Potassium and Rubidium Uptake in Freshwater Bivalves

Abstract

ABSTRACT Potassium transport characteristics were investigated in three species of freshwater bivalves: a corbiculid, Corbiculafluminea, and two unionids, Carunculina texasensis and Ligumia subrostrata. Using 42K, all three were found to take up potassium from dilute artificial pondwater ([K+] about 0.05 mmol l−1). The influx (Ji) was 0.72 μequivg−1 dry tissue h−1 in the corbiculid, significantly higher than the value of about 0.40/zequivg−1 dry tissueh−1 in the unionids. The K+ uptake displayed saturation kinetics in the range 0.05–0.36 mmol l−1: in Co. fluminea, there was a Jmax of 3.56 μequivg−1 dry tissueh−1 and the affinity coefficient (Km) was 0.27 mmol l−1; in Ca. texasensis, Jmax had a value of 1.8 μequivg−1 dry tissueh−1 and Km was 0.16 mmol l−1. Using K+-free artificial pondwater containing 0.03–0.04 mmol l−1 Rb+, the Rb+ influx was 0.41μequivg−1 dry tissueh−1 in the corbiculid and 0.28μequivg−1 dry tissueh−1 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 μequivg−1 dry tissueh−1, significantly higher than in Ca. texasensis, which had a Jmax of 0.84 μequivg−1 dry tissueh−1. The rubidium Km (approx. 0.05 mmol l−1) 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.