The generation of methane in marine sediments by methanogenic bacteria has been shown, in certain areas, to form undissolved gas voids and to provide a characteristic geoacoustic signature known as acoustic turbidity. The ether linked membrane lipids in the marine sediments of Holyhead Harbour and an intertidal site, are expected to primarily reflect methanogenic bacteria. The concentration of ether lipids was shown to be a good criterion for the determination of methanogenic biomass in pure cultures, since it correlated well
with other biomass determinants, including the cell dry weight of the organism. Alkane derivatives of the larger diether and acyclic and cyclic tetraethers gave the best reproducibility over the long analytical procedure. The preparation of a halogenated derivative of the diether offers an improved sensitivity by electron capture detection. An uncharacterised ether lipid derivative was shown to be an isomer of a three-cyclopentyl ring tetraether derivative. It is hypothesised that the high concentration of cyclic ether lipids in the sediments are formed by known methanogenic bacteria as a result of changing conditions in the sediment and are expected to occur soon after deposition. The concentration of methane below the sulphate reduction zone (SRZ) in Holyhead Harbour was shown to increase significantly and was calculated to exceed the solubility limit of the
pore water and form gas voids, which was also corroborated by sound velocity and X-ray measurements. Modelling of the methane distributions suggested that a methane removal term was necessary which may reflect anaerobic methane oxidation in the SRZ of the Holyhead sediments. Conversion factors of methane produced per ether lipid synthesised were calculated from pure cultures. Application of these conversion factors to the ether lipid data in the sediments of this study and an artificially prepared core revealed some interesting results. The concentration of ether lipids in the surface sediments was comparable to the deeper sediments, which was in contrast to the counts of viable methanogens and the methane concentration. Also, contrary to the literature the ether
lipids did not show a consistent increase below the SRZ. Due to the limited ether lipid data for near-shore marine sediments it is not known whether these results are typical or just reflect a variable depositional input to the sediments of Holyhead Harbour.