Microbialites are organosedimentary deposits formed by the interaction of benthic microbial communities with their environment (Burne & Moore, 1987) and provide the only continuous macroscopic record for life spanning its appearance in the Archean through to the present day (Riding, 2000). Biological, physical, and chemical processes combine to produce an internal structure characteristic of microbialites, which include laminated fabrics and clotted to unlayered fabrics (Kennard & James, 1986).

Molecular approaches [PCR-denaturing gradient gel electrophoresis (DGGE)] were used to determine whether three different vetiver (Chrysopogon zizanioides) genotypes, commercially used in Brazil and considered economically important over the world, select specific bacterial populations to coexist in their rhizospheres. DGGE profiles revealed that the predominant rhizospheric bacterial community hardly varies regarding the vetiver genotype. Moreover, using traditional cultivation methods, bacterial strains were isolated from the different rhizospheres. Colonies presenting different morphologies (83) were selected for determining their potential for plant growth promotion. More than half of the strains tested (57.8%) were amplified by PCR using nifH-based primers, specific for the enzyme nitrogenase reductase. The production of siderophores was observed in 88% of the strains, while the production of antimicrobial substances was detected in only 14.5% of the isolates when Micrococcus sp. was used as the indicator strain. Production of indole-3-acetic acid and the solubilization of phosphate were observed in 55.4% and 59% of the isolates, respectively. In total, 44 strains (53%) presented at least three characteristics of plant growth promotion and were submitted to amplified ribosomal DNA restriction analysis. Twenty-four genetic groups were formed at 100% similarity and one representative of each group was selected for their identification by partial 16S rRNA gene sequencing. They were affiliated with the genera Acinetobacter, Comamonas, Chryseobacterium, Klebsiella, Enterobacter, Pantoea, Dyella, Burkholderia, or Pseudomonas. These strains can be considered of great importance as possible biofertilizers in vetiver.

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Microbial mats are organosedimentary structures organized as multilayered carpets of microbial communities. Within the microbial mat microenvironment, the occurrence of different metabolic processes can lead to local chemistry alterations, inducing carbonate precipitation. Carbonate accretion in lithifying microbial mats typically induces microbialite formation, a poorly understood process, but studies have suggested that taxonomic composition of lithifying mats and their predominant metabolic pathways contribute. In contrast to lithifying mats, non‐lithifying mats, which do not form microbialites, can sporadically trap carbonate sand grains that are actively bound to the microbial mat through the production of extracellular polymeric substances. Both mat types occur in hypersaline lakes at Rottnest Island (Western Australia) and are currently under threat due to pollution and thus are being managed by the relevant government agency. Characterizing the microbial communities and functional genes of both mat types may help to develop strategies to better manage their ecosystem and elucidate microbialite formation processes. Metagenomics was used to compare the taxonomic and functional diversity of both mat types to determine whether differences in their taxonomy and functional capacity may influence their ability to form microbialites. Results revealed that both mat types harbor taxa (e.g., Firmicutes and Archaea), and functional genes (e.g., associated with photosynthesis, carbon, and sulfur cycles) that are known to play important roles in microbialite formation. This suggests that although non‐lithifying mats are not accreting carbonate, they have the potential to form microbialites. Further investigation is needed to determine whether environmental factors could be inhibiting carbonate precipitation within these mats.

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