Congratulations to Marc Garel (MEB) who defended his thesis on 24 September 2021

In front of a well-packed amphitheatre, Marc brilliantly presented his thesis on the following subject

Methods, procedures and hyperbaric techniques for the study of deep-sea microorganisms in in situ conditions

Thesis Director: Christian Tamburini, CNRS Research Director

Abstract

The ocean is largely deep, with the majority of its volume (>80%) located at depths greater than 1000 m, and is poorly sampled (<0.01%). It is characterised by high hydrostatic pressure, low temperature, high inorganic nutrients and low organic carbon concentrations. Substrate input is mainly provided in the form of organic matter (in particulate and/or dissolved form) by physical and biological processes. Bioavailable dissolved organic carbon (DOC) is mainly consumed in surface waters by prokaryotes, while most DOC in the deep ocean is recalcitrant, more difficult to degrade. Most measurements of metabolic activity and diversity of bathypelagic prokaryotes are generally underestimated due to technological limitations in recovering samples and maintaining them under in situ environmental conditions (high hydrostatic pressure, temperature, etc.). To study prokaryotes under in situ conditions, we have a hyperbaric technology not only to be used as a ready-to-use pressurised sampler, which can be adapted to be used on a CTD sampling carousel and capable of collecting, transferring and incubating samples under in situ conditions (high hydrostatic pressure and temperature), but also increasing the hydrostatic pressure along the water column.  The results highlight that by maintaining the pressure conditions, it is possible to sample piezophilic microorganisms representative of the deep environment. These results were confirmed by our experiment on the degradation of high molecular weight dissolved organic matter by a community adapted to the deep environment. Finally, we also showed the importance of environmental conditions for studying the fate of organic matter in the water column.

Mots clés : océan profond, haute pression, procaryotes, diversité, activité microbienne, échantillonnage in situ