Recent microbial mats in the Bano San Ignacio hot spring, Linares, Nuevo Leon

The Bano San Ignacio spring, located in Linares, Nuevo Leon, northeastern Mexico, represents a natural protected area that harbors not only endemic flora and fauna, but also a rich microbial life. This hydrothermal system is linked to the geotectonic evolution of underground continental waters trapped into a closed standard circuit. Microbial communities at Baoo San Ignacio (hereafter BSI), may be found either as gelatinous spheroidal mats, or as benthic stratiform mats associated with travertine terraces along the main channel. Microbial mats exhibit a marked fungal-like appearance and a well-defined macroscopic lamination. The textural diagnosis in BSI mats, characterized by optical microscopy and electronic scanning, shows that the extracellular matrix is organized as a three-dimensional network with abundant organic fibers where sediments may be trapped or bound. On the other hand, carbonate precipitation was observed only in small localized areas of the matrix and along some Phormidium-like filaments. The internal mat structure is organized as alternating layers of cyanobacteria, diatoms and variable-sized calcite. Although mat thickness is variable, the first centimeters of each mat correspond to Oscillatoriales belonging to the filamentous cyanobacteria, while pennate diatoms correspond to Amphora sp. and Nitzschia sp. As in many other hydrothermal springs mats, eukaryotic microorganisms are relatively scarce and many other bacteria have yet to be identified. At the microstructural level, diatoms may be playing a more significant role in mat structure and stability than traditionally acknowledged. The internal mat structure shows mineralized patches because diverse biologically-influenced sediments attach to polymeric fibers (some of them secreted by diatoms) from the EPS matrix; other polymeric fibers may even form organic bridges favoring microbial-mineral interactions. The description of microtextures from neutral environments in continental settings such as BSI holds a great astrobiological potential for the identification and detection of microbial signatures.