some other substance is oxidized:
This process can produce pyritic infillings and/or replacements of fossils in the sediment. Because of dis-equilbrium fractionation processes during sulphate-reduction, this sulphide is depleted in the stable isotope 34S by 4-46% compared with standard sea water.
A second group of sulphur-processing bacteria can convert this sulphur and sulphide back into sulphate, using oxygen as the electron acceptor. These sulphide-and sulphur-oxidizing bacteria may occur as biofilms above the sulphate-reducing zone. They also bloom copiously around hot, sulphide-rich submarine vents known as 'black smokers', where they play an important role in the food chain of the vent community. Their metabolism results in further 34S depletion of the sulphur isotopes (to - 60%), which can be measured in the fossil record.
This form of anaerobic respiration has a lower energy yield than aerobic respiration. Denitrification of this kind is widely found in the oxygen minimum zone of the oceans. The removal of nitrate from surface waters can act as a check on photosynthetic algal blooms and thereby prevent runaway anoxia.
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