The microbial food web along salinity gradients

The microbial food web was studied along a gradient of salinity in two sola r salterns used for the commercial production of salt. The different ponds in the salterns provide a wide range of ecosystems with food webs of differ ent complexities. Abundance of prokaryotes, cell volume, prokaryotic hetero trophic production, chlorophyll a, abundance of heterotrophic flagellates, ciliates and phytoplankton were determined in several ponds in each saltern . Increases in salinity resulted in a progressive reduction in the abundanc e and number of different groups of eukaryotic microorganisms present, but an increase in biomass of prokaryotes. Maximal activity of both phyto- and bacterioplankton was found at a salinity of around 100 parts per thousand, where there was also a maximum in chlorophyll a concentration. Growth rates of heterotrophic prokaryotes decreased with increasing salinity. Bacterivo ry disappeared above 250 parts per thousand salinity, whereas other loss fa ctors such as viral lysis appeared to be of minor importance throughout the gradient [Guixa-Boixereu et al. (1996) Aquat. Microb. Ecol. 11, 215-217]. (C) 2000 Federation of European Microbiological Societies. Published by Els evier Science B.V. All rights reserved.