Sedimentary rocks dating
Laguna Bacalar in Mexico's southern Yucatán Peninsula in the state of Quintana Roo, has an extensive formation of living giant microbialites (that is, stromatolites or thrombolites).The microbialite bed is over 10 km (6.2 mi) long with a vertical rise of several meters in some areas.Cyanobacteria use water, carbon dioxide, and sunlight to create their food.A layer of mucus often forms over mats of cyanobacterial cells.Very few ancient stromatolites contain fossilized microbes.While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic (non-biological) precipitation.Another pair of instances of freshwater stromatolites are at Pavilion and Kelly Lakes in British Columbia, Canada.Pavilion Lake has the largest known freshwater stromatolites and has been researched by NASA as part of xenobiology research.
One such location is Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia where excellent specimens are observed today, Pampa del Tamarugal National Reserve in Chile and another is Lagoa Salgada ("Salty Lake"), in the state of Rio Grande do Norte, Brazil, where modern stromatolites can be observed as bioherm (domal type) and beds.
Some Archean rock formations show macroscopic similarity to modern microbial structures, leading to the inference that these structures represent evidence of ancient life, namely stromatolites.
However, others regard these patterns as being due to natural material deposition or some other abiogenic mechanism.
Time lapse photography of modern microbial mat formation in a laboratory setting gives some revealing clues to the behavior of cyanobacteria in stromatolites. (2015) found that cyanobacteria exposed to localized beams of light moved towards the light, or expressed phototaxis, and increased their photosynthetic yield, which is necessary for survival.
In both light and dark conditions, the cyanobacteria form clumps that then expand outwards, with individual members remaining connected to the colony via long tendrils.