This paper investigates how archaea work in a microbial consortium with Sulphate-reducing Deltaproteobacteria to oxidize methane. I believe this topic to be important as it explores what happens to a lot of the methane that is produced in deep anoxic sediments, as well as how it is able to happen. Specifically, it examines the enzymes that are used in the anaerobic oxidation of methane (AOM) during sulphate reduction. There were three enzymes studied: ATP sulphurylase which activates sulphate to produce adenosine 5′-phosphosulfate (APS) and pyrophosphate (later hydrolysed to phosphate), APS reductase, which reduces APS to bisulphite, and dissimilatory sulphite reductase, which reduces sulphite to sulphide (Dsr).
AOM is performed by archaea that are closely related to methanogens and sulphate reducing bacteria. It is thought that they are able to perform methanogenesis in reverse as they are anaerobic methanogens (ANME) and so they have most of the genes that are required for methanogenesis and the methane purifying enzyme was similar to methyl-coenzyme M reductase, an enzyme used in methanogenesis. In some AOM environments archaeal cells have been found not to have any association with bacterial cells, implying that some archaea must be able to perform both methane oxidation and sulphate reduction.Fluorescence in situ hybridization (FISH) was used on the selected section of the microbial mat to detect the presence of consortia under a microscope and, as expected, the cells formed dense aggregates. Fluorescent stains with 16s rRNA probes were used, which showed that there was an archaeal core surrounded by either bacterial cells alone or a combination of archaeal and bacterial cells.
The results of this study have shown that ATP sulphurylase and APS reductase are most likely used by the archaea in the reduction of sulphate to sulphide but there is not sufficient evidence to suggest that Dsr is also used in the chain of reactions although this could be due to the experimental technique as much of DsrC, a subunit of Dsr that is important for the enzyme to function properly, was lost during extraction and purification. Changing the way that the enzymes are extracted and purified by experimenting with different saturations of ammonium sulphate precipitation or by reducing the speed of the centrifuge may help to reduce the amount of DsrC that is lost in the process and give more accurate results.
Reference: Basen, M et al. (2011), Bacterial enzymes for dissimilatory sulfate reduction in a marine microbial mat (Black Sea) mediating anaerobic oxidation of methane. Environmental Microbiology, Vol 13, Issue 5, pg 1370–1379
4 comments:
Hi Matt
I do like this subject. It is a very strange symbiosis the two organisms have and I wonder if either could survive without the other. I was interested in finding out more about the methane oxidation by the archaea and saw a paper in the references by Meyerdierks et al (2010) which tries to identify the pathway and what intermediate is past to the SRB. They conclude that the archaea seem to perform methanogenesis in reverse like you mentioned in your review but also that they are still unsure on the intermediate metabolite between the two organisms. The likely molecules being formate or acetate although I think it is still a bit unclear as to where the oxygen atoms from these two molecules come from. I am guessing either from H2O or maybe the SO4- that the bacteria reduce? They also argue that a direct electron transfer might also be taking place via a conductive matrix that connects the two species.
Reference
Meyerdierks, A. Kube, M. Kostadinov, I. Teeling, H. Glockner, F. Reinhardt, R and Amann, R. (2010) Metagenome and mRNA expression analyses of anaerobic methanotrophic archaea of the ANME-1 group. Environmental Microbiology. 12, 422-439.
I also ment to say that I think these papers highlight just how differcult it is to workout a microbes matabolic pathway.
Matt - this is a nice review of a very intersting topic. I think for the benefit of other readers, we need to clarify a couple of points. You say "AOM is performed by archaea that are closely related to methanogens and sulphate reducing bacteria". I think this may be a bit confusing - the archaea are related to methanonogens, of course the SRB are a completely different group. It seems like the authors imply that some archaea may be able to reduce sulphate on their own (without being in a consortium with SRB). However, I think the results show clearly that all the enzymes in this case are of bacterial origin although your last paragraph implies that the archaea are carrying this out. I don't think there's any evidence for this, based on the sequences of the enzymes.
Sorry. I meant that AOM is performed by archae that are related to methanogens with the help of sulphate reducing bacteria, not that they were related to both. And when I mentioned AOMs throughout the review I was referring to both the methane oxidising archae and the sulphate reducing bacteria as both are required for AOM. At the beginning I mentioned some archae had been found to do both processes but this was found in another study that the article has referenced and not in this study. I should have been more clear but was finding it hard to get the correct wording.
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