The Central Andes region displays unexplored ecosystems of shallow lakes and salt flats at mean altitudes of 3700 m. Being isolated and hostile, these so-called “High-Altitude Andean Lakes” (HAAL) are pristine and have been exposed to little human influence. HAAL proved to be a rich source of microbes showing interesting adaptations to life in extreme settings (poly-extremophiles) such as alkalinity, high concentrations of arsenic and dissolved salts, intense dryness, large daily ambient thermal amplitude, and extreme solar radiation levels. This work reviews HAAL microbiodiversity, taking into account different microbial niches, such as plankton, benthos, microbial mats and microbialites. The modern stromatolites and other microbialites discovered recently at HAAL are highlighted, as they provide unique modern—though quite imperfect—analogs of environments proxy for an earlier time in Earth's history (volcanic setting and profuse hydrothermal activity, low atmospheric O2 pressure, thin ozone layer and high UV exposure). Likewise, we stress the importance of HAAL microbes as model poly-extremophiles in the study of the molecular mechanisms underlying their resistance ability against UV and toxic or deleterious chemicals using genome mining and functional genomics. In future research directions, it will be necessary to exploit the full potential of HAAL poly-extremophiles in terms of their biotechnological applications. Current projects heading this way have yielded detailed molecular information and functional proof on novel extremoenzymes: i.e., DNA repair enzymes and arsenic efflux pumps for which medical and bioremediation applications, respectively, are envisaged. But still, much effort is required to unravel novel functions for this and other molecules that dwell in a unique biological treasure despite its being hidden high up, in the remote Andes.

The calibration of the erythemal irradiance measured by a Yankee Environmental System (YES) UVB-1 biometer is presented using two methods of calibration with a wide range of experimental solar zenith angles (SZAs) and ozone values. The calibration is performed through simultaneous spectral measurements by a calibrated double-monochromator Brewer MK-III spectrophotometer at "El Arenosillo" station, located in southwestern Spain. Because the range of spectral measurements of the Brewer spectrophotometer is 290-363 nm, a previously validated radiative transfer model was used to account for the erythemal contribution between 363 and 400 nm. Both methods are recommended by the World Meteorological Organization and we present and discuss here a wide range of results and features given by modified procedures applied to these two general methods. As is well established, the calibration factor for this type of radiometric system is dependent on atmospheric conditions, the most important of which are the ozone content and the SZA. Although the first method is insensitive to these two factors, we analyze this behavior in terms of the range used for the SZA and the use of two different mathematical approaches for its determination. The second method shows the dependence on SZA and ozone content and, thus, a polynomial as a function of SZA or a matrix including SZA and ozone content were determined as general calibration factors for the UV radiometric system. We must note that the angular responses of the YES radiometer and Brewer spectroradiometer have not been considered, because of the difficulty in correcting them. The results show in detail the advantages and drawbacks (and the corresponding associated error) given by the different approaches used for the determination of these calibration coefficients.

[1] The major factors causing differences between satellite-derived and ground-based ultraviolet (UV) erythemal irradiances and doses are discussed. Measurements totaling more than 4700 days during 1997-1999 were obtained at 8 stations (22°S-64°S) of the Argentine UV Monitoring Network. The satellite retrieval uses radiative transfer calculations for cloud-and aerosol-free conditions multiplied by correction factors for clouds and aerosols. Key parameters are total ozone, cloud optical depth, and surface albedo derived from Total Ozone Mapping Spectrometer (TOMS). When no aerosol correction is applied, systematic differences of satellite-derived erythemal irradiance relative to ground-based measurements amount to +1% at a tropical high-altitude Andean location, +10% at stations in the central Pampas, +5% at southern Patagonian sites, and À7% at the southernmost continental and Antarctic stations with varying snow cover. When an aerosol correction is applied by estimating ''minimum'' and ''maximum'' aerosol loading, the systematic differences are within ±10% for all ''snow-free'' stations. To reduce the differences at places with varying snow conditions, an ''average surfacealbedo climatology'' must be used instead the TOMS climatology of minimum albedo. Although the statistical uncertainty of the differences increases with TOMS reflectivity, the systematic difference is independent of TOMS reflectivity for most of the stations, so on average the comparison for cloudy situations is as good as for clear-sky conditions. The comparison for daily erythemal doses gives similar results with smaller statistical uncertainty. Measured uncertainties are in agreement with a theoretical analysis. For most locations, well-characterized ground-based instruments should agree with TOMS satellite estimations within 10% if aerosol corrections are known.