Microbial Community Structure

Investigation of methane oxidising bacteria using 13CH4 labelling of phospholipid fatty acids

This research was performed as part of an NERC funded CASE studentship in collaboration with the Centre for Ecology and Hydrology (CEH). Although some low affinity methanotrophs have been cultured, the majority of methane oxidising bacteria are not amenable to currently available methods of culturing, resulting in the need for a culture independent method of analysis. One such method is the combination of stable isotopic labelling and phospholipid fatty acid (PLFA) analysis. An example of the use of this technique is in the investigation of the methane oxidising community in the cover of a clay-capped landfill site. Soils were taken from four sections of a depth profile of the cap and incubated with 10,000 ppm methane of which either 1% was 13CH4 or all was unlabelled methane. Following incubation the PLFAs were extracted from the soils and analysed by GC/C/IRMS. From the data obtained it was possible to calculate the relative abundance of each of the PLFAs produced over the course of the incubation. These PLFAs are effectively a chemical signature of the methane oxidising community in the soil samples. As PLFAs can be used for chemotaxonomic characterisation of certain groups of bacteria it is possible to observe a change in the methane oxidising community with depth through the landfill cap. Figure 1 shows the relative abundance distributions of labelled PLFAs for the four sections of the cap. The top section of the profile is dominated by labelled C16 fatty acids suggesting that a population of Type I methanotrophs are performing the majority of the methane oxidation. Descending the profile, the deepest soil exhibits a different PLFA distribution, dominant in C18 fatty acids, suggesting a high abundance of Type II methanotrophs contributing to the methane oxidation. This method of microbial analysis can also be employed in the investigation of high affinity methanotrophic bacterial communities in forest soils by incubation with a lower concentration of labelled methane. Additionally, other labelled substrates may be utilised for the investigation of different functional groups of microbes involved in carbon cycling in soils and sediments.

References
Crossman, Z. C., Abraham, F. and Evershed, R. P. (2004) Stable isotope pulse-chasing and compound specific stable carbon isotope analysis of phospholipid fatty acids to assess methane oxidizing bacterial populations in landfill cover soils. Environmental Science and Technology 38, 1359-1367.

 

 

Methane assimilation bar charts

Figure 1