Origine, composition et destinée de la matière organique dissoute et ses interactions avec les communautés de procaryotes dans la mer du Labrador

Oceanic prokaryotes are key players in the carbon cycle by consuming dissolved organic mat-ter (DOM) produced by primary producers. As this organic matter is highly complex with varying degree of bioavailability, prokaryotic communities are highly diverse and diﬀerent taxa target certain types of organic compounds. By consuming this organic matter, prokary-otes reintroduce this carbon into the food web, a critical energy ﬂow in oligotrophic gyres. However, this consumption is not perfect and they release a lot of carbon as CO2 through respiration, but also as recalcitrant DOM. Thus, they contribute to carbon sequestration in aquatic ecosystems. The objective of this thesis is to characterize DOM bioavailability and its inﬂuence on the composition and metabolism of prokaryotic communities in the Labrador Sea, described as one of the Earth’s climate system tipping elements. More precisely, we quantify for the ﬁrst time how the spatial abundance distribution of prokaryotes inﬂuences ecosystem metabolism and organic matter association in the surface waters of the Labrador Sea. Lastly, we look at how DOM produced at the surface is transformed and sequestered following the Labrador Sea winter convective mixing. The oceanic carbon budget is still unbalanced. In order to better understand its carbon sources and bioavailability, we characterize DOM bioavailability across the aquatic contin-uum, from lakes to the open ocean. Using a meta-analysis, our results show that the propor-tion of labile organic matter, i.e. readily available for prokaryotes, is similar at around 6% in all aquatic ecosystems. However, the proportion of semi-labile organic matter, i.e requiring transformations to be consumed by prokaryotes, is highly related to terrestrial connectivity. The only ecosystems that did not follow these patterns were in a phytoplankton bloom pe-riod and had a high proportion of labile and semi-labile organic matter as their counterparts at equilibrium. Finally, we estimated that semi-labile organic matter could sustain 62% of prokaryotic biomass in lakes and coastal zones. Second, we evaluated the inﬂuence of DOM on prokaryotic metabolism and community composition. In order to determine organic matter composition, prokaryotic community composition and metabolic rates, we did three oceanic cruises in the Labrador Sea onboard the Hudson ship. By using spatial abundance distribution modelling of prokaryotes, we identiﬁed strong associations between groups of this novel approach and organic matter composition. We also proposed a framework to bridge the gap between prokaryotic diversity, microbial ecology, and biogeochemistry among methods and across scales. Lastly, we compared how prokaryotic communities from diﬀerent oceanic strata could se-quester carbon. When they consume organic matter, prokaryotes release a small amount in recalcitrant forms. Through this iterative process, called the microbial carbon pump, prokaryotes contribute to carbon sequestration by creating highly recalcitrant compounds that resist further degradation for hundreds of years. We have shown that all prokaryotes enable the microbial carbon pump, but that prokaryotes from deeper strata are more eﬃ-cient. Our results also conclusively show that the rare prokaryotic taxa are key players in the microbial carbon pump. This thesis contributes to better understand the carbon cycle in the Labrador Sea and in all aquatic ecosystems. We proposed a novel framework to relate biogeochemistry, prokaryotic diversity and microbial ecology which has been a challenge for decades. Moreover, we con-clusively showed for the ﬁrst time that the iterative process of the microbial carbon pump is related to prokaryotic succession. We also show that it happens in all oceanic strata, but that rare prokaryotes from the deep ocean are more eﬃcient to sequester carbon. Better understanding how DOM composition inﬂuences prokaryotes is of prime importance as they are the main drivers of the oceanic carbon cycle.

Keyphrases: Carbon cycle, Dissolved organic matter, Bioavailability, Prokaryotic communities, Labrador Sea, Aquatic ecosystems, Microbial carbon pump, Microbial diversity, Metabolism