TEMPERATURE-DEPENDENT PHAGOTROPHY AND PHOTOTROPHY IN A MIXOTROPHIC CHRYSOPHYTE

The roles of temperature and light on grazing and photosynthesis were examined for Dinobryon sociale, a common freshwater mixotrophic alga. Photosynthetic rate was determined for D. sociale adapted to temperatures of 8, 12, 16, and 20 degrees C under photosynthetically active radiation light irradiances of 25, 66, and 130 mu mol photons . m(-2) . s(-1), with concurrent measurement of bacterial ingestion at all temperatures under medium and high light (66 and 130 mu mol photons . m(-2) . s(-1)). Rates of ingestion and photosynthesis increased with temperature to a maximum at 16 degrees C under the two higher light regimes, and declined at 20 degrees C. Although both light and temperature had a marked effect on photosynthesis, there was no significant difference in bacterivory at medium and high irradiances at any given temperature. At the lowest light condition (25 mu mol photons . m(-2) . s(-1)), photosynthesis remained low and relatively stable at all temperatures. D. sociale acquired the majority of carbon from photosynthesis, although the low photosynthetic rate without a concurrent decline in feeding rate at 8 degrees C suggested 20%-30% of the carbon budget could be attributed to bacterivory at low temperatures. Grazing experiments in nutrient-modified media revealed that this mixotroph had increased ingestion rates when either dissolved nitrogen or phosphorus was decreased. This work increases our understanding of environmental effects on mixotrophic nutrition. Although the influence of abiotic factors on phagotrophy and phototrophy in pure heterotrophs and phototrophs has been well studied, much less is known for mixotrophic organisms.