REGULATION AND ROLE OF PHOTOSYNTHESIS IN THE COLONIAL SYMBIOTIC CILIATE OPHRYDIUM VERSATILE

Symbiotic photosynthesis of the colonial ciliate Ophrydium versatile f rom transparent temperate lakes was studied with O-2 microelectrodes a nd measurements of O-2 and C exchange in laboratory and field incubati ons, Oxygen gradients were steep just above the colony surface and sha llow toward the center of the colony, reflecting that O-2 metabolism i s high in the peripheral layer containing the ciliates and low in the jelly. Photosynthesis responses resembled those of unicellular and thi n multicellular algae by having relatively low light compensation poin t (25 mu mol photon m(-2) s(-1)), low light saturation point (190 mu m ol photon m(-2) s(-1)), and a sharp transition from light-limited to l ight-saturated photosynthesis. Inorganic C supply to Ophrydium support ed photosynthetic rates close to maximum in air-saturated alkaline lak e 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 zoochlorellae dominate d the O-2 metabolism of Ophrydium ciliates, resulting in much steeper O-2 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 mea sured growth rate, the respiration, and the substantial carbon content of the colony jelly, whereas particle filtration of ciliates presumab ly supplied the nutrients for net growth of the assemblage. The locati on of the ciliates at low density in the periphery of the colony and t he water movements generated by the ciliates are probably important fo r the efficient plant-like use of light and inorganic C by Ophrydium a nd the ability to form high densities of zoochlorellae.