The presence of yeasts in coastal and estuarine waters is known for a long time, but their presence in adjacent solar salterns has not been reported. This work focused on the occurrence, distribution and characterisation of yeasts in a solar saltern (Salina do Brito) at Alcochete, on the south bank of the Tagus estuary.
Strains from the genera Debaryomyces, Kluyveromyces, Pichia, Cryptococcus, Leucosporidium, Rhodotorula and Rhodosporidium were found to occur in the saltern brines. Debaryomyces prevailed among the ascomycetes and Rhodotorula and Cryptococcus were the basidiomycetous yeasts more often isolated. The occurrence and distribution of the yeast population presented the expected correlations with salinity, but neither correlated with other environmental factors such as temperature. The composition and the density of the yeast population agreed with the extreme-like nature of the habitat and the adjoining estuary. Both the diversity and the concentration of isolates decreased during the operation of the saltern in summer (the safra season), the time when the salinity gradient became steeper and the hydraulic residence time was much shorter. The autochthony of yeasts in the brines during the safra season remains an open question.
Six strains (two strains of Debaryomyces hansenii, Rhodotorula and Cryptococcus) were selected among the isolates for subsequent physiological characterisation. The tests evaluated growth tolerance and viability studies as a function of relevant brine salts and temperature. The toxic effects observed followed the order: MgCl2 >> NaCl  KCl. The resistance/tolerance patterns found were complex and depended on the environmental factor under investigation. As expected from the occurrence profile, Cryptococcus strains were the least tolerant/resistant. Although Debaryomyces hansenii PYCC 5092 showed to be slightly more growth tolerant to the salts employed than Rhodotorula mucilaginosa PYCC 5242, the latter was clearly more resistant under stringent conditions involving higher salt concentrations and temperature. Results from the physiological studies agreed with the fieldwork and suggest an allochthonous character for the yeast population during the safra season in the saltern.