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Predominant role of purple bacteria in stromatolite formation in Lake Dziani Dzaha, Mayotte

Using a combination of micro-imaging and microbial ecology techniques, a multi-disciplinary team (1) has highlighted the predominant role played by phototrophic alphaproteobacteria in the formation of stromatolites in Lake Dziani Dzaha, Mayotte. The precipitation of calcium carbonates in these stromatolites seems to be essentially linked to anoxygenic photosynthesis, opening up new perspectives for the analysis of the fossil record.

Predominant role of purple bacteria in stromatolite formation in Lake Dziani Dzaha, Mayotte

Publication date: 13/07/2018

Press, Research

Related themes : Earth System Science

Figure 1: Partially emerged stromatolites in Lake Dziani Dzaha, Mayotte. The green color of the water is linked to the presence of cyanobacteria, while the purple color is linked to the presence of phototrophic purple bacteria.

Stromatolites are laminated organosedimentary formations, i.e. a “stacking” of layers of organic and sedimentary matter, most often composed of carbonates, developed in shallow aquatic environments. They were very abundant in the Precambrian, but are much rarer today, their presence being restricted to a few lacustrine or marine environments. Fossil stromatolites are among the oldest traces of life on Earth. The oldest have been dated at around 3.5 billion years in the Pilbara region of Australia and Barberton in South Africa, and 3.7 billion years in the Isua region of Greenland.

The analysis of modern stromatolites has generally led to the hypothesis that fossil stromatolites were formed by the mineralization and lithification of microbial mats dominated by cyanobacteria. It was therefore assumed that cyanobacteria appeared very early in the history of life, and that oxygenic photosynthesis was a very ancient process already active at least 2.98 billion years ago. However, the absence of cyanobacterial microfossils associated with the oldest fossil stromatolites has led to the emergence of alternative hypotheses for stromatolite formation. Given that the Earth’s surface was predominantly anoxic during the Archean period (4 to 2.5 billion years ago), it has been proposed that the oldest stromatolites were formed by phototrophic, oxygen-deficient microorganisms. Molecular evidence suggests that anoxygenic photosynthesis predated oxygenic photosynthesis. However, although it has been shown with cultured purple bacteria that anoxygenic photosynthesis can promote calcium carbonate precipitation in the laboratory, the massive participation of anoxygenic phototrophic microorganisms in stromatolite formation had never been observed until now.

By combining confocal laser scanning microscopy, scanning electron microscopy, Raman spectroscopy, microbial diversity analysis using high-throughput sequencing of genes encoding bacterial and archaeal 16S RNA, and laser microdissection, the research team was able to show that photosynthetic purple bacteria were responsible for precipitating aragonite, the main calcium carbonate making up the stromatolites of Lake Dziani Dzaha in Mayotte. This observation therefore opens up the possibility that some stromatolites in the fossil record were formed by microbial communities dominated by anoxygenic phototrophic microorganisms.

This work was funded by the French National Research Agency (DZIANI project), the TOTAL Foundation and the Île-de-France region (SESAME project for financial support of the PARI high-resolution analytical platform).

Figure 2: A: stromatolite-associated biofilm from Lake Dziani Dzaha under confocal laser scanning microscopy, with the natural fluorescence of cyanobacteria belonging to the order Pleurocapsales in red and alphaproteobacteria belonging mainly to the Rhodobacteraceae family in green, stained with Syto9, a nucleic acid dye. B: scanning electron microscopy photo of a cross-section of the resin-embedded biofilm, showing a colony of Pleurocapsales in dark grey surrounded by aragonite-associated alphaproteobacteria in light grey (red star). The green star indicates the presence of hydromagnesite. The white arrows in both photos A and B highlight the presence of filamentous alphaproteobacteria, always positioned at the top of the biofilm towards the light.

(1) The following French institutions took part in this work:
UMR CNRS 7154 Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, Centre National de la Recherche Scientifique, Université Lyon 1, UMR CNRS 5557 / INRA 1418, UMR 7266 CNRS-Université de la Rochelle, Museum National d’Histoire Naturelle, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR 7590 CNRS Sorbonne Universités, Université Pierre et Marie Curie, Institut de Recherche pour le Développement UMR 206, UMR MARBEC, IRD, Ifremer, CNRS, Université de Montpellier, Sète, France

Réf : Gérard E, De Goeyse S, Hugoni M, Agogué H, Richard L, Milesi V, Guyot F, Lecourt L, Borensztajn S, Joseph MB, Leclerc T, Sarazin G, Jézéquel D, Leboulanger C, Ader M (2018) Key Role of Alphaproteobacteria and Cyanobacteria in the Formation of Stromatolites of Lake Dziani Dzaha (Mayotte, Western Indian Ocean). Frontiers in Microbiology, 9, 2018, DOI=10.3389/fmicb.2018.00796

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