Thesis Title: The coupling of carbon and iron cycles in the Southern Ocean through microbial metabolism
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 increase in phytoplankton biomass in response to Fe input and the consequent enhancement of the CO2 uptake were reported in several mesoscale Fe fertilization studies (reviewed in Boyd et al., 2007) and from naturally fertilized regions (Blain et al., 2007; Pollard et al., 2009). These previous studies have reached the same conclusion as to the importance of Fe for the biological pump of carbon in the Southern Ocean.
Despite the numerous studies conducted up to date, it is not well understood how Fe affects 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 because pathways such as glycolysis, the citric acid cycle and processes related to respiration rely on multiple Fe-containing enzymes. A deficiency in Fe ultimately results in a reduction in the metabolic activity with consequences on the energetic status of the organism and its ability to proliferate (Fourquez et al., 2014; Kirchman et al., 2000). An interesting feature that characterizes surface waters of the Southern Ocean is the low concentration of dissolved organic carbon (DOC, roughly 50 µM; Hansell et al., 2009). Fe and carbon represent therefore both potentially limiting elements for heterotrophic prokaryotes in Southern Ocean surface waters.
In environments where the bioavailability of these elements is low, heterotrophic microbes may adapt with different metabolic strategies. As an example, the expression of the isocitrate lyase gene was reported to vary in environments with contrasted Fe and carbon regimes. The glyoxylate shunt, induced by the isocitrate lyase gene, shortcircuits the tricarboxylic acid cycle, channeling organic carbon to biomass instead of the carbon dioxide production. This raises the question of whether the bioavailability of Fe and carbon regulate the glyoxylate shunt and how, in turn, this pathway affects microbial processing of organic matter.
The objectif of the proposed doctoral thesis is to perform a detailed study of physiological strategies of heterotrophic microbes as an adaptation to low carbon and iron availability. The main objectives are 1) to explore this question by the study of gene expression patterns of heterotrophic microbes in different nutrient regimes in the Southern Ocean and 2) to develop a new tool that allows to determine the bioavailability of carbon and Fe based on bacterial bioreporters. The PhD thesis candidate will use these complementary approaches on during oceanographic cruises in the Southern Ocean and in laboratory experiments in Banyuls sur mer (France).
This PhD thesis is part of the international projects PACBO (LEFE-CYBER; 2015-2016) and SOCLIM (Fondation BNP PARIBAS/UPMC/CNRS/IPEV; 2015-2018 ; (http://soclim.com) both focused on the study of the biological pump of CO2 in the Southern Ocean. Three oceanographic cruises (January-February 2016 ; October 2016 ; March 2017) will provide access to a variety of regions in the Southern Ocean (subtropical, subantarctic and antarctic zone) at different seasons of the year.
Egalement dans la rubrique
Le LOMIC en chiffres
6 Chercheurs CNRS
8 Personnels techniques
4 Post doctorants
8 Etudiants en thèse
4 Etudiants Master2