Precambrian sedimentary record shows a carbon isotopic anomaly, with a positive excursion of extreme intensity observed for δ13Ccarbonates (+8‰ to +14‰) over 200Ma. Although this excursion is the largest in intensity and duration, it remains poorly understood and differently interpreted, in terms of overall perturbation of the carbon cycle or local changes associated with methanogenesis.
To better understand these isotopic signatures, one of the research possible is to study a current analogue of these paleoenvironments. The Dziani Dzaha Lake located in Mayotte was recently identified as a potential analogue of Precambrian oceans based on a combination of analogies never reported yet to our knowledge:
- A reduced trophic to primary producers and prokaryotic mineralizers
- A permanent anoxia below 1.5 meter from the surface
- A significant degradation of organic matter by methanotrophic ways
- High values of δ13Ccarbonates of 10‰ to 15‰
The characterization of this system could provide some crucial information on the operation of past oceans. This PhD thesis is integrated into a larger project (ANR Dziani) which aims to perform a biogeochemical and microbiological integrated study of this unique ecosystem, and this thesis work will aim to highlight the relationship that exists between atypical isotopic values of δ13Ccarbonates in the lake and the activity of various metabolisms of microorganisms that are present. This work will test the assumption that methanogenesis can explain the very positive values of δ13C Precambrian and include two main tasks: (i) to determine the variability of carbon and nitrogen isotopic compositions in the biomass of water column and sediments, (ii) to perform the experimental determination of the isotopic fractionation associated with photosynthesis, nitrogen assimilation and metabolism of methanogenesis, from cultures under controlled conditions of microorganisms sampled in the lake.