Vertical physico-chemical gradients with distinct microbial communities in the hypersaline and heliothermal Lake Ursu (Sovata, Romania)

Abstract

The effect of vertical physico-chemical stratification on the planktonic microbial community composition of the deep, hypersaline and heliothermal Lake Ursu (Sovata, Romania) was examined in this study. On site and laboratory measurements were performed to determine the physical and chemical variables of the lake water, and culture-based and cultivation-independent techniques were applied to identify the members of microbial communities. The surface of the lake was characterized by a low salinity water layer while the deepest region was extremely saline (up to 300 g/L salinity). Many parameters (e.g. photosynthetically active radiation, dissolved oxygen concentration, pH, redox potential) changed dramatically from 2 to 4 m below the water surface in conjunction with the increasing salinity values. The water temperature reached a maximum at this depth. At around 3 m depth, there was a water layer with high (bacterio) chlorophyll content dominated by Prosthecochloris vibrioformis, a phototrophic green sulfur bacterium. Characteristic microbial communities with various prokaryotic taxa were identified along the different environmental parameters present in the different water layers. Some of these bacteria were known to be heterotrophic and therefore may be involved in the decomposition of lake organic material (e.g. Halomonas, Idiomarina and Pseudoalteromonas) while others in the transformation of sulfur compounds (e.g. Prosthecochloris). Eukaryotic microorganisms identified by molecular methods in the lake water belonged to genera of green algae (Mantionella and Picochlorum), and were restricted mainly to the upper layers.

Contrasting taxonomic stratification of microbial communities in two hypersaline meromictic lakes

“…Having a surface of 41 270 m 2 and a maximum depth of 18.2 m (Figure 1b), it was formed in a depression caused by an intense process of salt dissolution between 1875 and 1880 (Alexe, 2010;Máthé et al, 2014). Currently, it is exploited for recreational purposes during late-May to mid-August.…”

“…together with significant populations of unicellular algae (Máthé et al, 2014) and anoxygenic photosynthesizers (Supplementary Table S6) might be responsible for the primary productivity in the upper layer (down to 2.5 m deep) of Ursu Lake. We consider that the accumulated organic carbon might be subsequently metabolized by both heterotrophic aerobic (Actinobacteria, Bacteroidetes, Gammaproteobacteria and Planctomycetes) and anaerobic Bacteria (Halanaerobiales and Clostridiales) as well as Archaea (Halobacteriales).…”

“…14 Myr ago). These two temperate lakes are unique among the other Romanian salt lakes because of their heliothermy (that is, trapping of solar radiation by a 'saline lens'), size (that is, Ursu Lake is one of the largest heliothermal salt lakes in Europe), conservation status (that is, Fara Fund Lake is a protected area) and lake processes (that is, Ursu Lake is a natural lake whereas Fara Fund Lake is an artificial one) (Alexe, 2010;Máthé et al, 2014). In spite of their different genesis, they belong to the same type of meromictic lakes, where the water column overturn is hampered by strong salinity gradients.…”

“…Although studies investigating the microbiota and substance turnover in meromictic lakes are available (Biderre-Petit et al, 2011;Habicht et al, 2011;Hamilton et al, 2014), most of the ones concerning the hypersaline water bodies were performed on Arctic (Pouliot et al, 2009;Comeau et al, 2012) or Antarctic (Bowman et al, 2000;Lauro et al, 2011) lakes. Although information on temperate neutral hypersaline meromictic lakes microbiota are scarce, previous investigations on Ursu Lake prokaryotes (using community-level physiological profiling, culture-dependent and PCR-denaturing gradient gel electrophoresis methods) have shown the presence of active and diverse communities (Cristea et al, 2014;Máthé et al, 2014); yet, a detailed molecular diversity study is not available to date. The microbial diversity of Fara Fund Lake has not been previously investigated, and hence the lake could constitute a valuable site for monitoring longterm dynamics of microbial communities that are not under the impact of human activities.…”

Physiology and Molecular Phylogeny of Bacteria Isolated from Alkaline Distillery Lime

“…are ubiquitous, moderately halophilic/halotolerant Gram-negative bacteria found in saline lakes, saline soils, marine environments, and solar salt extraction facilities. Several alkaliphilic species of Halomonas can live in soda lakes and alkaline soils (Máthé et al, 2014). Halomonas hamiltonii, the closest relative of strains M3, H6, S14, and S16, is a halophilic, facultative alkaliphilic rod-shaped bacterium, isolated from patients' blood and dialysis machines in a nursing home (Kim et al, 2010).…”