Biotic and abiotic degradation of terrestrial organic matter discharged by Rivers (Rhône, Pô and Ebre) in Mediterranean Sea

Project description: 
Because riverine particulate organic matter (POM) consists of already highly degraded residues of terrestrial organisms (with a high content of lignin), it is generally considered as being refractory to decomposition in the ocean. It is surprising then that only a small fraction of the organic matter preserved in marine sediments appears to be land-derived. This suggests either that global budgets and distribution estimates are greatly in error, or that POM of terrestrial origin undergoes rapid and remarkably extensive remineralization at sea.

We intend during this thesis to study the degradation of TOM discharged by the Rhône River. Thanks to seasonal sampling of POM and different laboratory experiments, we will try to explain why biotic and abiotic degradation of TOM is so intensive in seawater. These works should contribute to a better knowledge of the behavior of TOM in the oceans and thus to a better understanding of the global cycle of carbon in coastal zones and its anthropic perturbations.

This thesis will be composed of two complementary tasks: 
- In the first task, the role played by metal ions in the induction of autoxidation of TOM will be studied. Autoxidation (spontaneous free radical reaction of organic compounds with oxygen), which has practically been ignored until now in the marine realm, can act not only on unsaturated lipids (such as sterols, unsaturated fatty acids, chlorophyll phytyl side-chain, alkenes, tocopherols and alkenones), but also on amino acids, sugars and polyphenols. It can also affect biopolymers and kerogen, inducing ring opening and chain cleavage, which may enhance bacterial degradation of these recalcitrant substrates. The aim of this task will be to determine if the intense autoxidation of TOM observed in seawater is induced by some metal ions (released in mixing waters) able to catalyze cleavage of hydroperoxides produced during the senescence of terrestrial higher plants. And if this is the case, to determine what are the ions implicated in this transformation. 
- The second task will concern the study of biodegradation of TOM by bacterial communities. The evolution of the diversity of bacterial communities along gradients of organic matter between terrestrial and aquatic systems, and the plasticity of estuarine organisms in terms of patterns of organic matter dissolved using (DOM) have been demonstrated. It has been theoretically proposed that along environmental gradients, there is a change in species according to their nutritional strategies based on the principle of competitive exclusion: the specialized species that have relatively well-defined niches and a narrow range of tolerance are replaced by generalist species that have broad niches and tolerate larger changes in the environment. And specialist species are likely to be found in simplified environments, while generalist species will be more distributed in environments containing a wider range of resources. This concept of compromise in the use of resources ("Jack-of-all-trades, master of none") has been widely applied to different ecosystems, but rarely tested in situ and in particular the marine environment. Thus, in this context, we can hypothesize that the bacterial assemblages of the marine environment use more effectively specific parts of the TOM that the bacterial assemblages of soils and rivers, where the sources of DOM are more various. The possibility of reactions of co-metabolism or priming effect (degradation of a recalcitrant substrate by microorganisms in the presence of a readily available substrate) cannot be excluded due to the high phytoplankton growth typically observed in river mouths.

We will try to answer the following questions: 
- How OM exported from earth system impacts the structure and functioning of aquatic bacterial communities? 
- How changes of specific and functional diversity of bacterial communities in the continuum riversea lead to greater efficiency in the degradation of TOM? 
- Is the degradation of TOM favored in the presence of an algal co-substrate? 
- Does autoxidation of TOM promote biodegradation?

This original project is based on a ‘cross-biome’ study, i.e. taking into account both the terrestrial and aquatic environments and exchanges of matter and organisms between these two environments.

This study is well integrated within OTMed’s WP1 (Air-Sea Interactions, Impact of contaminants on the Mediterranean Sea, Coastline changes) and WP2 (Mediterranean Soils - Management, Agriculture and Marine Ecosystem Functioning), as well as the Transversal WP1.

This study will be integrated to the MERMEX (https://mermex.com.univ-mrs.fr/) program and more particularly to the WP3-Rivers work package, whose one of the objectives is to study the behavior of TOM in seawater.