Nitrate Utilization by the Diatom Skeletonema costatum

Abstract

Nitrate uptake has been studied in nitrogen-deficient cells of the marine diatom Skektowema costatum. When Nitrogen appears to be the major nutrient limiting primary production in the oceans of the world (18), as well as in certain freshwater systems. Nitrogen is found in seawater as dissolved N2 (which cannot be fixed by most marine algae) and as inorganic ions: nitrate, nitrite, and ammonium. These ions are present in concentrations ranging from 0.01 to 50 pM for nitrate, from 0.01 to 5 Am for nitrite, and from 0.1 to 5 pM for ammonium. The concentration of organic forms is usually below 10 pm (15). When all three inorganic ions are present, ammonium is preferentially utilized (4). Nitrate and nitrite have to be reduced to ammonium by means of an energy-dependent enzyme system (nitrate reductase and nitrite reductase) prior to their assimilation by the cell (15).As nitrate is the most abundant form of N in seawater, marine phytoplankton, mostly composed of diatoms and dinoflagellates, utilizes nitrate as the main N source. Its uptake and reduction constitute major functions in these organisms, since N has been identified as the limiting factor controlling their growth. The relevance of the study of these processes arises from the fact that phytoplankton organisms are the major primary producers of the sea.The purpose of this paper is to characterize the kinetics of nitrate uptake in a phytoplankton organism of particular ecological significance because of its wide distribution, such as the diatom Skeletonema costatum. MATERIALS AND METHODSCulture Media and Conditions. Culture media were prepared according to Guillard and Ryther (8), with the modifications

Potassium Fluxes in Chlamydomonas reinhardtii (I.Kinetics and Electrical Potentials)

“…These authors interpreted the linear kinetics as the result of channel-mediated transport. Similar uptake kinetics have been observed for NO,-influx in barley (Siddiqi et al, 1990) and in the diatom Skeletonema costatum (Serra et al, 1978). Likewise, NH,+ influx in rice is mediated by saturable and linear transport systems (Wang et al, 1993).…”

Studies of the Uptake of Nitrate in Barley

“…1 (24) for maize. In the diatom Skeletonema costatum, a similar biphasic system for NO3-uptake has been reported (28). However, in this organism the linear system became evident at much lower concentrations (10-20 mmol m-3) than was the case for higher plants.…”

Steady-state Growth of the Marine Diatom Thalassiosira pseudonana

“…Neither the sizes of internal pools of nitrate, nitrite, and ammonia nor activities of permeases and reductases were measured. We do feel that any model of the mechanism of nitrate uptake for diatoms, such as that recently described by Serra et al (18,19), should be consistent with the metabolism described for continuous culture. In particular, understanding of the large variations in the ratio jNO2/jNO, affected by varying nutrient concentrations and light levels as shown in Figures 3 and 5 may provide insight into the regulation of the steps of nitrate assimilation.…”