The Southern Ocean is the largest High Nutrient Low Chlorophyll (HNLC) area where major nutrients are perennially present at high concentrations yet phytoplankton biomass remains low. Surface depletion in iron (Fe) was demonstrated to be the cause of these paradoxical HNLC conditions (Martin et al., 1990). The main objectives of this project are to explore this question by the study of gene expression patterns of heterotrophic microbes in different nutrient regimes in the Southern Ocean. The main focus are respiratory chain enzymes as 90% of these obtain iron. Together with the obtained dataset from the SOCLIM Cruise (October 2016, austral early spring) and the samples from the MOBYDICK Cruise (February-March 2018, late summer), the seasonal variability at two distinct oceanographic sites will be analyzed using a trans-omics approach. In order to better annotate metatranscriptomic and metaproteomic results, the analysis of the metagenomes is crucial which is the aim of this proposed project.