RESPIRATION, PHOTOSYNTHESIS AND CARBON METABOLISM IN PLANKTONIC CILIATES

Release of C-14-labelled carbon dioxide from uniformly labelled cells was used to measure respiration by individual ciliates in 2-h incubations in 1989 and 1990. In a strictly heterotrophic ciliate, Strobilidium spiralis (Leegaard, 1915), release of labelled carbon dioxide was equivalent to ca. 2.8% of cell C h-1 at 20-degrees-C, and there was no difference between rates in the dark and light. In the chloroplast-retaining ciliates Laboea strobila Lohmann, 1908, Strombidium conicum (Lohmann, 1908) Wulff, 1919 and Strombidium capitatum (Leegaard, 1915) Kahl, 1932, release of labelled carbon dioxide was less in the light than in the dark in experiments done at 15-degrees-C. In L. strobila release of radiolabel as carbon dioxide was equivalent to ca. 2.4% of cell C h-1 in the dark but ca. 1% at 50-mu-E m-2 s-1, an irradiance limiting to photosynthesis. In S. conicum release of radiolabel as carbon dioxide was equivalent to ca. 4.4% of cell C h-1 in the dark, but at an irradiance saturating to photosynthesis (250 to 300-mu-E m-2 s-1) there was no detectable release of labelled carbon dioxide. In S. capitatum release of radiolabel as carbon dioxide was equivalent to ca. 4.3% of cell C h-1 in the dark but at an irradiance saturating to photosynthesis was ca. 2.4% of cell C h-1. These data, combined with data from photosynthetic uptake experiments, indicate that C-14 uptake underestimates the total benefit of photosynthesis by 50% or more in chloroplast-retaining ciliates.