A microbial methane factory
The serene surface of Thale Noi, a vast freshwater wetland in southern Thailand, hides a turbulent microbial process. Below the water, in the oxygen-deprived peat soil, armies of microorganisms are decomposing immense stores of organic matter. This anaerobic digestion produces large quantities of methane (CH₄), a greenhouse gas with a warming potential many times that of carbon dioxide. The specific area of intense study is the Phru Khuan Khi Sian peat swamp, part of the larger Thale Noi complex. This site became Thailand's first Ramsar Wetland of International Importance in 1998.
The source of this methane production consists of archaea known as methanogens. In tropical peatlands like Thale Noi, the dominant methanogens are often hydrogenotrophic, meaning they produce methane by reacting hydrogen with carbon dioxide. Studies in Thai peat swamps have identified microbes from the class Methanomicrobia as prevalent. These organisms thrive in the waterlogged and anoxic conditions of the peat. The constant high temperatures and abundant organic material from decaying wetland plants create ideal conditions for methanogenesis year-round. While some methane is consumed by other microbes called methanotrophs, a substantial amount escapes into the atmosphere. The water level controls these emissions; when the water table is high, anaerobic conditions expand, and methane production increases.
The plant-powered pipeline
A significant portion of the methane produced in Thale Noi's sediment does not simply bubble up randomly. Instead, it is captured and transported directly into the atmosphere by the wetland's vegetation. Many aquatic plants have developed internal gas-transport systems, called aerenchyma, to supply oxygen to their submerged roots. These same structures act as conduits, or chimneys, for methane to bypass the oxygenated surface water where it might otherwise be oxidized by bacteria.
One of the most important plants in this process at Thale Noi is the Chinese water chestnut, Eleocharis dulcis. This emergent sedge forms dense beds across the wetland. Its straw-like stems, or culms, are highly efficient at venting methane from the peat soil where it is produced. Research on related plant species shows that gases can move through the aerenchyma via passive diffusion or even pressurized convective flow. This plant-mediated transport is a major pathway for methane emissions in many of the world's wetlands. The dense vegetation covering the 460-square-kilometer Thale Noi area provides a vast network of these natural pipelines. The combination of high methane production in the peat and efficient transport by plants makes Thale Noi a large natural source of atmospheric methane.