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Composition, role and fate of exopolymers in coastal sediments biofilms

Team 6: Source and transfer of organic matter in aquatic ecosystems


Year of defence: 

Biofilms are an assemblage of microorganisms embedded in a matrix of exopolymers or EPS (Extracellular Polymeric Substances), and are very common in tidal flats. EPS have numerous roles for both microorganisms and ecosystems, and yet factors controlling their dynamics and functions are still not fully understood. The aim of this work was to improve knowledge on EPS production and evolution in sediment depending on their biotic and abiotic environment, as well as the functions of these molecules in benthic systems. A one-year monitoring on a tidal flat was combined with experiments in controlled conditions to better characterise the interactions between microorganisms, EPS and their surroundings. Results show that EPS production in natural environment increases in summer, as a result of higher photosynthetic production. Besides, organisms from meiofauna and macrofauna are able to stimulate this production through bioturbation. EPS composition is not solely determined by the composition of producing assemblages; this also depends on other biotic parameters, such as EPS degradation by bacteria or EPS consumption by organisms from higher trophic levels. Yet, EPS are not universal trophic resources, as we demonstrated that some organisms are unable to assimilate them. Finally, EPS concentrations and composition in sediment influence their ability to stabilise sediment, in interaction with environmental parameters. Particularly, consumers of microphytobenthos can stimulate EPS production and therefore affect sediment stability. As a conclusion, EPS quantity and quality are at the heart of a complex interaction network, and they control EPS functions in intertidal areas. Because sediment stability is a crucial feature in these environments, organisms producing EPS, or modifying their concentration or composition in sediment, can be regarded as ecosystem engineers. 

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