Chilean high-altitude hot-spring sinters: a model system for UV screening mechanisms by early Precambrian cyanobacteria

Before the build-up of stratospheric ozone, Archean and early Proterozoic phototrophs existed in an environment subjected to highly elevated levels of ultraviolet (UV) radiation. Therefore, phototrophic life would have required a protective habitat that balanced UV attenuation and photosynthetically active radiation (PAR) transmission. Here we report on aspects of the phototroph geomicrobiology of El Tatio geothermal field, located at 4300 m in the Andes Mountains of northern Chile (22 °S), as an analogue system to early Precambrian environments. El Tatio microbes survive in a geochemical environment of rapidly precipitating amorphous silica (sinter) and unusually high solar radiation (including elevated UV-B flux) due to the high-altitude, low-latitude location. Cyanobacteria produce 10-mm-thick surface mats containing filaments encased in amorphous silica matrices up to 5 μm thick. Relative radiation absorbance of these silica matrices was UV-C > UV-B > UV-A > PAR, suggesting the silica provides a significant UV shield to the cyanobacteria. Cyanobacteria also occur in cryptoendolithic communities 1-10 mm below siliceous sinter surfaces, and in siliceous stromatolites, where viable cyanobacteria are found at least ∼10 mm below the sinter surface. UV-B was dramatically attenuated within ∼1 mm of the sinter surface, whereas UV-C (a frequency range absent today but present in the early Precambrian) was attenuated even more efficiently. PAR was attenuated the least, and minimum PAR levels required for photosynthesis penetrated 5-10 mm into the sinter. Thus, a favourable niche occurs between approximately 1-10 mm in siliceous sinters where there is a balance between PAR transmission and UV attenuation. These deposits also would have strongly attenuated Archean and early Precambrian levels of UV and thus, by analogy, cyanobacteria of early Precambrian shallow aquatic environments, inhabiting silicified biofilms and silica stromatolites, would have similarly been afforded protection against high-intensity UV radiation.