Table of Contents

  • This report focuses on the presence of methane oxidising bacteria (methanotrophs), their functional characteristics and diversity in different soil communities. The methanotrophs are responsible for removal of the greenhouse gas methane by biological oxidation of methane to carbon dioxide. In addition, they also have the capacity to degrade different halogenated hydrocarbons of environmental concern. The key function of methanotrophs as methane consumers and degraders of halogenated hydrocarbons combined with the fact that the methanotrophs are represented by two major groups of closely related microorganisms, bring them in the perspective of being potentially useful indicators of environmental perturbations.

  • In this paper we present data from our experimental work in the project “Methane oxidising bacteria as an integrated environmental indicator” commissioned by the Nordic Council of Ministers. Our data is then discussed with regard to the potential of methane oxidising bacteria as environmental indicators. The present research does not seek to reproduce monitoring surveys of methanotrophs or their processes that are already recognised as useful environmental indicators. Instead, it describes innovative and reliable methodologies that enable the study of methanotroph communities in soil both directly and by isolation of the bacteria. We emphasise the stability and functional adaptation of methanotroph communities to climate and to environmental pollution. This provides a better platform for the evaluation of methanotrophy as a microbial indicator in Nordic soils.

  • The basis for the use of environmental indicators is the relationship of societal and industrial development to key scientific measures of environmental impact. A number of different criteria characterise the individual indicators and may limit their use to specific situations. However, a general definition of a successful indicator is given by Nielsen & Windinge (2002) and Christensen & Møller (1992)

  • The methane oxidising bacteria are commonly known as methanotrophs. They are able to utilise the greenhouse gas methane, CH4, as their sole source of carbon and energy. Some methanotrophs are also reported to utilise methanol, CH3OH. Since methanotrophs use methane and utilise methanol and other 1-carbon compounds as intermediates in their energy metabolism, they are members of larger group of bacteria called the methylotrophs. The methylotrophs are obligate 1-carbon utilisers.

  • Methane is a greenhouse gas 30 times more powerful in absorbing infrared radiation than carbon dioxide, and its relative contribution to climate change is estimated to approx. 18% (cited from Hanson & Hanson 1996). Globally, approx. 500 Tg of atmospheric methane is produced each year. The source of methane is partly of industrial origin (agricultural and industrial emissions) and partly of natural origin. Methane is produced naturally in anaerobic environments by methanogenic bacteria belonging to the domain Archaea. Approximately 90% of the produced atmospheric methane is oxidised by photochemical reactions in the troposphere (see Fig. 3) while an estimated 2% is removed by microbial oxidation in soils (Hanson & Hanson 1996).

  • In this NCM project we have innovated two methods for isolation and quantification of methanotrophs. These are direct, precise, fast methods that enable a more reliable quantification of methanotrophs in environmental samples and allow simple isolation of a variety of species or genotypes from the sample communities.

  • The application of the new SSMS method to soil samples from a wetland near Roskilde and from Skellingsted landfill, Denmark, resulted in successful isolation of several methanotrophs. Skellingsted represents a comprehensively studied landfill with regard to methane oxidation (Kjeldsen & Fisher 1995, Christophersen et al. 2000, Scheutz & Kjeldsen 2003). Different methanotrophs were isolated from these sites and variation in the cell types is illustrated in Fig. 7.

  • It has been suggested that methanotrophs play a significant role in dehalogenation of different C-1 and C-2 compounds of anthropogenic origin. Recently published work demonstrates that microbial dehalogenation of HCFC gases occurs in soil layers at Skellingsted landfill, a site that also exhibits a high methane oxidation capacity (Kjeldsen & Fisher 1995, Christophersen et al. 2000, Scheutz & Kjeldsen 2003). The hydrochlorofluorocarbons HCFC-21 and HCFC-22 (Fig.) are supposed to be released either directly from insulation foams typically present in refrigerators, or are products of anaerobic dehalogenation of CFC gases (also present in refrigerators) that are even more harmful to the environment. The two HCFC gases are grouped as class II ozone-depleting substances and are now being phased out. They are harmful to the environment primarily because of their global warming potential rather than their ozone depletion potential. Although the compounds are being phased out, they will persist in the environment due to their stability and their slow release from industrial products, e.g. insulation foams (Kjeldsen & Scheutz, 2003).

  • Methanotrophs are the key organisms in biological oxidation of the greenhouse gas methane. In addition, methanotrophs participate in aerobic dehalogenation of a number of different chlorinated, fluorinated or brominated hydrocarbons of environmental concern. As potential environmental indicators, parameters linked to the community structure or activity of methanotrophs should at best be categorised as “state indicators”. However, provided that good baseline measures are obtained in future monitoring programmes, accumulated data will also be useful as indicators of impact or pressure. At present, no monitoring programmes include the structure or function of methanotrophs as routinely measured parameters. Research into this group of micro-organisms has been a difficult task. The lack of tools for simple, reliable quantification of methanotrophs and insufficient knowledge about the species composition of methanotrophs under different environmental conditions has restricted the use of these organisms in an indicator context.