Abstract
Little is known about the quantity and quality of dissolved organic matter (DOM) in waters from continental geothermal systems, with only a few reports available from the Yellowstone US National Park. In this study, we explored the chemodiversity of DOM in water samples collected from two geothermal hot springs from the Kenyan East African Rift Valley, a region extremely rich in fumaroles, geysers, and spouting springs, located in close proximity to volcanic lakes. The DOM characterization included in-depth assessments performed by negative electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Reduced, saturated and little aromatic DOM compounds were dominant in the hot spring waters collected from either the Ol Njorowa gorge (ON) or the south shore of the soda-saline Lake Elementaita (ELM). Oxygen-poor and sulfur-bearing DOM molecules prevailed in ON, probably reflecting abiotic sulfurization from sulfide-rich geofluids. Nitrogen-bearing aliphatic and protein-like molecules were abundant in ELM, possibly perfusing through the organic-rich sediments of the adjacent Lake Elementaita. Notably, the heat-altered DOM of ancient autochthonous derivation could represent an overlooked source of aliphatic organic carbon for connected lentic environments, with a potential direct impact on nutrient cycling in lakes that receive geothermal water inputs.
Highlights
Hydrothermal systems are considered active biogeochemical reactors with unique microbial communities and firm candidates for the origin of life [1]
The Dissolved organic carbon (DOC) concentrations estimated at ELM and Ol Njorowa gorge (ON) were within the same range of those reported from Yellowstone [28], suggesting that the two samples were not in contact with the lixiviates derived from terrestrial vegetation and soils, nor with the waters of Lake Elementaita that showed extremely high DOC concentrations [29]
Mass spectrometry evidenced that the SPE-dissolved organic matter (DOM) from ELM and ON was strongly reduced with an O/Cw ratio in the range of that reported for the Yellowstone hot springs (O/C < 0.4) [9], which was lower than the values typically reported for freshwaters (O/C > 0.45) [30,31]
Summary
Hydrothermal systems are considered active biogeochemical reactors with unique microbial communities and firm candidates for the origin of life [1]. Recent investigations of the carbon cycle in aquatic ecosystems have focused on the role of hydrothermal systems in the synthesis, processing and transformation of dissolved organic matter (DOM), the main carbon and energy source for the abundant heterotrophic fraction of the aquatic microbial community. DOM is a highly complex mixture of thousands of organic molecules which strictly interact with underwater light, selectively absorbing the radiance spectra and minerals through absorption-desorption processes [2,3]. In geothermal fluids under high temperatures, it was reported that oxygen-rich DOM molecules can be selectively removed with the loss of CO2 , carboxyl and hydroxyl groups [8]. Continental geothermal environments were overlooked and, to our best knowledge, a detailed DOM description is available only at Yellowstone Hot Springs [9]
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