Environmental effects on methane uptake in the marine environment

4.1 Introduction

Microbial processes are involved in the exchange of many trace gases between the atmosphere and various ecosystems. Although considerable research has focused on measuring methane emissions from major biological sources (Seiler and Conrad, 1987; Cicerone, 1987,1988; Conrad, 1996; see Chapter 1) much less is known about the magnitude of the factors controlling biological sinks of methane. It is imperative that a better understanding of the chemical and biochemical processes controlling the emission of methane is achieved, particularly in the development of models to predict the role of methane in climate change.

4.1.1 The role of methane oxidation

The flux of methane to the atmosphere is likely to be mediated to a large extent by methane-oxidising bacteria (see Chapter 3), of which many strains have been enriched and isolated from a number of environments (see Chapter 1, see Chapter 6). This biological oxidation of methane has been recognised as being globally important (Gal'chenko et al., 1978; Frenzel et al., 1990; Conrad and Rothfuss, 1991; DeAnglies and Scranton, 1991; Reeburgh et al., 1993; Prinn, 1994; Bender and Conrad, 1995) and the process plays an important role in reducing emissions of methane to the atmosphere (Kightley et al., 1995). These metabolic processes have the potential to be affected by environmental variables acting either on the enzyme activities that have been expressed by the resident methane-oxidising bacterial populations, on the synthesis of new enzyme activities, or the proliferation of particular methane-oxidising bacteria. (Conrad, 1996). Thus much interest has focused on the role of aerobic methane oxidation and on the ecological and anthropogenic practices that affect this process (Ojima et al., 1993).

Aerobic oxidation of methane requires the availability of oxygen, the presence of methane-oxidising bacteria and suitable physicochemical conditions that allow the bacteria to be active (King 1993). From field studies, physicochemical variables have been demonstrated to influence rates of methane oxidation in soil (Mancinelli, 1995), and have been shown to affect community structure (see Chapter 5). Major factors which influence the biological methane oxidation process include: oxygen, temperature, pH and N-content (Jones and Mortia, 1983; Bedard and Knowles, 1989; Adamsen and King, 1993; Goulding et al., 1995; Hutsch et al., 1994; Bronson and Mosier, 1994; Boeck and van Cleemput, 1996; see Chapter 3).

4.1.2 The importance of estuaries

Estuaries have historical and continuing importance to human activities, forming the dominant route for transportation to the ocean of material derived from the weathering of continents, and thus forms one of the most complex of all environments. The estuarine environment is amongst the most productive and sensitive of ecosystems. River runoff of nutrients and organic matter to the oceans forms a coupling between the terrestrial and marine ecosystems.

The distribution of nutrients and organic matter in estuarine waters are controlled, in common with other physiochemical properties (i.e. temperature, dissolved oxygen, and salinity), by the nature of the estuarine circulation, mixing and other physical processes, together with biological, sedimentological and chemical effects. The mixing of river water and seawater results in gradients of ionic strength, composition and pH. The mixing processes of the turbulent flow in estuaries bring about changes in concentrations of dissolved particles. The result is pronounced changes in physiochemical characteristics during the mixing of river water and seawater. There is close coupling between estuarine processes in the water column and sediments, which lead to a variety of reactions of geochemical significance (Burton, 1988). The principle processes affecting the distribution of chemical species in estuaries may be summarised in figure 4.0.

Figure 4.0 Schematic representation of important processes in estuarine chemistry

Modified from Burton, 1988

Anthropogenic activities in and around estuaries have often led to disposal of waste directly to the estuary, leading to many estuarine environments adjacent to human activity becoming polluted (Clark, 1992). These pollutants all contribute to the environmental degradation of coastal seas (Kersten et al., 1988). Deforestation and agriculture in the catchment area, as well as waste discharges can change the natural equilibrium in the estuary resulting in a stressed environment. The perturbation of the estuarine environment has, potentially, a defined effect on the methane oxidation capacity of the system.

4.1.3 Study Aims

While it is well established that the increased methane burden in the atmosphere can be largely attribute to human activities (see Chapter 1) there has been little work on environmental variables that might effect microbial oxidation of methane in the estuarine environment. Considering the extensive range of both methane concentrations and oxidation rates in the Tyne estuary (see Chapter 3), any changes in this highly unstable environment might either enhance or mitigate the net emission to the atmosphere. Although oxidation rates have been reported for a range of environments (see Chapter 1) very little is known about the controlling mechanisms.

The objectives of this study were: -

· To improve our understanding of methane exchange from the estuarine environment.

· To obtain a better understanding of the ecology of methanotrophs, in an attempt to elucidate the mechanisms that regulates methane oxidation in the estuarine environment.

The aim of this study was to observe methane oxidation rates under a variety of physiochemical regimens. Thus recognising what potential factors control methane oxidation

4.2 Results

The effects of physicochemical properties on methane oxidation rates were tested by measuring methane oxidation rates under a variety of physicochemical regimens (see section 2.7). Figure 4.1 shows the sampling point and a variety of physicochemical variables measured on the Tyne estuary and figure 4.2 shows the strategy used to observe the physicochemical regulation of methane oxidation. Samples were collected from the sampling position and all experiments were done in vitro (see section 2.6).

Figure 4.1 Position of and physicochemical variables of sampling station (October 1996).

Figure 4.2 Flow chart showing the protocols used to examine physicochemical effects on methane oxidation in the estuarine environment.

In the analyses of methane oxidation rates and the effects of environment variables all were measured in duplicate in the dark, unless otherwise stated. In examining the effect of environmental variables, methane oxidation, was estimated by measuring the cell matter and carbon dioxide, formed during the oxidation process of methane (see section 2.2) which provides the measurement for methane oxidation rates.