CHILEAN HIGH-ALTITUDE HOT SPRING SINTERS: A MODEL SYSTEM FOR UV SCREENING MECHANISMS BY EARLY PRECAMBRIAN CYANOBACTERIA

Due to low levels of oxygen in the Earth’s early atmosphere, insufficient ozone was present to shield early life from harmful UV irradiation. The El Tatio hot spring system located in the Andes of northern Chile provides a good analogue for the Precambrian ocean due to their similarity in chemistry (enriched in silica and other solutes) and biology (dominated by bacteria and cyanobacteria). Furthermore, the hot spring’s location at low latitude (Lat: 22^o S) and high altitude (~15000 ft) means the site is exposed to high levels of solar radiation, and is thus an appropriate system to study possible UV screening mechanisms utilized in the early Precambrian.

The silica sinters at El Tatio contained abundant silicified cyanobacteria living ~0.5 cm below the surface, both in silica stromatolite and cryptoendolithic communities. Culturing and phycoerythrin autofluorescence demonstrated these communities were viable. Cyanobacterial silicification often occurred upon the outer surfaces of thick extracellular sheaths; the sheaths protecting the more sensitive cell wall from silicification and probably also inhibiting dehydration. Additionally, silica stromatolites demonstrated a water potential of -5.69 MPa. When dried this value dropped to -88.9 MPa, indicating the hydrated nature of both the silica sinter and biofilms were likely vital in protecting the cyanobacteria from dehydration caused, in-part, by high solar radiation.

The light transmittance properties of thin sections of sinter determined using bulk PAR, UV-A, B and C analysis demonstrated that the sinter absorbed at least 3 times more UV-B, and at least 10 times more UV-C, than PAR (required for photosynthesis). Sufficient PAR for photosynthesis was able to penetrate the rock down to ~ 0.5 cm, corresponding to the depth at which viable cyanobacteria were common. Even 2 µm-thin silica precipitates formed upon glass slides left in discharge channels for 4 days demonstrated significant UV absorption capacity. UV/VIS spectroscopy of this precipitate revealed that discrete adsorption peaks corresponding to trace metals within the precipitate (e.g. Sb, As) also contributed to UV screening. This study illustrates how habitation within the silica stromatolites common to the early Precambrian may have protected cyanobacteria from harmful UV radiation.