Thesis of Maïwenn Herlédan

Defense of thesis of Maïwenn Herlédan - laboratory LOG

Abstract :

The objective of this PhD is to describe the current diversity of testate amoebae on the Kerguelen archipelago depending on climate and environmental context and to understand the recent evolution of the last 260 years by a paleoenvironmental approach combining microscopic observations, sedimentology and geochemistry. Located at the polar front in the sub-Antarctic zone, the Kerguelen archipelago is subject to extreme wind and precipitation conditions while being preserved from direct anthropic impacts. This makes it an ideal natural environment to study the impact of global climate change on the environment.
Two scientific campaigns on the Kerguelen archipelago in 2019 and 2021 have allowed us to collect samples of current plants (mainly bryophytes and azorelles), underlying soils and soil cores. The samples studied were selected to reflect the geographical and geological diversity of the archipelago, in terms of rainfall (west-east contrast), sedimentary nature (organic soil, alluvial sediments, etc.. ) and petrographic context (basalts - phonolites). Optical observations of the testate amoebae communities growing in the plant samples (101 samples) have allowed us to build a new database of testate amoebae in the archipelago. Sedimentological and geochemical analyses (water content, elemental and isotopic analyses, mineralogy) were carried out in parallel on 485 soil samples in order to characterize the environmental parameters.
This work has allowed to update and complete the 1981 database of 50 species with the description of 107 species of testate amoebae. Sedimentological analyses of soils revealed significant differences in terms of weathering between the western and eastern parts of the archipelago, reflecting contrasting rainfall patterns. This trend is also reflected in the testate amoebae assemblages whose diversity, variable according to the type of environment, is much greater in the west. The coupling of these different observations has made it possible to highlight the ecological affinities of certain species with specific environments, which depend mainly on the water content of the sediment, the type of environment and the vegetation, thus allowing the definition of indicator species. The use of this approach to reconstruct recent paleoenvironments from the different soil cores has allowed us to highlight local changes in the environment, which could be attributable to recent climate change. This study demonstrates the relevance of testate amoebae not only as an environmental indicator but also for tracing the recent evolution of paleoenvironmental conditions under climatic constraints.

Keywords : Climate change,Testate amoebae,Kerguelen,Subantarctic islands,Biodiversity,Paleoenvironment