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

Electrophysiological investigations have been carried out to study theuptake of nitrate and ammonium in the marine diatom Coscinodiscuswailesii. Double-barrelled glass microelectrodes were used torecord the responses of voltage (V) and resistance(R) between the inserted electrode tip and the outsideupon exposure to external nitrate or ammonium. Significant responses werefound to be restricted to certain periods following nitrogen-starvation,but consistent with the requirements of nitrogen uptake under physiologicalconditions. The response of V upon exposure to M concentrations of nitrate was a delayed andreversible depolarization which decreased in amplitude upon repetitiveexposures in the time range of a minute. For the ionic conditions ofnatural sea-water (about 500 mM Na⁺ and pH 8), theseeffects are thermodynamically consistent with a sodium-nitrate symportmechanism and inconsistent with the familiar proton-nitrate symportmechanism in glycophytic cells of vascular plants and fresh water algae(<1 mM Na⁺, pH <6). Replacement ofextracellular K⁺ in artificial sea-water (normal, 10mM K⁺) by ammonium caused no significantV response in non-starved cells; however,depolarizations were observed in N-starved cells, indicating an increase ofthe permeability ratioPammonium/Pk from about 1.0 toabout 1.2 under N-starvation.