The ocean covers more than 70 % of the Earth surface and represents the largest ecosystem on Earth. Bacteria are an important part of the marine food web not only in remineralisation but also since they utilize dissolved organic matter (DOM) and make the energy available to higher trophic levels. Despite their small size, the total bacterial biomass exceeds the combined mass of zooplankton and fishes.

Bacterial communities are often sensitive to disturbance and the composition changes in response to variations in environmental factors. The diversity of microorganisms is high and the biodiversity is composed of a few abundant and a long tail of rare taxa. The rare taxa contribute most to the diversity, but form a seed bank that is not growing or growing extremely slow. Experimental manipulations showed that the change in community composition in response to disturbance was, at least to some respect, accomplished by rare species becoming abundant. Changes in community composition and diversity did not have an affect on broad scale functions, e.g. utilization of dissolved organic carbon (DOC), indicating that bacterial communities are functionally redundant.

Microorganisms are generally believed to be easily dispersed globally because of their small size, high abundance and short generation time. However, changes in community composition have been shown both on temporal and spatial scales but there is limited information about which factors are most important for distribution of bacterial taxa. A study of in situ samples from the Sargasso Sea showed that a large extent of the variation in the abundance of broad bacterial taxa could be explained by environmental factors, whereas abundances generally did not differ between water masses. In addition, an investigation of the phylogenetic structure of bacterial communities from the Baltic Sea and Skagerrak showed that bacterial communities in specific environments usually are more phylogenetic related to each other than expected by chance. Together these results indicate that the environment is an important factor forming the community and that phylogenetically related bacteria are functionally related, sharing similar traits.