Acid brine shrimp: Metabolic strategies in osmotic and ionic adaptation

Acid salt lakes are found in several regions of Australia but are uncommonly abundant in the Yilgarn Block area of southwestern Australia. The chemical properties of the acid salt lakes are in general similar to those of shallow ephemeral alkaline salt lakes found in adjacent regions except for having a higher hydrogen ion concentration and an absence of carbonate and bicarbonate ions. The Australian brine shrimp, Parartemia, is the major zooplankter living in these salt lakes. Present investigation on two species of larval Parartemia reveal P. zietziana nauplii having high salt tolerance (LD₅₀ > 225 TDS) but a narrow pH range with an optimum lying near pH 8. In contrast, P. contracta collected from acid salt lakes had a more restricted salt tolerance (LD₅₀ < 100 TDS) but a wider range of pH tolerances with substantial survival below pH 3.5. Both species, P. zietziana and contracta, when placed in ouabain-laden salines, demonstrated decreased survival and indirectly indicated the presence of a ouabain-sensitive sodium pump. Direct enzymatic assay of the sodium pump (Na, K ATPase) in nauplii of P. zietziana gave specific activity values of 2.9 µM Pi/hr/mg protein supporting our working hypothesis that the nauplii of Parartermia have an osmoregulatory system similar to that found in larval Artemia which is dependent upon having large quantities of ATP to support the sodium pump. In Artemia larvae, the production of ATP is enhanced through a facultative pathway involving an aerobic glycolysis linked C-4 dicarboxylic shunt. The major CO₂ source for the C-4 acid shunt for alkaline brine shrimp has been found to be the dissolved bicarbonate/carbonate ions. In highly acidic saline lakes, these ions are missing. If acid brine shrimp are to survive in low pH ephemeral saline lakes, they must have evolved an additional proton pump and devised a mechanism to produce ATP from endogenous CO₂ substrates.