Limnology and plankton diversity of salt lakes from Transylvanian Basin (Romania): A review

In the present work, we review the current knowledge on genesis, limnology and biodiversity of salt lakes distributed around the inner contour of Eastern Carpathian arc (Transylvanian Basin, Central Romania). Transylvanian salt lakes formed on ancient halite (NaCl) deposits following natural processes or quarrying activities.  Most of these lakes are located in eastern (Sovata area), southern (Ocna Sibiului), and western (Turda-Cojocna) parts of the Transylvanian Basin, have small surfaces (0.1-4 ha), variable depths (2-100 m), are hypersaline (>10%, w/v, total salts, mainly NaCl) and permanently stratified. As consequence of steady salinity/density gradient, heat entrapment below surface layer (i.e., heliothermy) develops in several Transylvanian lakes. The physical and chemical water stratification is mirrored in the partition of plankton diversity. Lakes with less saline (2-10% salinity) water layers appear to harbor halotolerant representatives of phyto- (e.g., marine native Picochlorum spp. and Synechococcus spp.), zoo- (e.g., Moina salina), and bacterioplankton (e.g., Actinobacteria, Verrucomicobia), whereas halophilic plankton communities (e.g., green algae Dunaliella sp., brine shrimp Artemia sp., and members of Halobacteria class) dominate in the oxic surface of hypersaline (>10% salinity) lakes. Molecular approaches (e.g., PCR-DGGE, 16S rRNA gene-based clone libraries, and DNA metabarcoding) showed that the O₂-depleted bottom brines of deep meromictic Transylvanian lakes are inhabited by known extremely halophilic anaerobes (e.g. sulfate-reducing Delta-Proteobacteria, fermenting Clostridia, methanogenic and polymer-degrading archaea) in addition to representatives of uncultured/unclassified prokaryotic lineages. Overall, the plankton communities thriving in saline Transylvanian lakes seem to drive full biogeochemical cycling of main elements. However, the trophic interactions (i.e., food web structure and energy flow) as well as impact of human activities and predicted climate changes are yet to be assessed in these unique ecosystems with little or no match to analogous salt lakes worldwide.