A comparison of ammonium and nitrate as nitrogen sources for photolithotrophs

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−3 ammonium. 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.

The Uptake of Amino Acids by Microbes and Trees in Three Cold-Temperate Forests

“…NH 4 ϩ is positively charged and adsorbs readily onto the cation-exchange capacity of temperate forest soils rendering NH 4 ϩ largely immobile. By contrast, glycine and NO 3 Ϫ are neutral and negatively charged, respectively; they do not adsorb as readily onto exchange sites in soil; and both forms are mobile in soil water, often moving to existing root surfaces through transpirational water (Raven et al 1992, Chapin et al 2002. Therefore, plants may not produce as many carriers for the uptake of glycine and NO 3 Ϫ as they might for NH 4 ϩ , giving rise to the pattern of N uptake observed in this study.…”

Soluble and membrane‐associated nitrate reductases in the dinoflagellate Peridinium gatunense

“…In many natural environments low concentrations of ammonium represent the main nitrogen source. However, as a consequence of human activity, nitrate is dominant in many habitats (Raven et al, 1992). Thus a study of the assimilatory reduction of nitrate can provide valuable information on the strategies adopted by different photoautotrophs in order to proliferate.…”

Antarctic Sea Ice--a Habitat for Extremophiles

“…This dependency may be enhanced by the elevated pH in sea-ice brines increasing the dissociation of ammonium to ammonia that can diffuse directly into cells. (18,19). Ammonium levels can be as high as 150 M in sea ice (in seawater the values are generally Ͻ1 M), and such concentrations will also inhibit the assimilation of nitrate, owing to the inactivation of nitrate reductase by glutamine produced during the assimilation of ammonia (20).…”