Stromatolites in Crater-Lake Alchichica and Bacalar Lagoon

Extant stromatolites have been considered ecological similes to their ancient counterparts. We now know that these microbial assemblages are composed of a great diversity of microbes, which couple and intertwine their metabolic capabilities to create self-sustained microbial ecosystems. Presently, stromatolites thrive in a vast diversity of aquatic environments including freshwater, hypersaline, coastal lagoons, alkaline lakes, oligotrophic pools, abandoned pits, few marine systems, and brackish waters. In this chapter, we will summarize the research that has been done in two stromatolite-harboring sites in Mexico: the alkaline crater-lake Alchichica and the oligotrophic karstic coastal lagoon of Bacalar. Alchichica is located in the Transvolcanic belt in Central Mexico. It is a maar-alkaline crater lake (salinity 8.5 gl−1, pH 9.5) with water chemistry determined by high contents of carbonates, sodium, a high Mg/Ca ratio, and particularly low Ca2+ concentrations (~0.3 mM). Two main stromatolite-types, as defined by mineralogy, texture, and microbial composition, develop along its periphery from surface to over 30 m in depth. Alchichica is a modern environment that resembles Precambrian oceanic conditions. Stromatolites from Alchichica have been dated radiometrically in ~1.1–2.8 ka BP. Bacalar is a coastal lagoon located in the Yucatan Peninsula, which is a carbonate platform that emerged above sea level during the Oligocene. The Bacalar lagoon has high carbonate concentrations as a result of the influx of groundwater (salinity 1.2 gl−1, pH 7.6–8.3). It holds the largest freshwater stromatolite structures known, which have been radiometrically dated in ~6.8–9.2 ka BP. Their mineralogy, shape, and texture are similar along the lagoon’s coast, but microbial composition changes, possibly due to anthropogenic impact.