Energetics of Osmoregulation in an Intertidal Copepod: Effects of Anoxia and lipid Reserves on the Pattern of Free Amino Accumulation

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Abstract
The intertidal copepod Tigriopus californicus (Baker) adjusts its intracellular free amino acid (FAA) content to regulate cell volume following osmotic stress. The near absence of either proline or alanine in the medium and increase in oxygen consumption suggests that these free amino acids are oxidized during hyposmotic stress. The theoretical costs associated with the production of typical levels of proline and alanine following hyperosmotic stress (50 to 100% seawater) were estimated to be 11.cntdot.6% of daily energy use. However, a decline in oxygen consumption rate following hyperosmotic stress was recorded, and thus supports the view that proline synthesis can interfere with Krebs cycle turnover. The rate of alanine accumulation under anoxic conditions was nearly the same as that following hyperosmotic stress but no accumulation of proline was observed. When hyperosmotically stressed under anoxic conditions, nearly 50% more alanine was accumulated by T. californicus than when only experiencing anoxia. In contrast, T. californicus under anoxic conditions accumulated only 13% of the proline normally produced following hyperosmotic stress. Copepods having high lipid levels showed a greater reliance on proline than alanine (5:1) following hyperosmotic stress compared with copepods with little or no lipid reserves (1:1). This shift in FAA accumulation from proline to alanine under restricted energy supply (i.e. anoxia and low-lipid level) would be energetically advantageous since alanine is much less constly to synthesize. The proportion of proline and alanine produced during hyperosmotic stress may be regulated at the pyruvate branch point.

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