Symbiotic photosynthesis of the colonial ciliate Ophrydium versatile from transparent temperate lakes was studied with 0, microelectrodes and measurements of 0, and C exchange in laboratory and field incubations. Oxygen gradients were steep just above the colony surface and shallow toward the center of the colony, reflecting that 0, metabolism is high in the peripheral layer containing the ciliates and low in the jelly. Photosynthesis responses resembled those of unicellular and thin multicellular algae by having relatively low light compensation point (25 pmol photon m-2 s-l), low light saturation point (190 pm01 photon mA2 s-l), and a sharp transition from lightlimited to light-saturated photosynthesis. Inorganic C supply to Ophrydium supported photosynthetic rates close to maximum in air-saturated alkaline lake water. Photosynthesis patterns were similar in short-time laboratory experiments and diel outdoor experiments and showed no indication of photoinhibition and rhythmicity in the relationship of photosynthesis to irradiance during the day. The endosymbiotic zoochlorcllae dominated the 0, metabolism of Ophrydium ciliates, resulting in much steeper 0, gradients at the colony surface in the light than in the dark and a large diel surplus of gross photosynthesis relative to respiration. Symbiotic photosynthesis produced sufficient carbon to support the measured growth rate, the respiration, and the substantial carbon content of the colony jelly, whereas particle filtration of ciliates presumably supplied the nutrients for net growth of the assemblage. The location of th
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