Life under the Mediterranean Sea

A review of Stock, A et al (2011), Microbial eukaryotic life in the new hypersaline deep-sea basin Thetis, Springer.

This paper is related to the article Lee reviewed on the underwater lake Thetis. His article focused on the prokaryotic life, whereas this article focuses on the eukaryotes found in the lake.

Lake Thetis, a hypersaline basin found 3,258m below the Mediterranean Sea, was originally thought to be inhabitable due to its high salt concentrations (salinity of 348%). Salinity can impose cellular stress, great osmotic pressures and low water activity, making it a major ecological barrier, especially for unicellular eukaryotes, However, eukaryotic life has been found living in these conditions but most research has only focused on Bacteria and Archaea. This study aims to establish the eukaryotes that have successfully inhabited the hypersaline, anoxic Lake Thetis.

They used a eukaryote-specific probe, as well as amplifying and analysing cDNA which they obtained from numerous samples. They also used FISH and looked at SSU rRNA genes to identify indigenous protistan communities, comparing them to communities found in the overlaying interface and other deep hypersaline anoxic basins (DHABs) in the Mediterranean Sea.

The most dominant and taxonomically diverse eukaryote found was fungi, accounting for 37% of the total OTUs. The authors noted that the brine was a great source for unknown fungal diversity and importantly a hotspot for discovery of novel metabolic pathways and secondary metabolites important for various medical treatments. Only one phylotype was shared between the interface and brine samples, which suggests that most fungi in the brine are halophilic rather than halotolerant, supporting the existence of discrete fungal communities in the interface and the brine.

They also counted 0.69 x10⁴ protists per litre of anoxic brine, with distinct differences between the communities found in the interface and the brine. Stramenophiles and ciliates each accounted for 20% of protists in the samples. Interestingly, some of the cilliates identified differed at least phylogenetically from their marine forms, suggesting potential allopatric speciation through long-term separation by the environmental salinity gradient.

The also detected dinoflagellates which accounted for 24% of the sequences in the interface and 10% in the brine, however none of the OTUs were observed in both environments. They also found five haptophyte OTUs. Their relevance and ecology are not fully known, but they have been observed in other hypersaline basins, indicating that they might be common members of high-salt environments.

Generally, the higher taxon level community structure was similar to other terrestrial saline habitats such as solar salterns or salt lakes, but there were distinct differences on a lower level. They assume that the polyextreme conditions in the DHABs most likely select for even more specialized communities that include different phylogenetic lineages specifically adapted to the conditions.

Despite the findings, a major factor is whether the sequences detected derive from indigenous extremophiles,. This is because the amplification of cDNA was used, instead of DNA, which is not necessarily a marker of viability, especially in environments such as the deep-sea where numerous non-indigenous organisms and debris may accumulate. However, using FISH, they could confirm definite presence of protistan life in the brines. This means that further studies need to be done, using clade- or taxon-specific probes, to quantify the contribution of specific taxa in the samples.

Overall, a really good and very recent study, and also the first to count and visualise the number of protists in a DHAB brine, providing proof and showing that salinity limits for eukaryotic life still remain identified. I definitely recommend reading the full article as unfortunately I couldn’t include many of the interesting facts they found (such as fungi closely related to the ascomycete found in solar salterns!)