Inorganic nitrogen metabolism.

The ultimate source of nitrogen for all forms of life is inorganic nitro­ gen. The nitrogen atom has a variety of oxidation states ranging from the disputed oxidation level of plus six as represented by the presumed short half-lived NOs [Wells ( 1 ) ] through the oxidation states of plus five (N205 or its hydrated form, HNOs); plus four (NO.); plus three (N203 or its hydrated form, HN02), plus two (NO); plus one (N20, [HNO], H2N.O.; and N02'NH2 -nitrous oxide, nitroxyl, hyponitrous acid [the dimer of the hypothetical nitroxyl], and nitramide, respectively); zero (N2); minus one (NH20H); minus two (NH2NH2) ; and minus three (NHs). With the exception of the controversial plus six oxidation state, all of the above forms have been implicated in inorganic nitrogen metabolism with either intact organisms or cell-free preparations. The important bio­ logical questions concerning most of the inorganic nitrogen forms are: (a) what are the mechanisms of their absorption and stepwise conversion to either the organic form as represented by amino acids and proteins, or to the more oxidized states as occurs in nitrification; and (b) what other func­ tions do they perform in the metabolism of the living cell, for example, in terminal electron transport and possibly in coupled phosphorylation. In recent years, the various aspects of inorganic nitrogen metabolism have received increasing attention with notable results. Although past volumes of this A nnual Review of Microbiology contained articles concerned with only one phase of the subject, namely, nitrogen fixation [Virtanen (2); Wilson & Burris (3) ; Lockhead (4)], there were no papers dealing with the broad area of inorganic nitrogen function. The present reviewers have therefore given their attention to the whole field of inorganic nitrogen metabolism. This includes nitrogen nutrition, nitrate respiration, nitrate assimilation, denitrification, nitroaryl reduction, nitrification, and nitrogen