title: Metagenome-based diversity analyses suggest a significant contribution of non-cyanobacterial lineages to carbonate precipitation in modern microbialites.
abstract: Cyanobacteria are thought to play a key role in carbonate formation due to their metabolic activity, but other organisms carrying out oxygenic photosynthesis (photosynthetic eukaryotes) or other metabolisms (e.g., anoxygenic photosynthesis, sulfate reduction), may also contribute to carbonate formation. To obtain more quantitative information than that provided by more classical PCR-dependent methods, we studied the microbial diversity of microbialites from the Alchichica crater lake (Mexico) by mining for 16S/18S rRNA genes in metagenomes obtained by direct sequencing of environmental DNA. We studied samples collected at the Western (AL-W) and Northern (AL-N) shores of the lake and, at the latter site, along a depth gradient (1, 5, 10, and 15 m depth). The associated microbial communities were mainly composed of bacteria, most of which seemed heterotrophic, whereas archaea were negligible. Eukaryotes composed a relatively minor fraction dominated by photosynthetic lineages, diatoms in AL-W, influenced by Si-rich seepage waters, and green algae in AL-N samples. Members of the Gammaproteobacteria and Alphaproteobacteria classes of Proteobacteria, Cyanobacteria, and Bacteroidetes were the most abundant bacterial taxa, followed by Planctomycetes, Deltaproteobacteria (Proteobacteria), Verrucomicrobia, Actinobacteria, Firmicutes, and Chloroflexi. Community composition varied among sites and with depth. Although cyanobacteria were the most important bacterial group contributing to the carbonate precipitation potential, photosynthetic eukaryotes, anoxygenic photosynthesizers and sulfate reducers were also very abundant. Cyanobacteria affiliated to Pleurocapsales largely increased with depth. Scanning electron microscopy (SEM) observations showed considerable areas of aragonite-encrusted Pleurocapsa-like cyanobacteria at microscale. Multivariate statistical analyses showed a strong positive correlation of Pleurocapsales and Chroococcales with aragonite formation at macroscale, and suggest a potential causal link. Despite the previous identification of intracellularly calcifying cyanobacteria in Alchichica microbialites, most carbonate precipitation seems extracellular in this system.
authors: Unité d'Ecologie, Systématique et Evolution, CNRS UMR 8079, Université Paris-Sud Orsay, France.
authors: Aurélien
authors: A
authors: Saghaï
authors: Institut de Génétique et Microbiologie, CNRS UMR 8621, Université Paris-Sud Orsay, France.
authors: Yvan
authors: Y
authors: Zivanovic
authors: Institut de Minéralogie et de Physique des Matériaux et de Cosmochimie, CNRS UMR 7590, Université Pierre et Marie Curie Paris, France.
authors: Nina
authors: N
authors: Zeyen
authors: Unité d'Ecologie, Systématique et Evolution, CNRS UMR 8079, Université Paris-Sud Orsay, France.
authors: David
authors: D
authors: Moreira
authors: Institut de Minéralogie et de Physique des Matériaux et de Cosmochimie, CNRS UMR 7590, Université Pierre et Marie Curie Paris, France.
authors: Karim
authors: K
authors: Benzerara
authors: Unité d'Ecologie, Systématique et Evolution, CNRS UMR 8079, Université Paris-Sud Orsay, France.
authors: Philippe
authors: P
authors: Deschamps
authors: Unité d'Ecologie, Systématique et Evolution, CNRS UMR 8079, Université Paris-Sud Orsay, France.
authors: Paola
authors: P
authors: Bertolino
authors: Unité d'Ecologie, Systématique et Evolution, CNRS UMR 8079, Université Paris-Sud Orsay, France.
authors: Marie
authors: M
authors: Ragon
authors: Departamento de Ecología y Recursos Naturales, Universidad Nacional Autónoma de México Mexico City, Mexico.
authors: Rosaluz
authors: R
authors: Tavera
authors: Departamento de Ecología, Universidad Autónoma de Madrid Madrid, Spain.
authors: Ana I
authors: AI
authors: López-Archilla
authors: Unité d'Ecologie, Systématique et Evolution, CNRS UMR 8079, Université Paris-Sud Orsay, France.
authors: Purificación
authors: P
authors: López-García
keywords: anoxygenic photosynthesis; biomineralization; cyanobacteria; diatoms; green algae; metagenomics; stromatolite; sulfate-reduction