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 22Na+ 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 22Na+ labelling gave lower estimates of the Na+1] than a rinsing technique using isotonic Ca(NO3)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 22Na+ 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 22Na+ 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).