Marine life, Marine nitrogen fixation cycle

Marine life and nitrogen cycle fix
I live by an estuary and this stretch of water is inhabited by lots of fish who have seals as predators. The local fishermen dislike the seals and would rather they left.
In the last month their wishes have come true and a pod of killer whales has appeared on the scene and they are feasting on the baby seals. The baby seals are very vulnerable as they begin to swim out of their mothers care.
The natural balance of he shore life will no doubt change as a result of these new visitors. Killer whales are the third most common mammal on the planet after man and the rat.
The cycle of life is ultimately dependent upon the massive food cycles. One such is the nitrogen fixation that takes place in the oceans.
The flow of nitrogen compounds between the oceans and the atmosphere is central to life, as nitrogen is a fundamental component of bio-mass and is essential for many biological processes. Much is known about the nitrogen cycle of the oceans, but there unanswered question.
Evidence is developing which shows that the primary process which puts nitrogen compounds into the sea i.e. biological nitrogen fixation is intimately associated, with marine nitrogen removal. The ratio of nitrogen to phosphorus in sea water may be the central factor that regulates nitrogen fixation.
Biological nitrogen fixation, the enzyme-catalysed reduction of nitrogen gas (N:) continually adds nitrogen to the sea in the form of compounds that can be used as nutrients. Nitrogen fixation is commonly associated with certain cyanobacteria that live in the warm, sunlit surface waters of low-latitude oceans’. These photosynthetic bacteria tap the immense reservoir of dissolved N: gas in sea water, but their growth is often limited by the scarcity of other nutrients such as phosphorus and iron . A ratio of iron to phosphorous of 16:1 is the norm.
Fixed nitrogen is eventually converted to nitrate by nitrifying bacteria. Ultimately, the loss of nitrogen from the ocean occurs by denitrification, a process that converts nitrogen compounds such as nitrate back to N-. Denitrification occurs mostly at depths of 200-700 metres in the ‘oxygen minimum zones’ (OMZs) of the ocean — that is, in the eastern tropical north Pacific, the eastern tropical south Pacific Ocean , the Arabian Sea and in marine sediments.
The unusually high growth rates of nitrogen fixers in the north Atlantic result from the high rate of growth of nitrogen fixers stimulated by the high flux of iron containing dust blown in the wind from North Africa
Capone and Knapp Nature 2007, January 11, vol 445, pp 159-160
Deutsch et al Nature, 2007, , vol 445, 163-167.

Martin Eastwood
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