In environmental biotechnology, microbial transformations of organic molecules are often carried out by a microbial community as opposed to one single type of microbe. Within these communities, microorganisms are forced to interact with each other. In the most general case, microbes compete for space, a resource needed by all community members. More specific interactions may involve competition for shared substrates or a mutual requirement for each other's metabolic products. Understanding the network of interactions between the members of an existing community may enable us to introduce and maintain in an augmented community a thermodynamically possible but currently unrealized ecosystem function. This reasoning was successfully used to engineer a granular, phototrophic and methane-consuming community, able to convert dissolved methane into biomass without externally supplied oxygen. Despite competition for oxygen with traditional heterotrophic respiration, the initially introduced methanotrophic activity was maintained over prolonged times in a continuously operated reactor system. Microbial community analysis revealed that a more complex foodweb established than anticipated. Methane oxidation likely involved subsequent steps of partial oxidations by different bacteria, and possibly even intergranule dependencies. Theoretical considerations are a necessary starting point but need to be sufficiently backed up by experiments to account for unexpected behaviour.