OSMOREGULATION IN A MARINE CILIATE, MIAMIENSIS AVIDUS. I. REGULATION OF INORGANIC IONS AND WATER

¹ Marine Biological Laboratory, Woods Hole, Massachusetts

1. The euryhaline marine ciliate, Miamiensis avidus, was investigated for its ability to regulate solutes and water when exposed to different external salinities.

2. In 100% sea water-culture medium, M. avidus had the following inorganic ion concentrations (mM/kg cells): Na⁺—87.9; K⁺—73.7; Ca⁺⁺3.7;—Mg⁺⁺—28.5; Cl^-—60.8.

3. In 100% sea water-culture medium, [Na⁺]_i, [Cl^-]_i, [Mg⁺⁺]_i and [Ca⁺⁺]_i were lower than the environmental values and [K⁺]_i was greater than [K⁺]_o. [Na⁺]_i and [Cl^-]_i changed with changes in external salinity, but were kept lower than [Na⁺]_o and [Cl^-]_o. [K⁺]_i, [Mg⁺⁺]_i and [Ca⁺⁺]_i were maintained at fairly constant internal concentrations.

4. The contractile vacuole output was related to external osmolarity. Osmolarities greater than that of 100% sea water resulted in decreased vacuole output. In dilute sea water, output increased.

5. Cell volume determinations indicated a return toward the original volume after swelling or shrinking caused by transfer to media of different osmolarities.

6. The results suggest that M. avidus maintains itself hyperosmotic to the environment at all salinities. The contractile vacuole regulates cell volume by expelling water that enters passively.

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