Methane (CH4) is a powerful greenhouse gas (1), and microbes play an important role in its the cycling. In natural aquatic ecosystems, most of the microbial CH4 production occurs in deep anoxic sediment (2). The produced CH4 diffuses upwards in the sediment and is removed by methane oxidizing microorganisms (so called methanotrophs) (2, 3). In the sediment surface CH4 is efficiently removed by aerobic methanotrophs (i.e. requiring oxygen) (4). Because these bacteria are located in the sediment surface they are affected by the activity of animals, such as burrowing (i.e. bioturbation) that affects oxygen and organic matter availability and sediment chemistry. Even micro-animals (i.e. meiofauna, < 1 mm size) such as roundworms are known to create burrows, feed on bacteria, and their activity has previously been shown to affect nitrogen cycling in sediments (5). However, how meiofauna and larger animals affect the activity of methanotrophs and their potential to remove CH4 are still not well understood. Shallow coastal waters are hotspots for methane emissions from marine systems. These areas represent ~3% of the oceans but potentially up to 50% of the marine CH4 emissions and higher when considering CH4 escape via bubbling (6). However, it is still unclear what are the dominant ecological factors explaining such high emissions. In recent years, research has focused on environmental drivers of anaerobic CH4 oxidation occurring in deep anoxic sediment, however much less understood are the biotic factors that regulate the activity of aerobic methanotrophs in the sediment surface. The goal of the project is therefore to investigate how biotic drivers such as animal activity mediates aerobic methane oxidation and its consequences to sediment-to-water CH4 fluxes. The project will increase knowledge on to what extent micro- and macro animals in the sediment surface influences the activity of aerobic methanotrophs and subsequently CH4 emissions from coastal sediments. This is important because eutrophication and global warming directly affects animal biodiversity in coastal sediments, which might therefore also increase CH4 emissions from these environments. This project has been funded by BalticWaters2030