Abstract
This study describes, for the first time, the water chemistry and microbial diversity in Dziani Dzaha, a tropical crater lake located on Mayotte Island (Comoros archipelago, Western Indian Ocean). The lake water had a high level of dissolved matter and high alkalinity (10.6–14.5 g L-1 eq. CO32-, i.e. 160–220 mM compare to around 2–2.5 in seawater), with salinity up to 52 psu, 1.5 higher than seawater. Hierarchical clustering discriminated Dziani Dzaha water from other alkaline, saline lakes, highlighting its thalassohaline nature. The phytoplankton biomass was very high, with a total chlorophyll a concentration of 524 to 875 μg chl a L-1 depending on the survey, homogeneously distributed from surface to bottom (4 m). Throughout the whole water column the photosynthetic biomass was dominated (>97% of total biovolume) by the filamentous cyanobacteria Arthrospira sp. with a straight morphotype. In situ daily photosynthetic oxygen production ranged from 17.3 to 22.2 g O2 m-2 d-1, consistent with experimental production / irradiance measurements and modeling. Heterotrophic bacterioplankton was extremely abundant, with cell densities up to 1.5 108 cells mL-1 in the whole water column. Isolation and culture of 59 Eubacteria strains revealed the prevalence of alkaliphilic and halophilic organisms together with taxa unknown to date, based on 16S rRNA gene analysis. A single cloning-sequencing approach using archaeal 16S rDNA gene primers unveiled the presence of diverse extremophilic Euryarchaeota. The water chemistry of Dziani Dzaha Lake supports the hypothesis that it was derived from seawater and strongly modified by geological conditions and microbial activities that increased the alkalinity. Dziani Dzaha has a unique consortium of cyanobacteria, phytoplankton, heterotrophic Eubacteria and Archaea, with very few unicellular protozoa, that will deserve further deep analysis to unravel its uncommon diversity. A single taxon, belonging to the genus Arthrospira, was found responsible for almost all photosynthetic primary production.
Highlights
Saline inland waters often provide model ecosystems for studying microbial ecology, addressing numerous fundamental questions ranging from microbial diversity to the limits of life in extreme environments [1, 2]
The physical and chemical characteristics of Dziani Dzaha are significantly different from other tropical soda and saline lakes, mainly owing to its extremely high alkalinity and the relative similarity of the dissolved element composition to seawater [74]
Microbial life flourishes in this lake, and most of the carbon flow is driven by cyanobacterial photosynthetic activity
Summary
Saline inland waters often provide model ecosystems for studying microbial ecology, addressing numerous fundamental questions ranging from microbial diversity to the limits of life in extreme environments [1, 2]. Most saline lakes are endorheic inland water bodies and, as the dissolved mineral composition originates from the evaporation of continental water, are classified as athalassohaline environments [3], from the ancient Greek Θάλασσα (ocean) and άλαB (salt), with the α-privative prefix a- (not). These soda lakes are typically alkaline (pH between 9 and 11) and brackish to hypersaline. Marine salterns are typically host to eukaryotic microalgae such as Dunaliella salina, with larger, herbivorous metazoan organisms such as brine shrimps Artemia spp
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