Description du sujet de thèse

Solving the role of Biogenic Volatile Organic Compounds (BVOCs) as defense molecules to cope with herbivory in seagrass beds from marine environments
Biogenic Volatile Organic Compounds (BVOCs), emitted by terrestrial and marine ecosystems, are the primary source of carbon released into the atmosphere. Global emissions are estimated at 1 PgC.yr-1, representing 90 % of VOC emissions at the global scale and the main producers of BVOCs are plant species. These compounds are involved in plants' responses to abiotic and biotic stresses and are also important chemical cues in inter- and intra-species interactions. For instance, herbivory or pathogens can induce changes in the plant volatilome (i.e. composition of BVOC blend), inducing pests' repulsion or attracting natural predators of herbivore species. Therefore, BVOCs play important roles in ecosystem functioning, and deciphering how plants respond to environmental factors through BVOCs emissions is crucial to the future of ecosystems in a changing world. Nevertheless, much of our knowledge of BVOCs is based on terrestrial ecosystems, while marine and coastal environments have been barely studied. Our previous studies on Mediterranean seagrasses showed a high diversity of volatilome with the presence of terpenoids but also fatty acids and derivatives, benzenoids or sulfur compounds. Besides, our first quantification of emission rates on Posidonia oceanica highlighted strong emissions of oxygenated BVOC (e.g. acetone, up to 4 μg.gDW-1.h-1) and dimethyl sulfide (DMS, up to 10 μg.gDW-1.h-1). This compound has been shown to be released in greater quantities when herbivores fed on macroalgae, affecting consumption rates. Moreover, this compound seems to be an important chemical cue since it has been shown that fish larvae or birds are attracted by DMS-containing seawater. Nevertheless, our knowledge of BVOC emissions from marine species remains still very limited and the main factors influencing their magnitude and composition are still not understood. As BVOCs are identified as defense compounds against (a)biotic stresses in terrestrial environments, they may have a similar role in marine and coastal ecosystems. Since herbivory is a structuring factor for seagrass beds dynamics, understanding the interactions between both organisms is needed. Besides BVOCs, non-volatile metabolites such as phenolics are also important defense compounds in terrestrial environments. Thus, data on all defense compounds will give new insights into the resistance and resilience levels of these ecosystems confronted with this stress. To implement future conservation measures, it is therefore essential to investigate metabolome changes (volatile and non-volatile) from marine and coastal organisms, allowing a better understanding of the functioning of these ecosystems.

Contexte de travail

The global objective of this PhD is resolving the role of BVOCs as grazing defense compounds in seagrass beds through the following questions:
Question 1: Do marine organisms exposed to grazing pressure change their BVOC emissions ?
Question 2: Does the metabolome (especially non-volatile specialized metabolites) of marine organisms change when exposed to grazing pressure ?
Question 3: Do potential key BVOCs influence grazers' behavior ?
Question 4: What is the diffusion and evolution of marine BVOC when they act as chemical cues ?
Question 5: Can BVOC prime the defenses of healthy individuals from the same species ?

The candidate will need to have an interdisciplinary profile, including skills in ecology and/or marine ecology and notions in analytical chemistry as well as statistical approaches (univariate and multivariate analyses). The main focus of this PhD will be on BVOC emissions (GC-MS and PTR-MS) and, to a lesser extent, on phenolic compounds (UHPLC-MS). If the candidate does not have experience in BVOC sampling and analyses, he/she will be trained by PhD supervisors (C. Fernandez and J. Kammer) and the technical staff (C. Lecareux, S. Greff and B. Temime-Roussel). The PhD student associated with this project will plan the experiments and run them (e.g. grazing, choice and priming tests). He/she will assess the dataset by performing all the BVOC analyses, their identification and their quantification. The candidate will also have to perform the statistical analyses to highlight significant findings. He/she will have to work with the entire consortium and will have to organize meetings between all participants (with the support of the PIs of the project) to agree on experimental design and present the project results and achievement during the entire PhD. He/she will also write the corresponding papers in peer-reviewed international journals. Then, the candidate will also have to present his/her results at national and international conferences (through posters or oral communications). Besides, for the entire project, the candidate will receive help from :
- Diving service from OSU Institut Pytheas to sample seagrasses
- Common service « dispositifs expérimentaux » for the seagrass keeping and sea urchins rearing. This service will also be helpful for choice tests as well
- Common service « Métabolomique et chimie des Substances Naturelles » to perform GC-MS and UHPLC- qToF-MS for chemical analyses of volatile and non-volatile metabolites.
- Massalya platform for online chemical analysis

Contraintes et risques

RAS