Thesis presented March 03, 2021

Abstract:
Algae populate nearly every type of environment, from freshwaters and oceans to soil, rock surfaces, ice, snow, etc. They are primary producers at the base of trophic networks, and can play a pioneering role in the conquest of new habitats. In temperate regions, mountain environments are characterized by a sharp gradient of decreasing temperatures, increasing light and UV exposition, and low water content in the soil or low nutrient content in the snow and ice. Green eukaryotic microalgae dominate freshwater and aeroterrestrial environments. Their biodiversity in the Alps is still mostly unknown, and the factors that drive it, poorly understood. Mechanisms of adaptation of green algae to the vast array of conditions in the Alps are barely explored.
To narrow the gap in our knowledge, we addressed the biodiversity of green algae in the French Alps. We focused on Chlorophyta and its main class, the Chlorophyceae. Two new markers were designed for a metabarcoding study of Chlorophyta and Chlorophyceae in soil, lakes and snow. The level of Chlorophyta DNA proved to be extremely low in soil samples, dominated by Trebouxiophyceae. Sampling of soil over several altitude gradients from 1,000 to 3,000 m, at five distinct locations in the French Alps, showed that altitude was not a major driver of green algae α-biodiversity, but was determinant along with pH, nitrogen and carbon/nitrogen soil content for the distribution at genus and species levels. Lakes had more balanced proportions and the highest diversity of algae overall, with more Chlorophyceae and Trebouxiophyceae than Ulvophyceaea.
Chlorophyceae constituted almost all snow algae at a selected site at 2,500 m altitude, including species of the Sanguina genus. Snow algae in the region of the Col du Lautaret were collected from 2017 to 2019, cultured in the laboratory, isolated and axenized. The fifteen strains isolated constitute a new Lautaret Culture Collection. The classical snow alga, Chlamydomonas nivalis, was not found in snow sites therefore a C. nivalis strain from the University of Texas collection was used as a reference. We made a preliminary analysis of physiological responses of multiple strains, giving insight for future comprehensive characterizations. The full genome of C. nivalis and three isolated strains were sequenced and revealed potential new species. This project confirmed the use of two new green algae markers, paves the way for more thorough aeroterrestrial algae studies and will allow in depth molecular biology of some selected snow algae strains in the future.

Keywords:
Molecular phylogeny, High-altitude ecosystems, glycolipids, biodiversity