Potassium and Rubidium Uptake in Freshwater Bivalves

Potassium transport characteristics were investigated in three species of freshwater bivalves: a corbiculid, Corbiculafluminea, and two unionids, Carunculina texasensis and Ligumia subrostrata. Using K, all three were found to take up potassium from dilute artificial pondwater ([K] about O.OSmmolP). The influx (7i) was 0.72/iequivg"drytissueh^ in the corbiculid, significantly higher than the value of about O.^jiequivg^drytissuerT in the unionids. The K uptake displayed saturation kinetics in the range 0.05-0.36 mmolP: in Co. fluminea, there was a 7max of 3.56/lequivg" 1 dry tissue h" and the affinity coefficient (Km) was O^Tmmoll"; in Ca. texasensis, Jmax had a value of 1.8 jiequivg^dry tissue h ' 1 and Km was 0.16mmoll~ . Using K-free artificial pondwater containing 0.03-0.04 mmoi r 1 Rb, the Rb influx was 0.41/lequivg" dry tissue h" in the corbiculid and 0.28 //equivg" dry tissue h" in Ca. texasensis. All animals lost K during the rubidium flux studies, and since they contained no Rb, the Rb efflux was zero and the net flux was equal to the influx. The Jmax values for Rb + were lower than the corresponding values for potassium: in Co. fluminea, Jmax was 1.4/^equivg" dry tissueh", significantly higher than in Ca. texasensis, which had a/m a j of 0.84/xequivg" 1 dry tissue h". The rubidium Km (approx. O.OSmmoll" ) values were significantly lower than corresponding values for potassium. Salt depletion increased the rubidium transport rate fourfold for both Co. fluminea and Ca. texasensis. High rates of net K uptake may account for the bivalves' inability to tolerate elevated environmental potassium.