Electrophysiology of the marine diatom Coscinodiscus wailesii III. Uptake of nitrate and ammonium

Abstract

Electrophysiological investigations have been carried out to study the uptake of nitrate and ammonium in the marine diatom Coscinodiscus wailesii. Double-barrelled glass microelectrodes were used to record the responses of voltage (V) and resistance (R) between the inserted electrode tip and the outside upon exposure to external nitrate or ammonium. Significant responses were found to be restricted to certain periods following nitrogen-starvation, but consistent with the requirements of nitrogen uptake under physiological conditions. The response of V upon exposure to µM concentrations of nitrate was a delayed and reversible depolarization which decreased in amplitude upon repetitive exposures in the time range of a minute. For the ionic conditions of natural sea-water (about 500 mM Na+ and pH 8), these effects are thermodynamically consistent with a sodium-nitrate symport mechanism and inconsistent with the familiar proton-nitrate symport mechanism in glycophytic cells of vascular plants and fresh water algae (<1 mM Na+, pH <6). Replacement of extracellular K+ in artificial sea-water (normal, 10 mM K+) by ammonium caused no significant V response in non-starved cells; however, depolarizations were observed in N-starved cells, indicating an increase of the permeability ratio Pammonium/PK from about 1.0 to about 1.2 under N-starvation.