Page 24 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 1. General introduction
We live in a microbial world. Microorganisms, while invisible to the naked eye, were the first life forms on our planet, shaping the world as we know it. During Earth's history, a set of metabolic processes evolved exclusively in microorganisms that thrived in the absence of oxygen. These processes changed the chemical speciation of all major elements (Falkowski, Fenchel and Delong 2008; Stolz 2017). Our present-day environment is thus the integrated result of microbial experimentation that has allowed life to develop and persist, despite major environmental changes. Among the most urgent scientific questions are which key groups of microorganisms drive the relevant reactions, how do these microorganisms interact with each other and their geochemical environment, and how do they impact the Earth system (Anantharaman et al. 2016; Thompson et al. 2017). In a warming world, especially, the microbial processes that influence climate change are of special interest, not only in order to answer fundamental research questions but also to address urgent societal challenges. Since the onset of the Industrial Revolution, we observe major changes in the world around us. Permafrost is thawing, ice sheets are melting, and global temperatures are on the rise. Humanity should be put on notice that the impacts of climate change will depend heavily on the responses of microorganisms, which are crucial for a sustainable future on our planet (Cavicchioli et al. 2019).
In order to answer one of the urgent questions, this thesis focuses on microbial dynamics of the methane (CH4) cycle in a changing world. Methane is a potent greenhouse gas with complex biological and geochemical feedback mechanisms. Permafrost landscapes thaw, wetland CH4 emissions increase, and human activities result in additional CH4 sources. The last decades, we have gained more insights on global CH4 sources and sinks (Intergovernmental Panel on Climate Change and World Meteorological Organization 1990; Myhre et al. 2013; Nisbet et al. 2019). However, we still know little about the microbes that control several major CH4 fluxes. The next paragraphs focus on CH4 and its role in the Earth’s climate. After this, the microbes involved in CH4 cycling and the ecosystems discussed in this thesis are presented.
Methane is the most abundant reduced compound in the atmosphere and it plays an important role in the Earth's carbon (C) cycle. This cycle consists of continuous transformations of C between the organic and inorganic pools in the atmosphere, terrestrial biosphere, hydrosphere, and geosphere. Atmospheric carbon dioxide (CO2), the fully oxidized form of C, is fixed by the
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