Proline Betaine and Amino Acid Accumulation in Sea Slugs (Elysia chlorotica) Exposed to Extreme Hyperosmotic Conditions

Elysia chlorotica do not exhibit a volume recovery following a hyperosmotically induced shrinkage. However, in response to hyperosmotic conditions, the tissues of E. chlorotica accumulate amino acids and proline betaine. The major amino acids (Ala, Pro, Gin, and Glu) all have unique time courses of concentration change. Alanine alone accumulates rapidly; the others increase in concentration only days or weeks later. Furthermore, all the amino acids ultimately return to prestress levels, even though the stress continues. At its peak, Ala contributes 190 μmol/g dry wt. The other amino acids never contribute more than 40 μmol/g dry wt to the osmolyte pool. Proline betaine begins to accumulate between 4 and 7 days after the stress. It reaches peak concentrations at 14 days and, unlike the amino acids, remains elevated. Although the intracellular proline betaine concentrations are much higher (651 μmol/ g dry wt) than those of the amino acids, it can not balance the entire external osmotic increase and, as a result, does not provide sufficient solute for a volume recovery. The delayed time course of the proline betaine accumulation is consistent with the hypothesis that action of the genome is required for the synthesis. Finally, Elysia lacking chloroplasts have similar proline betaine concentrations to normal animals, indicating that the molecule is not a chloroplast product.