A Load of Hot Air : The Anaerobic Oxidisation of Methane by Macroscopic Biofilms

Marine sediments contain a high proportion of the world's methane carbon in the form of gas hydrate which is constantly leached into the atmosphere. The release of methane from the sediment is, in part, regulated by the anaerobic oxidation of methane (AOM) by communities of archaea and bacteria. This study concentrates on a new sulphate-methane transition (SMT) setting in the macroscopic biofilm forming microbial communities of the Indian and northeast Pacific Ocean.

Macroscopic pink to orange biofilms (~0.5cm³ in volume) were collected from the marine sediment in three different settings across India and the West coast of Canada and the US. Firstly, the samples were analysed using pore water analysis to determine the sulphate and methane profiles for each site. The SMT depth (the depth where sulphate and methane concentrations converge at minimum values) ranged from 18 to 20 mbsf (meters below the sea floor) at the site in India compared to 1.4 to 5 mbsf at the site in Canada. These depths corresponded to the depths of biofilm occurrence. To identify the taxa of the microbial community of the samples DNA was extracted and examined using PCR-mediated t-RFLP analysis to allow comparison of the prominent DNA sequences. A total of 133 distinct archaea and 148 bacteria were detected with 52 archaea and 56 bacteria being shared between biofilms and reference samples (fragments of biofilms collected). 16S rRNA indicated that the major bacterial groups identified in the samples were shown to be Deltaproteobacteria (71% of all clones) and Chloroflexi (10% of all clones). The archaea communities mainly contained Euryarchaeota and Crenarchaeota for both sites located offshore of India and Vancouver of which 4 phylotypes of the ANME-1 clade were detected). Microarray analysis of the samples also detected several different phylotypes from each site.

As the AOM communities are restricted to growth in environments containing both sulphate and methane then environments containing higher concentrations (such as seep-dominant SMT environments) lead to higher abundance of AOM biofilms. The authors postulate that fractures on the sea bed serve as pathways allowing the flow of methane which was proved by their observation of methane bubbles rising from the seafloor near the sampling sites. These fracture may therefore increase the potential for the growth of AOM biofilms. The carbon composition of the biofilm from the SMT suggests that the production of biofilms is associated with AOM. The authors conclude that, although rare in normal marine environments, these biofilms show distinct communities represented by the ANME-1 clade that occur in methane-rich marine sediments.

Knowledge and further research of these biofilm communities could give insights into whether environmental stresses, such as pH and temperature change, could lead to a change in the biofilm diversity impacting on the moderation of methane gas from marine sediments.

A review of : Briggs, B.R., Pohlman, J.W., Torres, M., Riedel, M., Brodie, E.L., Colwell, F.S.; 2011; Macroscopic Biofilms in Fracture-Dominated Sediment That Anaerobically Oxidise Methane; App. & Env. Micro.; Oct. 2011; pp 6780-87.