Patrons saisonniers de transformation du carbone et efficacité métabolique des communautés bactériennes du golfe d’Amundsen, Arctique canadien

Arctic ecosystems are undergoing rapid changes, primarily due to unprecedented climatic warming as a function of anthropogenic activities, which threaten their short-term stability. One of the most dramatic impacts has been the loss and change in annual sea ice. Understanding and predicting how these systems will evolve is crucial, especially if considering how carbon (C) fluxes from these ecosystems – either net sinks or net CO2 sources for the atmosphere – could have important repercussions on global climate. The objective of this thesis is to establish a seasonal portrait of bacterial activity to characterize its contribution to Arctic carbon fluxes. Specifically, we quantify for the first time microbial respiration in sea-ice and the water column and explore the use of photoheterotrophy by microorganism over an annual cycle in the Amundsen Gulf of the Arctic Ocean. These components of carbon cycling remain poorly understood and infrequently directly measured. As a consequence they are either extrapolated or omitted from models, despite their significant role in C dynamics not only in the Arctic, but also in marine systems in general. First, we characterise respiration in sea-ice microbial communities (CR). An understanding of respiration rates is essential for accurate estimation of C fluxes, but the role of respiration in sea ice is poorly understood. This work represents the first comprehensive evaluation of respiration in polar sea ice to date.  Using novel O2 consumption measurements in sea-ice, we found high respiration rates in sea-ice, 2 to 3 times higher than in the water column and bacterial production rates up to 25 times higher. These results show that microbial respiration can consume a significant portion of sea ice primary production (PP) and play a key role in biogenic CO2 fluxes between sea-ice and the atmosphere (Nguyen and Maranger, 2011). Second, we measure respiration of pelagic microbial communities of Amundsen Gulf over an eight-month period, including under the winter ice-cover. By measuring directly O2 consumption, we show high CR, measurable over the whole year and greatly surpassing C inputs from PP. Globally, high C consumption by microbial communities supports a high reliance on internal recycling of local PP. These observations have important consequences on our understanding of the CO2 sequestering potential of the Arctic Ocean (Nguyen et al., 2012) Finally, we describe the seasonal patterns in presence (DNA) and expression (RNA) of the proteorhodospin (PR) gene, involved in bacterial photoheterotrophy. The PR gene, combined with the retinal chromophore, allows bacteria to capture energy from light towards energetic or sensory purposes. This additional energy source could contribute to the survival and proliferation of bacterial communities expressing the gene in the highly variable polar environment. Although PR has been found in many oceans, this study represents a unique time-series that follows the seasonal distribution and expression of the gene in a natural marine system. We show that the PR gene was present over the whole study period and widely distributed in diverse bacterial communities. Surprisingly, we observed continued PR expression over winter, in the absence of sunlight. This suggests either that the PR’s expression does not depend on light or, that other very localized photon sources could justify PR expression for detection and sensory functions (Nguyen et al., submitted to the ISME journal). This thesis contributes to the understanding of Arctic carbon cycling and includes several novel elements such as the characterization of respiration and bacteria growth efficiency in both pelagic and sea-ice habitats. The use of an alternative C pathway by bacteria in the Polar ocean was also explored for the first-time in a time-series. The observed sustained expression of the PR gene in the Arctic could modulate C consumption by respiration and justify its inclusion in future models of C cycling. In a context of climate change, it is clear that bacterial activity has been underestimated and how this will change in a warmer Arctic will have a significant impact in the ecosystem’s overall C budget.

Keyphrases: respiration, Arctic ocean, Sea-ice, biogeochemistry, Carbon cycling, seasonal patterns, bacterial production, growth efficiency, functional diversity, proteorhodopsin