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 an inefficient biological pump of carbon in the Southern Ocean. While the effect of Fe on phytoplankton has been investigated in detail, it is not well understood how this and potentially other trace metals affect heterotrophic microbial metabolism. Fe and carbon are tightly coupled in a suite of metabolic processes crucial for growth. Fe plays a pivotal role in the carbon metabolism, in particular because processes related to respiration rely on multiple Fe-containing enzymes. A deficiency in Fe ultimately results in a reduction in the metabolic activity and the transformation of organic carbon. These observations raise the question of the mechanisms used by microbial taxa to acquire and metabolize Fe and organic carbon in cellular processes. The main objectives of this project is to study the temporal and spatial variability of physiological strategies of heterotrophic microbes to utilize iron and organic carbon.