A MIXOTROPHIC CILIATE AS A MAJOR CONTRIBUTOR TO PLANKTON PHOTOSYNTHESIS IN AUSTRALIAN LAKES

A large mixotrophic ciliate (similar to 200 mu m long) of the genus St enter is a common constituent of the protozooplankton of Australian la kes. We investigated the photosynthetic rates of populations of this c iliate from two lakes, one in the Australian Capital Territory and the other on the New South Wales/Victorian border, in relation to photosy nthesis 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. I ndividual 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 hig hest rates of photosynthesis at high photon fluxes, typical of the sur face waters. Vertical distribution patterns of Stenter in the water co lumn of one of the lakes supported these physiological data. Southern hemisphere lakes seem to have a protozooplankton that may contain subs tantial numbers of a large ciliate capable of contributing a significa nt portion of carbon fixation in the plankton.