This study investigates prokaryotic energy metabolism in the Arctic Ocean and the Baltic Sea. We will analyse metabarcoding and metatranscriptomic data from prokaryotes collected in 1.2 µm-filtered samples from the Swedish Synoptic Arctic Ocean 2021 expedition. In addition, the project will evaluate a microcosm experiment investigating the effects of extreme salinity and temperature in the Baltic Sea. We will focus on the expression of genes related to maintenance respiration, which is relevant to determining prokaryotic growth efficiency.