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sub-section  The Nitrogen Cycle  The main features of the
nitrogen cycle must be considered because of the outstanding
importance of nitrogen to all forms of life and because of the
great general interest it holds in illustrating the successive
ecological implications with respect to bacteria.  It has been
pointed out (chapter VI) that nitrogen exists in the sea in
combination with other elements, for example, in ammonia (NH3),
and as oxides of nitrogen in nitrite ion (NO2), and nitrate ion
(NO3). Nitrogen enters into the composition of all living
things. It is one of the building blocks (nutrients) used by
plants in forming the complex protein molecules of their bodies
from which animals must derive their nitrogen. However, not all
forms of nitrogen can be used by plants, hence the complex
nitrogenous compounds found in both plants and animals must,
upon death of the organisms, be decomposed, along with their
products of excretion, to chemically si