The characteristics of the world around us vary from one location to the next. This is also true of aquatic environments, where bottom-dwelling microorganisms must cope with variation in temperature, salinity, dissolved oxygen, and nutrients. Unfortunately, little is known about how these microbial communities and their functional genes respond to environmental changes. A team of researchers at Stockholm University recently set out to do just that by collecting samples of sediment at 59 sites spanning 1,145 km across the Baltic Sea. They characterized the environmental attributes and microbial community at each site using genetic sequencing and other laboratory techniques. The researchers found that salinity and dissolved oxygen content had the greatest effects on the microbes making up each community with the communities in oxygen-deficient “dead” zones being particularly dissimilar to those with higher dissolved oxygen content. The team also found evidence of adaptation to local environments in terms of the functions carried out by the microbes. These results lend important insight into how environmental factors shape microbial communities and how they may be affected in the future by ongoing environmental change.