SODIUM TRANSPORT IN ENTEROMORPHA INTESTINALIS (L.) LINK

S ummary Sodium transport was studied in the marine euryhaline alga, Enteromorpha intestinalis in seawater (465 mM Na + and in low salinity medium [Artificial Cape Banks Spring Water (ACBSW), 25.5 mM Cr, 20.4 mM Na + , 0.5 mM K + ]. The intracellular Na + concentration ([Na + 1 ]) of E. intestinalis was so low that it was difficult to detect using chemical and 22 Na + methods. Consequently, intracellular Na + fluxes were also difficult to measure. Most of the Na + of the Enteromorpha tissue was bound to the fixed negative charges of the cell wall and this binding has, in previous studies, led to great overestimates of the intracellular Na + of this plant‐Data of 22 Na + labelling gave lower estimates of the Na + 1 ] than a rinsing technique using isotonic Ca(NO 3 ) 2 . The overall mean [Na + 1 ] of seawater plants was only 5.5 ± 1.4 mM, with a value of 0.623 ± 0.163 mM Na + in ACBSW plants. With one exception, all the seawater 22 Na + experiments indicated a single intracellular exchange phase, i.e. a separate vacuolar phase could not be detected. The data on plants grown at low salinity could be interpreted as having either a single intracellular phase or two intracellular phases because of the problem of cell wall Na + exchange. No significant difference was found in total 22 Na + uptake or exchange fluxes in the light and dark in seawater‐grown plants but there may have been a light effect on low salinity plants. The Na + flux in Enteromorpha plants in seawater was about 3 nmol m −2 s −1 and in low salinity plants was about 0.2 nmol m −2 s −1 . Sodium in Enteromorpha is far from electrochemical equilibrium (more than –100 mV) in plants in both seawater and ACBSW medium so that Na + is actively excluded from the cells. The plasmalemma has a very low Na + permeability (seawater, 3 pm s −1 ; ACBSW plants, either 3 or 100 pm s −1 depending on which compartmentation model is accepted).