VIRAL LYSIS AND BACTERIVORY AS PROKARYOTIC LOSS FACTORS ALONG A SALINITY GRADIENT

We estimated prokaryotic mortality due to viruses and bacterivores thr ough salinity gradients in 2 solar salterns. In each saltern system, s uccessive ponds provided steady state environments with a range of sal inities from 37 to 372 parts per thousand. Prokaryotic and viral abund ance increased with salinity, reaching about 10(8) prokaryotic cells m l(-1)and 10(9) virus-like particles (VLP) ml(-1) at salinities higher than 250 parts per thousand. Prokaryotic doubling times became longer than 2 d above 250 parts per thousand salinity until the end of the gr adient. Bacterivory accounted for all the production at lower saliniti es but it was found to be zero at the highest salinities. The percenta ge of visibly infected cells was not different among the ponds where i nfected cells could be detected and it was always lower than 4%. From the percentage of infected cells and using conversion factors from the literature we estimated rates of prokaryotic mortality due to viral l ysis: about 0.6 to 2 x 10(6) prokaryotes ml(-1) were lysed daily by Te viruses in the salterns. This number represented a low percentage of prokaryotic abundance and production compared to the prokaryotic losse s due to bacterivores (0.2 to 4 x 10(7) bacteria ml(-1) d(-1)). Howeve r, viral production reached values higher than 10(8) VLP ml(-1) d(-1) above 250 parts per thousand salinity, due to the large burst size (20 0 viruses cell(-1)) found in a particular morphotype of prokaryotes, t he square archaea. These archaea represented more than 25% of the prok aryotic assemblage above 250 parts per thousand salinity. At this poin t they became the prokaryotic morphotype with the largest percentage o f infected cells (1 to 10% of square archaea with visible phages insid e) A lemon-shaped virus (similar to one described for some other group s of archaea) was found infecting square archaea, its abundance increa sed in the saltiest ponds together with that of the square archaea. In this system viruses did not exert a strong control over the prokaryot ic abundance and growth rate.