A mixotrophic ciliate as a major contributor to plankton photosynthesis in Australian lakes

A large mixotrophic ciliate (~200 μm long) of the genus Stentor is a common constituent of the protozooplankton of Australian lakes. We investigated the photosynthetic rates of populations of this ciliate from two lakes, one in the Australian Capital Territory and the other on the New South Wales/Victorian border, in relation to photosynthesis by the whole phytoplankton community. The concentration of the ciliate varied between 192 and 4,267 cells liter -1 during the study period of May-January (the austral winter, spring, and autumn) and it contributed between 4.3 and 69.3% of total plankton photosynthesis. Individual photosynthetic rates ranged between 1.03 ± 0.8 and 3.98 ± 0.6 ng C cell -1 h -1 and individual Chl a content between 925 ± 62 to 1,461 ± 63 pg cell -1 , giving assimilation numbers of 1.00-2.74. Light-response curves indicated that the ciliate achieved its highest rates of photosynthesis at high photon fluxes, typical of the surface waters. Vertical distribution patterns of Stentor in the water column of one of the lakes supported these physiological data. Southern hemisphere lakes seem to have a protozooplankton that may contain substantial numbers of a large ciliate capable of contributing a significant portion of carbon fixation in the plankton.