Microscopic unicellular organisms display a wealth of diversity, and occupy many different roles on Earth. Due to their ubiquitous distribution and high numbers, what they do and when they do it are of vital importance for the biogeochemical cycles on Earth. A large and important group of microbes are the heterotrophic bacteria and archaea, from here on collectively referred to as prokaryotes. They are specialized in consuming and re-mineralize organic material converting it into biomass and inorganic nutrients and carbon.

The oceans function as a vast reservoir of dissolved organic matter (DOM), which contain organic bound carbon equal in size to atmospheric carbon dioxide. Prokaryotes mediate the fate of a large part of marine DOM, which is their principal source of energy and substrate. However, a large fraction is also left behind in the water column persisting for millennia, and prokaryotes may hold the key to understanding the mechanisms controlling the cycling of DOM within marine waters.

In the thesis presented here, the aim was to investigate the activity and composition of prokaryotes to determine their functional role in DOM utilization. The thesis incorporates a range of study systems – ranging from bacterioplankton communities in seasonally variable coastal ecosystems, a manipulated pelagic food web, to a mathematical model of free-living prokaryotes and extracellular enzyme strategies. The results characterize links between community dynamics and function in prokaryotes, and emphasize extracellular enzymes as a key trait of the prokaryote-DOM interface. Understanding the chemical ecology of extracellular enzymes may reveal new insights to DOM cycling in the oceans.