Metabolism of photosynthate in the chloroplast-retaining ciliate Laboea strobila

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Abstract
Assimilation of photosynthate into 4 chemical classes of compounds was compared in the chloroplast-retaining ciliate Laboea strobila and the algae from which its chloroplasts were derived (Isochrysis galbana and Pyrenomas salina). In laboratory cultures, the ratio of allocation of radiolabel into polysaccharide and protein was higher in L. strobila compared to microalgae following a 6 h incubation at irradiance which were saturating or limiting to photosynthesis. In field populations collected in Vineyard Sound, Massachusetts, USA, and in laboratory cultures of L. strobila, maximum rates of polysaccharide production from photosynthate were equivalent to ca 7 to 10% h-1 of the pool of polysaccharide carbon. In contrast, maximum lipid and protein production from photosynthate were usually .ltoreq. 1% h-1 of lipid and protein carbon pools, respectively. About half the radiolabel which accumulated during a 3 h incubation in light was respired or excreted when ciliates were placed in light in the presence of the photosynthetic inhibitor dichlorophenyldimethylurea (DCMU), or in the dark for 18 h. Net loss of radiolabel occurred largely from polysaccharide. Mixotrophic ciliates may have higher trophic efficiencies than heterotrophic ciliates because respiratory and excretory needs are supplemented by photosynthesis.