Microbial distributions across methane-associated geochemical zones in marine sediments

In deep marine sediments along continental margins, methane is one of the dominant features that determine the microbial community. Sediments containing methane are spilt into three separate geochemical zones according to methane concentration and gas hydrate stability. The uppermost zone in the sediment, sulphate methane transition zone (SMTZ), is where minimal levels of methane merge with sulphate from overlying water. Below the SMTZ, gas hydrate (methane ice) is formed, creating the gas hydrate stability zone (GHSZ) and beneath this is the free gas zone (FGZ). Methane gas is present here as the methane concentration exceeds its solubility.

Microbial communities in the SMTZ are globally important as they perform anaerobic oxidation of methane, consuming nearly 90% of all methane produced in deeper sediments. This layer of sediment contains a distinct community of Deltaproteobacteria and anaerobic methanotrophs (ANME).

The objective of this study was to investigate the microbial community composition in different geochemical zones in relation to methane and other abiotic factors of the sediment. Cores were taken from two different sites from the Ulleung Basin. DNA was extracted and a PhyloChip micro-assay and a terminal restriction fragment length polymorphism (T-RFLP) was used to assess the microbial community structure.

It was concluded that the three different geochemical zones are significantly different in terms of the microbial community however there was some overlap in community structure. From the PhyloChip analysis, SMTZ was found to have 74 taxa related to Deltaproteobacteria and the archaeal taxa detected was related to Thermoplasmata. In the GHSZ a lower diversity of 65 taxa were found related to Vibrio-like taxa. Little DNA was extracted from FGZ samples and therefore analysis was very limited.

Archaeal genes were not found below the SMTZ which was unusual as archaeal methanogens were expected as the production of methane was present in the sediment but there could have been possible complications with detecting the archaea. Another possibility is that the methanogenic zone was not sampled, as it usually occurs below the SMTZ but in the upper 50 metres below the seafloor.

ANME are commonly associated with the SMTZ but was not found in the Ulleung Basin, but the Marine Benthic Group-B was detected instead which suggests that other microbial groups are involved in the oxidation of methane. Another explanation would be that the samples missed the area of high ANME abundance.

From the GHSZ, two distinct microbial groups were discovered, separated along the hydrate saturation of the nearest hydrate layer vector. The presence or physical properties associated with hydrate was thought to affect microbial distributions at varying distances from the source. Vibrio-types are one of the species that prefer sediments closer to the hydrate containing sediment.

Applying these significant patterns to other sedimentary environments will be very difficult as the microbial community can vary greatly according to location therefore assumptions should not be made on the basis of previous studies.

 

Briggs, B.R., Graw, M., Brodie, E.L., Bahk, J., Kim, S., Hyun, J., Kim, J., Torres, M. and Colwell, F.S. (2013) Microbial distributions detected by an oligonucleotide microarray across geochemical zones associated with methane in marine sediments from the Ulleung Basin. Marine and Petroleum Geology. 47: 147-154