Potassium and Rubidium Uptake in Freshwater Bivalves

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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 mmoll+1). The influx (Ji) was 0.72μequiv g−1dry tissue h−1 in the corbiculid, significantly higher than the value of about 0.40μequiv g−1dry tissuer g−1 in the unionids. The K+ uptake displayed saturation kinetics in the range 0.05-0.36 mmoll−1: in Co. fluminea, there was a Jmax of 3.56μequiv g−1 dry tissue h−1 and the affinity coefficient (Km) was 0.27mmoll−1; in Ca. texasensis, Jmax had a value of 1.8 μequiv g−1dry tissue h−1 and Km was 0.16mmoll−1. Using K+-free artificial pondwater containing 0.03-0.04 mmoll−1 Rb+, the Rb+ influx was 0.41μequiv g−1 dry tissue h−1 in the corbiculid and 0.28 μequiv g−1 dry tissue h−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 1.4μequiv g−1 dry tissueh+1, significantly higher than in Ca. texasensis, which had Jmax of 0.84μequiv g−1 dry tissue h−1. The rubidium Km (approx. 0.05 mmoll−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.

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