Microbialites have played an important role in the early history of life on Earth. Their fossilized forms represent the oldest evidence of life on our planet dating back to 3500 Ma. Extant microbialites have been suggested to be highly productive and diverse communities with an evident role in the cycling of major elements, and in contributing to carbonate precipitation. Although their ecological and evolutionary importance has been recognized, the study of their genetic diversity is yet scanty. The main goal of this study was to analyse microbial genetic diversity of microbialites living in different types of environments throughout Mexico, including desert ponds, coastal lagoons and a crater‐lake. We followed a pyrosequencing approach of hypervariable regions of the 16S rRNA gene. Results showed that microbialite communities were very diverse (H′ = 6–7) and showed geographic variation in composition, as well as an environmental effect related to pH and conductivity, which together explained 33% of the genetic variation. All microbialites had similar proportions of major bacterial and archaeal phyla.

The present study describes patterns and rates of N 2 fixation and associated diversity (based on the nifH gene) of microbialite and mat-forming bacterial consortia from different aquatic environments in Mexico. All of the communities shared a diurnal pattern of N 2 fixation with peak activity during the day while showing a unique genetic composition. Our study suggests the importance of heterocystous cyanobacteria as the main diazotrophic organisms, although nifH sequences were also affiliated to Oscillatoriales, Chroococcales, and several Pro-teobacteria. Genetic composition did not relate to the environmental variables analysed, although rates of nitrogenase activity associated to microbial C and N explained 11% of the variation among consortia. KEY WORDS: N 2 fixation · Microbialites · Microbial mats · nifH · Diversity · Community ecology Resale or republication not permitted without written consent of the publisher Aquat Microb Ecol 67: 15-24, 2012 Editorial responsibility: Douglas Capone,

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