Hydration State, Metabolism, and Hatching of Mono Lake Artemia Cysts

Artemia monica, the only macrozooplankton in Mono Lake, California, is unique among brine shrimp in that it produces encysted diapause embryos that sink to the lake bottom where they overwinter. Currently, the lake's salinity is about twice as high as it was 40 years ago and, at equilibrium, it is projected to fluctuate between 169-248 g/l. Here we describe the effects of salinity on the termination of diapause, hatching, carbohydrate metabolism, and hydration of the cysts. As expected, hatching is much more sensitive to salinity than is termination of diapause. Carbohydrate metabolism, which involves the conversion of trehalose to glycerol and is required for hatching, responds to increasing salinity as reported in other Artemia species: increasing amounts of glycerol must be synthesized as salinity is raised. The unfreezable water in these embryos is 0.29 g H2O/gram dry weight (gdw) cysts, similar to values reported for other biological systems. This result and previous studies suggest that water probably becomes limiting at hydration levels of about 0.60 g H2O/gdw cysts. In Mono Lake water, the cysts reach this critical hydration at a salinity between 140-160 g/l, equivalent to approximately 3780-4330 mOsm/ kg. We conclude that Artemia monica will cease to exist within this salinity range and doubt that it can hatch beyond this limit, which is imposed by the requirement of metabolic processes for minimal amounts of cellular water.