Impact of the seismo-volcanic crisis on Mayotte’s unique thalassohaline crater lake, the Dziani Dzaha – SUBSILAKE

Since 2018, the underwater rise of a volcano 50 km east of the Mayotte archipelago has caused numerous earthquakes and increased the islands subsidence rate. The Dziani Dzaha, a thalassohaline volcanic crater lake is located on Petite-Terre, the second largest island of the archipelago. This lake has undergone seawater entries since the onset of the seismo volcanic crisis, probably as a because of the conjunction of its proximity to the ocean, and of the subsidence and bedrock fracturing.
The SUBSILAKE project consortium has been studied this lake from 2010 to 2017. The results they obtained showed that it was until 2017 exclusively microbial, and at its ecological climax given that the structure, diversity and main functions of its microbial communities were stable through time. The Dziani Dzaha was also found to present an original geochemistry, attesting of its isolation from seawater, and a carbon cycle dominated by photosynthesis and methanogenesis. These two processes are very active and contribute to the strong methane emissions from the lake to the atmosphere. Because the isotopic composition of methane is extremely negative (-65‰), its degassing from the lake induces a dissolved inorganic carbon isotopic signature highly positive (+13‰), which is rare on Earth today but was quite common in several Precambrian environments.
The central hypothesis of the SUBSILAKE project postulates that seawater entries will push the Dziani Dzaha out of its ecological climax thereby inducing various types of response of its microbial communities. In November 2020, during a field expedition dedicated to the magmatic gases emissions of the Dziani Dzaha, we documented a rise of the lake level of 40 cm and a salinity decrease from 65 to 55 psu. This seawater entry is a unique and unexpected opportunity to learn how a natural microbial ecosystem responds to drastic changes in its environment. Microbial communities and the functions they perform in ecosystems have been shown to respond quickly to environmental changes. Three categories of responses of the microbial ecosystem can be expected: resistance, resilience and alternative stable state.
The SUBSILAKE project proposes a multidisciplinary approach, interfacing geochemistry and microbial ecology, in order to document the ecosystem response to this perturbation. This will be done by detecting changes in the microbial biodiversity and the ecosystem functioning, but also by describing and anticipating the consequences of more profound modifications of the lake as a result of larger seawater entries. The chosen strategy to achieve this goal is to i) carry out on-site sampling and measurements to document possible changes and to enrich the knowledge base already acquired; ii) experimentally test in micro- and mesocosms the consequences of sulfate inputs from seawater, in particular the modifications induced in the structure of microbial communities and biogeochemical functioning; and iii) model the microbial interactions shaped by the environment, to predict their response to seawater entries.
The anticipate that the impacts of the SUBSILAKE project will be a (i) better knowledge of the original Dziani Dzaha ecosystem as such, (ii) an improved understanding of its biogeochemical functioning to strengthen its relevance as a current model of the Precambrian oceans, but also (iii) the production of predictive models of microbial response to an environmental perturbation through the joint implementation of field, experimental and modeling tools. Finally, (iv) social and societal repercussions are envisaged, whether in the field of biotechnologies, or in the appropriation of the Dziani Dzaha ecosystem as a teaching model in the high schools of Mayotte.