Microbial responses to terrestrial dissolved organic matter input in freshwater and marine ecosystems in a changing environment – RESTORE

A continuing challenge for scientists is to understand how multiple interactive stressor factors affect biological relationships and subsequently ecosystem functioning. It is largely recognized that local watersheds act as critical controls of aquatic ecosystem qualities. Inputs of terrestrial dissolved organic matter (tDOM) in aquatic ecosystems have increased over the past decades due to various factors, such as land use intensification, changes in climate, hydrology, and atmospheric deposition, or higher tDOM solubility associated to lower sulfate deposition in Northern Hemisphere. Predicted increases in runoff of tDOM and global changes (e.g., temperature increase) will have potentially important impacts on energy fluxes and food-web efficiency of aquatic ecosystems. Moreover, DOM in aquatic ecosystems could provide an important feedback mechanism in the climate system, because minor changes in the DOM pool would considerably impact atmospheric CO2 and the radiation balance on earth. Yet the cycling of DOM in the aquatic ecosystems is not well understood. A shift of aquatic ecosystems to a more heterotrophic state is expected by the degradation of tDOM and the brownification effect (light shielding). However, the interactive effects with temperature, the possibility of a priming effect, the resilience of the system and the consequences on ecosystem services are largely unknown.

RESTORE brings together specialists in general ecology, oceanography and limnology, microbiology, modelling and management of experimental infrastructures. For the first time, the responses of freshwater and marine ecosystems to separate or combined disturbances (inputs of tDOM and temperature) will be compared using a unique combination of experimental approaches.

Mesocosm experiments (3-12 weeks) will be performed using the facilities of the CEREEP-Ecotron IDF and MEDIMEER platforms to study the responses of freshwater and marine ecosystems, respectively. Additionally, microcosms experiments (batch cultures and chemostats) will be performed with both marine and freshwater to study more specifically the role of priming effect using stable isotope labelled 13C-lignin. Microbial responses will be measured in term of functionalities, composition and trophic transfert using a complete set of parameters including cutting-edge technologies (e.g., metabarcoding, metagenomic, isotope tracing, ultrahigh-resolution Fourier transform ion cyclotron mass spectrometry [FTICR MS] to characterize DOM). Models will be developed and used to implement virtual experiences to test hypotheses that have so far never been tested experimentally and that go beyond the experimental possibilities of the project (for technical and financial reasons).

Results obtained will allow us to (1) describe the modification in structure and functionalities of aquatic microbial communities after tDOM introduction in combination to an upshift in temperatures; (2) decipher the effect of priming on the fate of tDOM in different environmental contexts with the identification of the microbial actors and the degrading enzymes ; and (3) identify the changes in the microbial co-occurrence network and model these changes in freshwater and marine water ecosystems. Our results could be used to develop the formulation of policy associated with freshwater and marine water management in the EU to conserve aquatic resources impacted by global change.