Rui ZHANG

PhD Title: Temporal and spatial variability in the microbial metabolism of carbon and trace metals in the Southern Ocean

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 Fecontaining 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. Several recent studies have provided novel insight to the gene inventories and expression patterns of related pathways in a diverse range of microbial taxa. Fe-limitation has been shown to induce the glyoxylate shunt in heterotrophic bacterial model organisms, a pattern that was also observed for SAR11 in the HNLC Southern Ocean. The glyoxylate shunt bypasses two decarboxylation steps and the coupled release of CO2 and reducing equivalents (NADH2) of the TCA cycle, with important consequences on ATP production and processing of organic carbon. The object of this research is to study the temporal and spatial variability of physiological strategies of heterotrophic microbes to process organic carbon under trace element limiting conditions. I will mainly utilize Meta-omics methodologies providing insight to the metabolic potential of taxonomically diverse marine microbes, and thereby address the fundamental question of which microbial taxa participate to biogeochemical fluxes and what are their environmental drivers in a changing state of the Southern Ocean.

This PhD thesis is part of the multidisciplinary projects SOCLIM – Southern Ocean and Climate (www.soclim.com) and SWINGS - (South West Indian Geotraces Section).