Some Factors Controlling the Distribution of Two Pond‐Dwelling Ciliates with Algal Symbionts (Frontonia vernalis and Euplotes daidaleos)1

Abstract

ABSTRACTThe vertical distributions of two pond‐dwelling zoochlorellae‐bearing ciliates (Euplotes daidaleos Diller & Kounaris, 1966 and Frontonia vernalis Ehrenberg, 1838) were monitored over a 24‐h period. Both species maintained peak abundance at a low O2 level (usually < 1 mg/liter). They did not migrate in response to the changing light level. Experiments with laboratory cultures indicated that the characteristic distribution in an O2 gradient in the dark was largely controlled by the oxygen tension. The increased motility in anoxia and high pO2 was independent of large changes in pCO2 and pH. Ciliates living in anoxia or a very low pO2 would migrate out of the dark and into the dimly lit (10 μE m‐2 sec‐1) part of a glass cell because there they could photosynthesize, produce O2, and create a suitable oxygenated microenvironment; a further increase in the light level caused a slow migration out of the light. Similar migrations were observed when the light level remained low but the pO2 was artificially raised. Ciliates suspended in 1 μM DCMU (an inhibitor of photosynthetic O2 evolution) took longer to migrate into the light and they did not avoid high light levels (> 100, μE m‐2 sec‐1)‐ Frontonia suspended in water with a pO2 of 1% aggregated at a low light level (1 μE m‐2 sec‐1); peak daytime abundance in the pond occurred at about this light level. Frontonia vernalis tends to swim vertically upwards (anterior end up) when suspended in anoxic water. This apparent negative geotaxis compensates for the high sedimentation velocity (0.36 mm sec‐1) of this large ciliate and facilitates its aggregation at the metalimnion. The O2 tension appears to be the principal factor controlling the vertical distributions of both species. Occasional, enhanced convection within the metalimnion has a secondary influence. Light influences the vertical profile only if it promotes photosynthesis and increases the intracellular pO2.