Saltern evaporation ponds as model systems for the study of primary production processes under hypersaline conditions

Multi-pond solar salterns, which are used worldwide for salt production along tropical and subtropical coastal areas, present an environment with increasing salt concentrations, from seawater to NaCl saturation. Characteristic salt-adapted microbial communities are found along the salinity gradient. In ponds of intermediate salinity (100 to 250 g l^-1), most of the primary production occurs in benthic microbial mats dominated by different types of unicellular and filamentous Cyanobacteria (Aphanothece, Microcoleus, Phormidium and others), sometimes in association with diatoms. In crystallizer ponds, the unicellular green alga Dunaliella is the sole primary producer that lives in association with dense communities of heterotrophic halophilic Archaea that color the brines red. This basic pattern is common to all saltern systems, in spite of local variations in climate and nutrient availability. Photosynthetic activities of benthic cyanobacterial mats in the evaporation ponds and of endoevaporitic microbial communities within the gypsum crust that precipitates at intermediate salinities have been extensively studied in salterns at different locations, using oxygen microelectrodes and other techniques adapted to the study of benthic communities. These environments are generally highly productive, although most of the oxygen produced during daytime by the Cyanobacteria is recycled within the mats rather than exchanged with the overlying water and the atmosphere. Surprisingly few attempts have been made thus far to estimate the photosynthetic activity of Dunaliella, which is often present in numbers between 10³ and 10⁵ cells ml ^-1 in the heavily saltstressed environment of crystallizer ponds, so that the dynamics of the system is largely unknown.