Embryos (cysts) of the brine shrimp Artemia enter a profound, yet reversible, state of metabolic arrest in response to cellular dehydration. We have monitored metabolic activity during this transition in embryos from the Great Salt Lake population by using microcalorimetric measurements of heat dissipation. Embryo hydration states can be precisely controlled by immersing cysts in solutions of varying ionic strength. When developing embryos were incubated in a 2.0moll−1NaCl solution, heat dissipation fell after 20 h to 1.13 mWg−1 dry mass, or 21 % of the value obtained when embryos were in control solutions of 0.25 moll−1. At higher ionic concentrations, heat dissipation declined to as low as 3% of control values. Recovery from dehydration was rapid. Energy flow increased to 135% of control values within 2h after returning cysts to the control medium.

These metabolic transitions were correlated with embryo hydration levels measured across the same dehydration series. Total cyst water ranged from 112±2.6 gH2O 100 g−1 dry mass in 0.25 mol l−1 NaCl to 46±0.6 gH2O 100 g−1 dry mass in 5.0 mol l−1 NaCl. At the first point where heat dissipation was markedly suppressed (the 2.0 mol l−1 incubation), cyst water content was 72.8 ±0.9 gH2O 100 g−1 dry mass. This water content is similar to the ‘critical’ hydration level required to suppress carbohydrate catabolism and respiration in San Francisco Bay Artemia embryos (Clegg, 1976a,b). However, hydration characteristics of the two populations differed in solutions of lower ionic concentration.

Total osmotic pressure in fully hydrated cysts was 1300 mosmol kg−1 H2O. A comprehensive inventory of the internal osmolytes indicated that inorganic ions (Na+, K+, Cl, Mg2+, Ca2+, Pi) accounted for 21% of the osmotic activity and 1.48% of embryo dry mass. Organic solutes (trehalose, glycerol, ninhydrinpositive substances, and trimethylamine-N-oxide+betaine) contributed 60% of the osmotic pressure and 22% of the dry mass. Macromolecular components (protein, lipids, glycogen and DNA) were also quantified and formed the bulk of embryo mass. Taken together, 97.4% of the cyst dry mass was identified. At the cellular dehydration state promoting metabolic arrest, the concentrations of inorganic and organic osmolytes were 480–590 mmol kg−1 H2O and 1200–1480 mmol kg−1 H2O, respectively. The influence of these osmolyte concentrations is considered in the context of macromolecular assembly and metabolic control.

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