The role and distribution of the autotrophic ciliate Mesodinium rubrum (Myrionecta rubra) in three Antarctic saline lakes

Abstract

The microbial plankton of three saline lakes (Highway Lake, Ace Lake and Lake Abraxas) in the Vestfold Hills oasis, eastern Antarctica was investigated during the austral summer of 1993. The three lakes span the salinity range from brackish to seawater. In the two deeper meromictic lakes (Ace and Abraxas) the distribution of the microbial components was related to the chemical and physical characteristics of the water column. Heterotrophic flagellates and ciliates were most abundant above the chemocline in oxygenated epilimnetic waters, while bacteria numbers increased dramatically in the water below the chemocline. In Highway Lake, where the water column has a uniform conductivity and temperature, vertical distribution of the microbial plankton displayed a different pattern. Abundances of bacteria in epilimnetic water ranged between 1.02-1.2 x 10 8 l -1 in Ace Lake and 3.6-3.8 x 10 8 l -1 in Lake Abraxas. In Highway Lake bacterial concentrations averaged 0.54-1.06 x 10 8 l -1 . Nanoflagellate concentrations in Ace and Highway Lakes were in the region of 450-3000 x 10 3 l -1 , while in Lake Abraxas they were much lower : 92-790 x 10 3 l -1 , which reflected the much lower bacterial food resources in this lake. The ciliate concentration in Highway Lake was extremely high, on occasions in excess of 200 x 10 3 l -1 . Mesodinium rubrum was abundant in all three lakes during November and December and ranged from 34-48 % of the total ciliate community in Highway Lake, 83-99% in Ace Lake and 82-88 % in Lake Abraxas. Primary production measurements in Ace Lake and Highway Lake indicate that Mesodinium may be contributing between 15-40% of carbon fixation in the phytoplankton.