Between 1994-1995 a series of very unusual lakes were discovered in the eastern Mediterranean Sea. What was special about them is the fact that they are situated over 3 km beneath the Mediterranean Sea surface and consists of a dense brine which was left behind when the Mediterranean Sea evaporated millions of years ago. Deep hypersaline anoxic basins (DHABs) are kept separate from overlying sea water because the brine is up to 30% denser, this also results in a very thin interface (1.5-2m) between the two bodies. DHABs themselves can be several 100 metres deep, completely lack any oxygen and be between 7-10 times the salinity of sea water. Such environments were thought to be too harsh for any life to exist but many studies have shown that they harbour high numbers of microbes, particularly the thin interface between the brine lake and the sea water. Although, due to the extreme depths there are still many mysteries associated with these lakes.
In 2008 another DHAB was discovered in the eastern Mediterranean by La Cono et al (2011) which they named Thetis. Thetis is found at a depth of ~3258m and is around 157m deep itself. This study was undertaken in order to investigate the prokaryotic communities of this DHAB and to compare them to the geochemical gradient running from the sea water, through the lake interface and into the brine itself. This was done by taking water samples at multiple depths which were used for DNA fingerprinting as well as 16rRNA and cDNA clone library analysis. Epifluorescence was used to estimate cell numbers and important metabolic genes were also identified to further understand the ecology of these DHABs.
The geochemical results showed a fivefold increase in salt concentration in just a 1m section of the upper interface. This sharp increase in anions caused a large decrease in redox potential, +213 mV in the overlying water to -340 mV 1m into the interface. This was thought to be enough to prevent any cellular life but this study once again showed that this is not the case. Prokaryotic cell numbers showed to be highest in the interface section, around 2.11x106 ml-1 compared to 9.63x104 in the sea water. Prokaryotic cells in the deeper brine averaged 7.11x104. The identified bacteria belonged to 11 different taxa while archaea were found to be less diverse. All were extreme halophiles and many had not been discovered in DHAB before, about 40% of phylotypes.
The results of metabolic activity concluded that the Thetis ecosystem is being driven by dark primary production of chemolithoautotrophic cells in the upper interface layer. This appears to depend largely on sulphur cycling as well as CO2 fixation. This also appears to make up for the fact that it is very difficult for organic matter from the upper sea water to penetrate the dense DHAB. Methanogenesis and methane oxidation seem to also be a major metabolic process in Thetis but confined more to the lower levels which also highlighted the stratification of different metabolic pathways.
In conclusion, like many over deep sea environments Thetis are populated by groups of prokaryotes most of which have only been found there. This is despite the fact that it was previously thought to be unable to support life. Whether DHABs were colonized after their formation or if the microbes were present from the start and then adapted is still unknown. I think it was a shame that this paper did not look at any eukaryotic cells that might have been present, as they have been found at other DHABs. As highlighted in many other blogs this term the extreme depths prove to be a big hurdle in researching this area of microbiology.
A Review of:
La Cono, V. Smedile, F. Bortoluzzi, G. Arcadi, E. Maimone, G. Messina, E. Borghini, M. Oliveri, E. Mazzola, S. L’Haridon, S. Toffin, L. Genovese, L. Ferrer, M. Giuliano, L. Golyshin, P and Yakimov, M (2011) Unveiling microbial life in new deep-sea hypersaline lake Thetis. Part 1: Prokaryotes and environmental settings. Environmental Microbiology. 13, 2250-2268.
5 comments:
Hi Lee,
A really interesting article, it’s amazing how microbes are able to adapt and colonise in such extreme environments, not to mention how technology has developed so that we can begin to explore environments like these.
As you said, they only focused on prokaryotes in the article which is a shame as I found a paper focusing just on eukaryotic life in Thetis and they found a large amount of unknown eukaryotic organisms, including interesting results that suggest that some of the organisms have adapted to the brines. They found that fungi was the most diverse eukaryote and said that these deep sea brines are hotspots for unknown fungal diversity. Definitely something that needs more research! They also found 0.6x104 protists per litre in the brine, which were mainly ciliates. Interestingly they found a high occurrence of closely-related ciliate sequences which were exclusive to this and other Mediterranean brine basins, suggesting that the organisms have specific adaptations to the habitats.
More detailed research is definitely needed, it would be really interesting to see how some of these organisms have adapted and to discover the vast amount of unknown species down there.
For anyone who is interested the second part of this study was published at a later date but is only online at the moment so there are no page numbers yet. It focused more on the metagenomics of Thetis.
Ferrer et al (2011) Unveiling microbial life in the new deep-sea hypersaline Lake Thetis. Part II: a metagenomic study. Environmental Microbiology. 13 (12) pp ??
Hi Jelena
Thanks for the comment.
That paper sounds really interesting! It was a real shame that this one did not discuss eukaryote at all. I wander if the reason why the microbes at the different lakes seem to be closley related because they have a commom ancestors from way back before the lakes were formed and they were not isolated from eachother. Like you said, a good area for research. Could you send me the reference for the one you read please, sounds really interesting. Unless you thinking of doing a blog on it and want to keep it secret for now :-).
Hope your lab session went well today by the way.
haha thanks, I saw you working hard in the lab today too, hope all is going well!
Ofcourse you can have the reference, it's
Stock, A., Breiner H.W., Pachiadaki, M., Edgcomb, V., Filker, S., La Cono, V., Yakimov M.M. and Stoeck, T., (2011) Microbial eukaryote life in the new hypersaline deep-sea basin Thetis, PubMed
I can't seem to find a volume or page number though :S but I did just notice that our papers have a few of the same authors, so I guess they thought it was a shame not to look at eukaryotes too!
I also agree, it seems likely that there was one common ancestor. From what I understand, these lakes are formed by the shallow waters separating from the ocean and then drying out leaving thick layers of salt/minerals and then being submerged again by the water returning. I wonder if the microbes had inhabited the lakes before they dried out and then managed to survive and adapt to the brines or whether they only appeared when the ocean returned. In which case, why did they choose to inhabit the hypersaline lakes instead of just remaining in the slightly-salty ocean..Interesting!
Ha ha, saw me working not sur about the hard part. I missed the seminar today because I had to be in the lab.
Thanks for the reference, I will give it a read. I think when the vol and page numbers are not avalible its because it has not been published yet. Its good in a way as it means its really new. You should do a post on it.
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