Bernard, C., Escalas, A., Villeriot, N., Agogué, H., Hugoni, M., Duval, C., et al. (2019). Very Low Phytoplankton Diversity in a Tropical Saline-Alkaline Lake, with Co-dominance of Arthrospira fusiformis (Cyanobacteria) and Picocystis salinarum (Chlorophyta). Microb Ecol, 78(3), 603–617.
Résumé: Lake Dziani Dzaha (Mayotte Island, Indian Ocean) is a tropical thalassohaline lake which geochemical and biological conditions make it a unique aquatic ecosystem considered as a modern analogue of Precambrian environments. In the present study, we focused on the diversity of phytoplanktonic communities, which produce very high and stable biomass (mean2014–2015 = 652 ± 179 μg chlorophyll a L−1). As predicted by classical community ecology paradigms, and as observed in similar environments, a single species is expected to dominate the phytoplanktonic communities. To test this hypothesis, we sampled water column in the deepest part of the lake (18 m) during rainy and dry seasons for two consecutive years. Phytoplanktonic communities were characterized using a combination of metagenomic, microscopy-based and flow cytometry approaches, and we used statistical modeling to identify the environmental factors determining the abundance of dominant organisms. As hypothesized, the overall diversity of the phytoplanktonic communities was very low (15 OTUs), but we observed a co-dominance of two, and not only one, OTUs, viz., Arthrospira fusiformis (Cyanobacteria) and Picocystis salinarum (Chlorophyta). We observed a decrease in the abundance of these co-dominant taxa along the depth profile and identified the adverse environmental factors driving this decline. The functional traits measured on isolated strains of these two taxa (i.e., size, pigment composition, and concentration) are then compared and discussed to explain their capacity to cope with the extreme environmental conditions encountered in the aphotic, anoxic, and sulfidic layers of the water column of Lake Dziani Dzaha.
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Hugoni, M., Escalas, A., Bernard, C., Nicolas, S., Jézéquel, D., Vazzoler, F., et al. (2018). Spatiotemporal variations in microbial diversity across the three domains of life in a tropical thalassohaline lake (Dziani Dzaha, Mayotte Island). Molecular Ecology, 27(23), 4775–4786.
Résumé: Thalassohaline ecosystems are hypersaline environments originating from seawater in which sodium chloride is the most abundant salt and the pH is alkaline. Studies focusing on microbial diversity in thalassohaline lakes are still scarce compared with those on athalassohaline lakes such as soda lakes that have no marine origin. In this work, we investigated multiple facets of bacterial, archaeal and eukaryotic diversity in the thalassohaline Lake Dziani Dzaha using a metabarcoding approach. We showed that bacterial and archaeal diversity were mainly affected by contrasting physicochemical conditions retrieved at different depths. While photosynthetic microorganisms were dominant in surface layers, chemotrophic phyla (Firmicutes or Bacteroidetes) and archaeal methanogens dominated deeper layers. In contrast, eukaryotic diversity was constant regardless of depth and was affected by seasonality. A detailed focus on eukaryotic communities showed that this constant diversity profile was the consequence of the high predominance of Picocystis salinarum, while nondominant eukaryotic groups displayed seasonal diversity turnover. Altogether, our results provided an extensive description of the diversity of the three domains of life in an unexplored extreme environment and showed clear differences in the responses of prokaryotic and eukaryotic communities to environmental conditions.
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