Cryoconite holes are vertical columns of meltwater within the shallow subsurface of glaciers. In the McMurdo Dry Valleys (MDV) of Antarctica cryoconite holes are a source of meltwater and harbor microbial communities in an otherwise arid environment with low biologic activity. The holes form as sediments on the ice surface, which are darker than the surrounding ice, are preferentially heated by solar radiation. The warm sediments melt the underlying ice and migrate downwards. An ice lid forms, isolating them from the below-freezing atmosphere enabling them to remain thawed. In this study, field observations, laboratory experiments, and numerical modeling are used to characterize the fundamental variables controlling cryoconite hole development. Field and laboratory results show that solar radiation drives cryoconite hole melting by controlling the energy available to the cryoconite and to warm the surrounding ice. Holes deepen further in warmer ice. Laboratory results show that at temperatures of -10º C at least 405 (W m-2) are needed to warm the cryoconite sufficiently to melt surrounding ice. Numerical modeling shows that increased radiation flux into the subsurface and warmer air temperatures cause cryoconite to descend deeper and the meltwater-filled holes to enlarge, while increased surface ablation decreases their average depth. Cryoconite holes thaw sooner and refreeze later when the optical properties of the ice facilitate greater radiation transmission. Cryoconite warms the ice significantly more than ice without cryoconite. Within the melt-filled hole, the heat capacity of the water keeps the surrounding ice warm for several weeks after the cryoconite-free ice has cooled. The cryoconite itself is last to completely freeze.

This thesis compares the habitat and community ecology of cylindrical meltholes from the surface of two polar glaciers. These holes (termed cryoconite holes) are formed when wind-blown dust gathers in small depressions in the ice causing vertical melting by absorption of more radiation than the surrounding ice. The communities are complex microbial consortia of heterotrophic bacteria, cyanobacteria, eukaryotic algae, and protists. Samples were taken from cryoconite holes on Canada Glacier, Taylor Valley, Antarctica (77°37'S, 162°55'E) and on White Glacier, Axel Heiberg Island, Nunavut Territory, Canada (79°27'N, 90°40'W). Water from Canada Glacier cryoconite holes contained significantly higher concentrations of nutrients and had higher pH values and conductivities, relative to the White Glacier meltwater. Cryoconite communities on the Canada Glacier were dominated by cyanobacteria, either coccoid or filamentous, while the White Glacier cryoconite holes showed an abundance of either saccoderm desmids or filamentous cyanobacteria. Canada Glacier communities were found to be associated with environmental gradients whereas White Glacier cryoconite ecosystems were not.

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Titre de l'écran-titre (visionné le 9 avril 2024) / Recouvrant la glace surélevée de Ward Hunt (traduction libre de Ward Hunt Ice Rise, WHIR) (Nunavut, Canada), des trous reconnus comme étant des points chauds de diversité microbienne avec des taux d'infection virale très élevés y sont retrouvés. La WHIR est actuellement stable, mais elle fait face aux conséquences des changements climatiques drastiques. Ce milieu naturel est éloigné des activités anthropiques et peut servir de référence avant d'être irréversiblement impacté. De plus, les communautés microbiennes des cryoconites ont su développer des gènes de résistance aux antibiotiques (GRA) loin de l'influence anthropique. Ainsi, les objectifs seront de caractériser la diversité et l'abondance virale et bactérienne dans l'eau de fonte et les sédiments des cryoconites. De plus, la présence de GRA, associés ou non aux virus sera identifiée. Les hypothèses sont les suivantes ; i) que les sédiments présentent une richesse relativement élevée de taxons viraux et bactériens par rapport à l'eau de fonte ; ii) que l'eau de fonte et les sédiments hébergeront des taxons spécifiques, entraînant un indice de dissimilarité élevé ; et iii) que les sédiments agiront comme un réservoir naturel de multiples GRA et que les virus joueront un rôle dans la dissémination de ces gènes. Une analyse par métagénomique a permis de conclure que l'eau de fonte présente une diversité microbienne plus élevée en comparaison avec les sédiments et que huit familles de GRA ont été retrouvées dans les sédiments de cryoconites, mais aucun GRA n'a été associé aux virus. Cette étude apporte donc de nouvelles données sur la diversité microbienne et recense la présence de GRA des cryoconites de l'Arctique canadien situés sur la WHIR. / Covering the Ward Hunt Ice Rise (Nunavut, Canada), meltwater-filled holes recognized as hotspots of microbial and viral diversity are present. While the Ward Hunt Ice Rise (WHIR) is currently stable, it is experiencing drastic climatic changes. Consequently, this pristine and remote environment can serve as a reference point for the future impacts of climate change. Moreover, cryoconite communities may support antimicrobial resistance gene (ARG) profiles, which have developed in isolation from anthropogenic antimicrobial pollution. Identifying environmentally intrinsic ARGs could serve as a comparative baseline to future community change. The objectives of this work were to characterize viral and bacterial diversity and abundance in meltwater and sediments. Additionally, the presence of ARGs within cryoconites was assessed, as was their association or lack thereof, with viral genomes. The hypotheses are: i) that sediments exhibit a relatively high richness of viral and bacterial taxa compared to meltwater; ii) that meltwater and sediments potentially harbor specific taxa, leading to a high dissimilarity index; and iii) it is hypothesized that sediments will act as a natural reservoir for multiple ARGs, with viruses playing a role in the dissemination of these genes. Metagenomic analyses revealed that meltwater represents the highest microbial diversity compared to sediments. Eight families of ARGs were identified in cryoconite sediments, but none were associated with viruses. This study provides new insights into microbial diversity and documents the presence of ARGs from Canadian Arctic cryoconites in the LIA.