Influence of aphotic haloclines and euxinia on organic biomarkers and microbial communities in a thalassohaline and alkaline volcanic crater lake

Abstract

Studies on microbial communities, and their associated organic biomarkers, that are found thriving in the aphotic euxinic waters in modern stratified ecosystems are scarce compared to those undertaken in euxinic photic zones. The Dziani Dzaha (Mayotte, Indian Ocean) is a tropical, saline, alkaline crater lake that has recently been presented as a modern analogue of Proterozoic Oceans due to its thalassohaline classification (having water of marine origin) and specific biogeochemical characteristics. Continuous intense photosynthetic production and microbial mineralization keeps most of the water column permanently aphotic and anoxic preventing the development of an euxinic (sulfidic and anoxic) photic zone despite a high sulfide/sulfate ratio and the presence of permanent or seasonal haloclines. In this study, the molecular composition of the organic matter in Lake Dziani Dzaha was investigated and compared to the microbial diversity evaluated through 16S rRNA gene amplicon sequencing, over two contrasting seasons (rainy vs. dry) that influence water column stratification. Depth profiles of organic biomarker concentrations (chlorophyll-a and lipid biomarkers) and bacterial and archaeal OTU abundances appeared to be strongly dependent on the presence of aphotic haloclines and euxinia.OTU abundances revealed the importance of specific haloalkaliphilic bacterial and archaeal assemblages in phytoplanktonic biomass recycling and the biogeochemical functioning of the lake, suggesting new haloalkaline non-phototrophic anaerobic microbial precursors for some of the lipid biomarkers. Uncultured Firmicutes from the family Syntrophomonadaceae (Clostridiales), and Bacteroidetes from the ML635J-40 aquatic group, emerged as abundant chemotrophic bacterial members in the anoxic or euxinic waters, and were probably responsible for the production of short-chain n-alkenes, wax esters, diplopterol and tetrahymanol. Halocline-dependent euxinia also had a strong impact on the archaeal community which was dominated by Woesearchaeota in the sulfide-free waters. In the euxinic waters, methanogenic Euryarchaeota from the Methanomicrobia, Thermoplasmata and WSA2 classes dominated and were likely at the origin of common hydrocarbon biomarkers of methanogens (phytane, pentamethyleicosenes, and partially hydrogenated squalene).

Two modes of gypsum replacement by carbonate and native sulfur in the Lorca Basin, SE Spain

“…Organic sulfur compounds provide further insight into the original sedimentary input present during bacterial sulfate reduction, as reactive organic compounds are otherwise easily degraded. Sulfurization under euxinic conditions in the water column would probably have preserved compounds specific of anoxygenic phototrophic bacteria or organisms living in stratified water bodies, such as isorenieratane (Sinninghe Damsté et al, 1995;Wakeham et al, 2012;Sabino et al, 2021) and tetrahymanol or its degradation product gammacerane (Repeta et al, 1989;Van Gemerden and Mas, 1995;Wakeham et al, 2007;Sala et al, 2021), respectively. Both lipids were not observed in the bitumen and the desulfurized asphaltenes, arguing against water column euxinia and stratification at the time of deposition.…”

Brine: Genesis and Sustainable Resource Recovery Worldwide

“…In arid regions, brine can be formed in closed basins where the rate of water evaporation exceeds the rate of inflow. Naturally closed basins usually experience a variety of geotectonic processes, such as continental rifting, volcanism, and block thrusting (59)(60)(61). Three major types of closed basins are commonly reported to contain brines: (a) ephemeral saline lake basins, such as Death Valley in California; (b) large perennial basins, such as the Dead Sea; and (c) shallow perennial basins, such as Great Salt Lake in Utah (62,63).…”

Impact of the seismo-volcanic crisis offshore Mayotte on the Dziani Dzaha Lake

“…On the Petite Terre Island in Mayotte, one of the latest eruptive events proposed at around 7 and 4 ky is probably at the origin of the Dziani Dzaha Lake formation [Zinke et al, 2003]. For the last decade, this atypical lacustrine system has been extensively studied [Leboulanger et al, 2017, Cellamare et al, 2018, Gérard et al, 2018, Milesi et al, 2019, Aucher et al, 2020, Jovovic et al, 2020, Sala et al, 2022. Based on its water chemistry, the lake was most likely initially filled with seawater [Sarazin et al, 2021], and has since evolved as a closed system.…”