Despite the abundance of the element in the atmosphere, relative inertness of nitrogen gas rather restricts most photautotrophic exploitation to nitrogen compounds. The element is also poorly represented in the Earth's crust: its occurrence is largely restricted to biogenic layers in sedimentary rocks. The principal forms of combined nitrogen available to photoautotrophs are the ions nitrate, nitrite and ammonium (NO3- , NO2- and NH4+), although this may not be exclusively true for all phytoplankton (see below). Very little of the available resource, either in lakes or in the sea, is due to direct atmosphere-to-water linkages: most of the sources of combined nitrogen in water are imported from terrestrial systems or are recycled within the aquatic system. The ready solubility of most inorganic nitrogen compounds, the rarity of their occurrence in secondary polymers and the redox sensitivity of their ionic configurations assist the frequency of transformations and relocation. The biogeo-chemical cycling of nitrogen is mediated mainly by organisms. Accordingly, its turnover is regulated predominantly at the physiological level, and so is extremely rapid when compared to the cycling of other elements such as phosphorus or silicon.
Of the three main sources of inorganic combined nitrogen, it is the highest-oxide form that occurs most widely in solution in lakes and seas. In the deep oceans, nitrate concentrations are generally in the range 20-40 | M (280-560 mg N m-3) but, towards the surface (the upper 50-100 m or so), they may be drawn down severely as a result of algal and microbial uptake, to levels close to the limits of conventional analysis (~1 |M). Among the most nitrate-deficient waters are those of the North Pacific Gyre, the subtropical Atlantic (including the Sargasso) and Indian Oceans (McCarthy, 1980). At the other extreme, temperate shelf waters, especially those influenced by large fluvial outfalls, may have nitrate levels of 60-70 | M. In lakes and rivers, especially in the temperate regions, the nitrate availability may reach 50-65 |M in late winter (generally the time of minimum biological demand, slowest terrestrial denitrification and maximum leaching: George, 2002). In regions subject to intensive modern agriculture and relatively heavy applications of nitrogen fertiliser, leachate may raise the dissolved inorganic nitrogen concentration in receiving river waters to up to 1 mM (14 g N m-3). However, on the ancient continents at lower latitudes and, especially, in arid regions, the amount of nitrate lost from catchment topsoils is usually small and subject to further microbial denitrification. Thus, receiving waters tend to be relatively more deficient in nitrate (1-10 |M, 15-150 mg N m-3) than in phosphorus (Reynolds, 1997a). Even at temperate latitudes, however, barren upland catchments may be capable of delivering only low concentrations of nitrate (<15 | M). There is considerable evidence (Soto et al., 1994; Diaz and Pedrozo, 1996) of a nitrogen-regulated carrying capacity in the oligotrophic lakes of Patagonia and the southern Andes (total N <300 |g N L-1, some <100 |g N L-1; equivalent to 7-20 |M as nitrate supplied). These observations prompt questions about comparative current deliveries of nitrate in northern-hemisphere rainfall, which may have been relatively more augmented by industrial airfill than in the southern hemisphere.
Nitrate ions are sensitive to the low-redox conditions (<+300 mV) in sediments, the deep water of stratified, eutrophic lakes and seas and in other (usually polluted) waters experiencing high biochemical oxygen demand. Reduction to lower oxides (nitrite), to nitrogen gas and ammonia is accelerated through microbial oxidation of organic carbon and its requirement for alternative electron acceptors to the diminishing quantities of oxygen. Specifically, the activites of the denitrifying nitrate reducers like Thiobacillus deni-trificans and various pseudomonad bacteria result in the venting of nitrogen gas to the atmosphere. Nitrate ammonification occurs through the agency of facultatively anaerobic bacteria, such as Aeromonas, Bacillus, Flavobacterium and
Vibrio, first reducing nitrate to nitrite. This may be excreted or, under appropriate conditions, some of these organisms reduce the nitrite further, to hydroxylamine (NH2OH) and ammonium (Atlas and Bartha, 1993).
The ammonium ions are more soluble (so less volatile) than nitrogen, hence the reduction is more of a transformation within the pool of inorganic nitrogen, denoted by DIN (dissolved inorganic N), rather than a loss therefrom. Whereas nitrate may dominate the DIN fraction in the open water of seas and lakes, in-situ biologically mediated redox transformations may lead to the accumulation of comparable quantities of nitrite and, especially, ammonium (to >1 g N m-3, 70 | M) in microaerophilous or anoxic environments. Ammonium is typically also present in oxic, unpolluted surface waters, though rarely in excess of ~150 mg N m-3 (or <10 |M) (Reynolds, 1984a).
The sources of nitrogen available to phyto-plankton may be supplemented by certain dissolved and bioavailable organic nitrogen compounds (DON). These include urea (McCarthy, 1972), which is produced mainly as an excretory metabolite of animal protein metabolism, as well as through the bacterial degradation of purines and pyrimidines. McCarthy's (1980) compilation of urea concentrations recorded in the literature reveals concentrations under 3 | g-atoms N L-1 (<3 |M N) in the sea and up to ~9 |M in some North American rivers. Other sources of organic nitrogen directly available to phyto-plankton include the small amounts (generally <1 |M N) of free amino acid present in lakes and seas (McCarthy, 1980). The relevant deam-inases are said to be produced by microalgae only under conditions of DIN deficiency (Saubert, 1957; Turpin, 1988).
Of course, the size and dynamics of the DON pool is of additional indirect relevance to the pelagic function. Far from being refractory, DON is frequently the major source of nitrogen available to planktic microbes (>80% of the nitrogen available in oceanic surface waters is organic: Antia et al., 1991) and some of it is evidently metabolised rapidly (in days rather than weeks; see the review of Berman and Bronk, 2003). Plank-tic algae and cyanobacteria may contribute to the
DON pool as well as benefit from the microbial liberation of DIN.
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