The production of biogenic gases in the marine environment.

Wilson, Sam (2007). The production of biogenic gases in the marine environment. PhD thesis The Open University.



The biogenic trace gases dimethylsulphide (DMS) and methane play a major role in the Earth’s climate and atmospheric chemistry. DMS makes a significant negative contribution to radiative forcing and methane is an important greenhouse gas. The marine environment is an important source of both DMS and methane and this thesis investigated the interactions between methylated sulphur compounds and methanogenesis within micro-environments in the upper-water column.

The production of DMS is closely associated with dimethylsulphoniopropionate (DMSP) and dimethylsulfoxide (DMSO) although comparatively little is known about the dynamics of DMSO in the marine environment. This study analysed the production of DMSO by a range of marine phytoplankton species in comparison with DMSP. Algal concentrations of DMSO were taxon-dependent and an average DMSPp:DMSOp ratio of 4.85 was calculated for the phytoplankton species analysed. The consistent presence of DMSO at ~20 % of total intracellular methylated sulphur (DMSP + DMSO) suggests it is an important algal constituent. The fate of DMSP and DMSO produced by algal cells was analysed in a series of grazing experiments with the copepods Temora longicornis and Acartis clausi, and the dinoflagellate Scrippsiella trochoidea. The relative importance of assimilation, sloppy feeding, DMS production and excretion of faecal pellets as sinks for algal-DMSP was assessed.

In comparison to the ambient marine environment, both algal cells and copepod faecal pellets were identified as micro-habitats of elevated DMSP and DMSO concentrations. Millimolar concentrations of DMSP were measured in algal cells and micromolar concentrations of DMSP were recorded in copepod faecal pellets. These hotspots of DMSP and DMSO may be associated with chemical processes that differ from bulk seawater characteristics, such as the transformation of DMSO as revealed in coastal pelagic particulate material. Furthermore, oxygen depletion at the micro-scale could facilitate the presence of anaerobic bacteria, or anaerobic microbial activity, in the predominantly aerobic pelagic environment. The occurrence of methanogenic Archaea specifically associated with copepod faecal pellets from mono-species cultures of copepods, environmental samples and total pelagic particulates was investigated using 16S rRNA gene libraries. Clusters of sequences closely related to Methanogenium, Methanobacterium and Methanolobus were recovered. This work revealed that CO2-reducing methanogens can exist in the pelagic environment where it was previously assumed methylotrophic methanogenesis dominated.

This work demonstrated that the methylated sulphur compounds represent a metabolic substrate for methanogens in the upper water column. The addition of DMSP, DMSG and DMS to samples collected from the upper water column stimulated methanogenesis when incubated under anaerobic conditions. The addition of inhibitors suggest that other anaerobic microbes e.g. sulphate-reducing bacteria, may play a key role in this process. The potential for DMSP and DMSO to serve as precursors for methane, a potent greenhouse gas, as well as DMS, has important implications when considering the emissions of these compounds from the marine environment and their role in the Earth’s climate.

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