Electrophysiology of the marine diatom Coscinodiscus wailesii II. Potassium currents

Abstract

upon changes in [K+] o , are well described by a threestate reaction cycle for catalysis of K+ translocation The dominating mechanism of K+ uniport through the with a steady activity. plasmalemma of Coscinodiscus wailesii has been studied in some detail as part of a general study of Key words: Coscinodiscus, current‐voltage relationships, ionic relations in marine diatoms. Electrical measure- diatoms, inward rectification, phytoplankton, potassium ments with double-barrelled glass-microelectrodes selectivity. have been made in intact cells (diameter #250 mm, height #100 mm) bathed in artificial sea-water in Introduction which [K+] has been changed from 3 mM to 100 mM. Using a modified Goldman equation, these results Marine diatoms are of global importance as they accomprovide an estimate of [K+] i of about 400 mM and a plish about 20‐25% of the earth’s assimilation ( Werner, selectivity for K+ over Na+ and Cl’, which could spon- 1977). However, very little is known about their physitaneously vary by orders of magnitude and reach ology and especially their membranes. Gradmann and values of about 1000. Voltage-clamp experiments have Boyd (1995) have reported the first measurements of the been carried out in these states of high K+ selectivity membrane voltage of the marine diatom Coscinodiscus using bipolar staircase command voltages over a range radiatus. Methodological improvements of the medium from ’180 to +60 mV. The resulting steady-state cur- exchange and voltage-clamp system (Boyd and rent‐voltage relationships have inward rectifying sig- Gradmann, 1999a) and using a the larger species moid characteristics with a negative saturation current Coscinodiscus wailesii enabled a more detailed electroaround ’300 nA, a positive saturation current of about physiological characterization of the plasmalemma of 30 nA, and a slope conductance of the order of 1 mS these diatoms. Currently, this characterization consists of at free running voltages <’60 mV. Temporal a unique collection of new phenomena (Boyd and responses of the clamp currents upon rectangular volt- Gradmann, 1999a), of an electrophysiological investiage steps were basically rectangular, i.e. they did not gation of N-nutrition (Boyd and Gradmann, 1999b) and show the familiar relaxation kinetics of voltage- of this study which focuses on K+ relations. Since K+ induced activation/inactivation. The sigmoid steady- currents frequently dominate the electrophysiological state current‐voltage relationships could not be behaviour of biological membranes, the results of this described by a usual model of constant-field currents study are expected (a) to allow a comparison with known through a voltage-gated pore, where the positive cur- membranes, and (b) to provide reliable background rent of an inward rectifier would not saturate but knowledge for the interpretation of the electrophysiolovanish. Alternatively, the observed steady-state inward gical phenomena which comprise movement of ions other than K+.