AMMONIA AND AMMONIUM FLUXES BETWEEN PHOTOLITHOTROPHS AND THE ENVIRONMENT IN RELATION TO THE GLOBAL NITROGEN-CYCLE

Ammonium (used here to signify NH4+ plus NH3) is the immediate inorganic precursor of organic nitrogen in photolithotrophs. In marine habitats ammonium is also the major exogenous nitrogen source, and the same is probably true of terrestrial habitats. In addition to this major role of ammonium as exogenous nitrogen source, it is also quantitatively very important as an endogenous nitrogen source with nitrate or N2 as exogenous nitrogen source, and as a recycled nitrogen compound in photorespiration and in phenylpropanoid synthesis. The shoots of terrestrial plants have higher NH3 compensation partial pressures than most natural soils, and especially higher than that of the sea. However, long-distance transport of gaseous NH3 (e.g. from continents to oceans) is a negligible component of the 'natural' global nitrogen cycle. Current concepts of evolution of the atmosphere and biosphere do not involve high partial pressure of NH3 in the atmosphere: any ammonium produced in inorganic or biological processes is removed from the atmosphere by rain-out, or, over larger time-scales, by the action of ultraviolet radiation (especially before the ozone screen came into being) and of hydroxyl radicals (especially after oxygenation of the atmosphere). In addition to posing problems for the origin of life and arguing for an early rather than a late evolution of diazotrophy, a low NH3 partial pressure renders implausible arguments that phenylpropanoid synthesis was restricted early in evolution by the effect of high ammonia in reversing phenylalanine ammonia-lyase.