Stromatolites and Oncolites from Desert Springs

The Cuatro Ciénegas Basin (Coahuila, Mexico) is a complex karstic system in which the underlying Cretaceous limestone, dolomites and gypsum formations are actively dissolved by an aquifer of distant origin. This results in the formation of innumerable springs, surface and underwater streams, caves and sinkholes pozas, which are famous for their beauty and the biological diversity they harbor. Within the frame of a large multidisplinary effort funded by NASA’s National Astrobiology Institute, scientist at ASU are looking at the food-web stocihiometry, biosignatures, grazer interactions, and microbial populations of these springs.

Cyanobacteria are often dominant primary producers in calcareous freshwater springs. In most cases, they occur as sessile, benthic or epiphytic dwellers, and are also typically associated with the precipitation of the microcrystalline calcite, that often results in the formation of macroscopic stromatolitic structures and rolling oncolites. These systems allow us to study the interactions between microbial metabolism and carbonate precipiation, in a manner that may help us understand present and past microbialites.

Jet-Suspended, Calcite Ballasted Waterwarts

While studying the Cuatro Ciénegas microbialites, we noted planktonic populations of marble-sized colonies of blue-green algae developing at Escobedo’s Warm Spring, a sheltered, small, fast-flowing spring. There, cm-sized waterwarts were kept in suspension by the upwelling waters of a central 6-m deep well. Waterwarts were built by an Aphanothecelike unicellular cyanobacterium and supported a community of epiphytes that included filamentous cyanobacteria and diatoms, but were free of heterotrophic bacteria on the inside. Waterwarts contained orderly arrangements of mineral crystallites, made up of microcrystalline low-magnesium calcite, with high levels of Strontium and Sulfur. An analysis of the hydrological properties of the spring well and the waterwarts demonstrated that both, large colony size and the presence of controlled amounts of mineral ballast, are required to prevent the population from being washed out of the well. The mechanisms by which controlled nucleation of extracellular calcite is achieved remain to be explored.