Abstract
Global warming is continuing without delay and this is caused by the accumulation of greenhouse gases in the atmosphere. Methane is a strong greenhouse gas, 25 times stronger compared to CO2. The increase in methane concentrations in the atmosphere is largely the result of human influences, but there are also
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natural sources of methane, as for instance peat bogs. Peat bogs store an enormous amount of carbon, three times as much as tropical rain forests, yet are also responsible for about 10% of the methane flux to the atmosphere. This methane flux is substantial, however, it is already strongly reduced by methane-oxidizing bacteria (methanotrophs), which live in symbiosis with peat moss (Sphagnum). This thesis comprises the investigation of Sphagnum-associated methanotrophy on a global scale and down to a molecular level, aiming to further the understanding of the interactions between the methane cycle and a changing climate. The work described in this thesis demonstrates that Sphagnum-associated methane oxidation occurs ubiquitously across the globe. Labelling experiments showed that methane-derived carbon is transferred, via methane-oxidizing bacteria, into plant material in different Sphagnum mosses. This way, carbon is efficiently recycled. Climate change projections indicate that areas at mid- to high latitude, where peat bogs are situated, will become increasingly wetter and warmer. It was shown in this theses that, with increasing temperature and increasing humidity, methane oxidation rates rise. At the same time, the balance between methane oxidation and methane production becomes more critical, as methanotrophs cannot compensate for the increase in methane production that occurs concurrently. Methanotrophs act as an efficient filter for the escape of methane, reducing diffusive methane emissions up to 98%. The efficiency of this filter, however, strongly declines at increasing temperatures, from 98% at 5ºC to only 50% at 25ºC. Even though methane consumption increased with increasing temperature, methanotrophs were not able to compensate for the increase in methane production, at least on the time scale of the experiment. Therefore, global warming may be enhanced by increased methane release from peat bogs. Biomarkers can be used to assess the long term influence of environmental change on peat bogs. The stable carbon isotopic composition of hopenes and the BHPs aminobacteriohopanetetrol and aminobacteriohopanepentol have been shown to be the most suitable biomarkers to assess methanotrophy in peat bogs. Methanotrophic biomarkers in a peat record from Misten Bog indicated that methanotrophic activity is strongly influenced by humidity, being higher during cold, wet periods compared warm and dry episodes over the past 1200 years. Since methane emissions increase with increasing water level, this implies that peat bogs probably generated highest methane emissions during cold periods, providing a negative feedback for the prevailing colder conditions. What the future will bring thus critically depends on the extent of the increase in temperature and precipitation. It is, however, clear that methanotrophs play a vital role in reducing methane emissions from peat bogs worldwide and this role may become increasingly important when the climate changes in the near future.
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