A comparison of ammonium and nitrate as nitrogen sources for photolithotrophs

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Abstract
Summary: Potential advantages of ammonium relative to nitrate assimilation (assuming equal nitrogen supply in the two forms), deduced from known biochemical pathways and the site of nitrate assimilation in vascular land plants, include (i) a greater maximum specific growth rate, (ii) lower costs of photons and (in transpiring plants) water per unit carbon assimilated, and (iii) lower costs of iron, manganese and molybdenum per unit carbon assimilated per unit time. Actual measurements show that the growth rate and photon cost predictions are often, but not invariably, borne out; while data on the water cost of growth almost invariably contradict the prediction by showing a lower water cost from nitrate‐supplied than ammonium‐supplied plants. Few data seem to be available that test the predictions as to metal costs of growth; some of the predictions are borne out. These possible or realised advantages to the photolithotroph of using ammonium must be considered in the context of the relative availability of the two sources of combined nitrogen. In the oceans nitrification of ammonium produced in mineralization does not compete well with photolithotrophic assimilation of ammonium in the euphotic zone, so ammonium is the major combined nitrogen source for ‘recycled’ primary production, while ‘new’ production largely uses nitrate produced in deep, dark water. When both nitrogen sources are available to phytoplankton organisms ammonium is almost invariably preferred, with complete suppression of nitrate use with as little as 1–2 mmol m−3ammonium. In terrestrial habitats the nitrification of ammonium produced in mineralization is not photoinhibited (as can occur in the surface water of the ocean) but is subject to inhibition by anoxia, low pH and (possibly) plant‐produced defence compounds. However, the lower diffusion coefficient for ammonium than for nitrate in soils means that nitrogen‐limited plant growth as a given rate needs a higher mean dissolved ammonium concentration in the soil than is the case for nitrate when the soil contains only one of these nitrogen sources and root distribution and morphology are unaffected by the nitrogen source. With equal mean ammonium and nitrate concentrations, a nitrogen‐limited plant supplied solely with ammonium would need a more extensive root/root hair/mycorrhiza system to attain the same nitrogen uptake rate on a per plant basis as would a nitrate‐supplied plant, with consequences for resource allocation by, and growth rate of, the ammonium‐grown plant. In addition to the larger mean area‐based ammonium assimilation rate by photolithotrophs in the oceans, consideration of the interactions among ammonium diffusion coefficient, ammonium diffusion distance, organism surface area per unit biomass and the organism maximum specific growth rate in the ocean relative to soils can plausibly account for the lower mean ammonium concentrations in the ocean than in soils.