LIU Yan

LIU Yan Doctorante UPMC (ED129) Laboratoire d'Océanographie Microbienne (LOMIC) - UMR 7621 CNRS-UPMC Avenue Fontaulé - 66650 Banyuls sur mer, France Tél: 33 (0)4 68 88 73 77 E-mail: yan.liu @ obs-banyuls.fr (yan.liu @ obs-banyuls.fr)

Thesis title: Microbial respiration and organic matter in the Southern Ocean

The biological pump of carbon dioxide (CO2) is recognized as one of the main drivers of the global carbon cycle. Its amplitude and variability can have a large impact on climate. As a consequence, considerable efforts were undertaken to describe its functioning and its response to a changing environment. Up to date, most research has focused on the processes that transform CO2 to particulate organic matter and its export to depth. By contrast, microbial respiration, the process that returns organic to inorganic carbon has received much less attention. The environmental factors that control respiration of organic matter can differ from those controlling its production, the biological sinks and sources of CO2 are therefore partly decoupled. For this reason, identifying the mechanisms that govern microbial respiration and its dynamics deserve dedicated studies.

At present, field data of respiration measurements lag far behind those of primary production. Also, experimental studies that aim to better understand the mechanisms controlling microbial respiration are scarce. This is particularly the case for the Southern Ocean. While the spatial and temporal variability of primary production can be estimated from satellite obseravtions, microbial respiration still needs to be explored in this remote ocean. A particular characteristic of the Southern Ocean is that the concentrations of dissolved organic carbon (DOC) in surface waters are the lowest in the global ocean. This is due to the permanent upwelling of refractory organic matter from the deep ocean, and the low primary production limited by the micronutrient iron. Consequently, the bioavailability of dissolved organic matter (DOM) is a key factor that controls microbial respiration in the Southern Ocean.

The objectif of the proposed doctoral thesis is to perform a detailed study of microbial respiration in the Southern Ocean, in relation to the bioavailability of DOM and the composition of the prokaryotic community. The main objectives are 1) to obtain new data on microbial respiration using a suite of methodological approaches (O2/N2 in situ measurements, O2 in vitro measurements, enzymatic essays) in combination with the caracterization of the DOM (FT-ICR-MS) and the prokaryotic community composition (high throughput sequencing) to better understand its spatial and temporal variability, 2) to develop a new tool that allows to determine the bioavailability of DOM based on bacterial bioreporters and their utilization to better understand how the bioavailability determines microbial respiration. The PhD thesis candidate will use these complementary approaches 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 BARIPAS/UPMC/CNRS/IPEV; 2015-2017 ; (http://lomic.obs-banyuls.fr/fr/publications_workshops_conferences/revues_de_presse/projet_soclim.html), both focused on the study of the biological pump of CO2 in the Southern Ocean. Three oceanographic cruises are scheduled (January-February 2016 ; October 2016 ; March 2017), that will provide access to a variety of regions in the Southern Ocean (subtropical, subantarctic and antarctic zone) at different seasons of the year.