Anaerobic oxidation of methane: a new model

Methane is a potent greenhouse gas; there is evidence from geological surveys that significant releases of methane in the past have caused dramatic climate change. There is a high concentration of methane stored under the ocean trapped into, or underneath, sediment. The oxidation of methane in ocean sediments to some extent prevents the release of this greenhouse gas into the atmosphere. For some time it was a mystery as to how methane oxidation occurred in anoxic, anaerobic conditions typically found in sediments; in 2000 Boetius et al. discovered that a “consortium” of marine microbes mediate this process. The anaerobic oxidation of methane was thought to be facilitated by coupling it to the reduction of sulphate. FISH and NanoSIMS technology revealed that a consortium of methane-oxidizing archaea was surrounded by sulphate reducing bacteria (see Fig.1. taken from Colin’s lecture notes). This association of microbes can almost be envisioned as one multicellular organism.



Figure 1. Boetius et al. (2000) original methane oxidation model.

 The paper I have reviewed, Milucka et al. (2012), have made an interesting discovery which changes the way we think about the model proposed by Boetius et al. (2000). Miluacka et al. (2012) used also NanoSIMS as well as confocal micro-Raman spectroscopy and other experimental methods to convincingly show that the archaea alone can oxidize the methane and reduce sulphate to zero-valent sulphur compounds (S⁰) via an unknown pathway (See Fig. 2. taken from Milucka et al. (2012)). Furthermore it is possible that the archaea can reduce sulphate to hydrogen sulphide but the evidence for this wasn’t conclusive. If the arachaea can carry out both methane oxidation and sulphate reduction the obvious question is why are they so closely associated with the sulphur reducing bacteria? The study partly addresses this question by demonstrating that the bacteria are reducing hydrogen sulphide; they propose a model in which the zero-valent sulphur leaves the archaen cells where it then reacts with sulphide to form compounds such as disulphide and hydrogen sulphide which the bacteria reduce via fermentation. If this is the case it still is difficult to understand what advantage, if any, the archaea get from being so closely associated with the sulphur reducing bacteria. More experimental work is required to unpick the “consortium” relationship which was originally thought to be symbiotic and even compared to a multicellular organism.
 

 

                            Figure 2. Milucka et al. (2012) new model for methane oxidation.

This study fits neatly into our lecture on microbial activity in sediments in which we learnt about the Boetius et al. (2000) model. Milucka et al. (2012) have convincingly disproved this original model and re-opened an exciting area of study with many unanswered questions yet to be researched. Whilst the finding was convincing and didn’t rely solely on NanoSIMS, I still question the accuracy of this technique and would like to see the quantification of likely rate of error in the future.

Boetius, A., Ravenschlag, K., Schubert, C. J., Rickert, D., Widdel, F., Gieseke, A.,Amann, R., et al. (2000). A marine microbial consortium apparently mediating anaerobic oxidation of methane. Nature, 407(6804), 623–6.

http://www.nature.com/nature/journal/v407/n6804/full/407623a0.html

Milucka, J., Ferdelman, T. G., Polerecky, L., Franzke, D., Wegener, G., Schmid, M., Lieberwirth, I., et al. (2012). Zero-valent sulphur is a key intermediate in marine methane oxidation. Nature, 491(7425), 541–546.

http://www.nature.com/nature/journal/v491/n7425/full/nature11656.html