Viral lysis and bacterivory as prokaryotic loss factors along a salinity gradient

We estimated prokaryotic mortality due to viruses and bacterivores through salinity gradients in 2 solar salterns. In each saltern system, successive ponds provided steady state environments with a range of salinities from 37 to 372 parts per thousand. Prokaryotic and viral abundance increased with salinity, reaching about 10(8) prokaryotic cells ml(-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 gradient. Bacterivory accounted for all the production at lower salinities but it was found to be zero at the highest salinities. The percentage of visibly infected cells was not different among the ponds where infected 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 lysis: 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 losses due to bacterivores (0.2 to 4 x 10(7) bacteria ml(-1) d(-1)). However, 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 (200 viruses cell(-1)) found in a particular morphotype of prokaryotes, the square archaea. These archaea represented more than 25% of the prokaryotic assemblage above 250 parts per thousand salinity. At this point they became the prokaryotic morphotype with the largest percentage of infected cells (1 to 10% of square archaea with visible phages inside) A lemon-shaped virus (similar to one described for some other groups of archaea) was found infecting square archaea, its abundance increased in the saltiest ponds together with that of the square archaea. In this system viruses did not exert a strong control over the prokaryotic abundance and growth rate.