ENVIRONMENTAL MICROBIOLOGY Hypolithic Cyanobacteria Supported Mainly by Fog in the Coastal Range of the Atacama Desert Armando Azúa-Bustos & Carlos González-Silva & Rodrigo A. Mancilla & Loreto Salas & Benito Gómez-Silva & Christopher P. McKay & Rafael Vicuña Received: 26 May 2010 / Accepted: 25 November 2010 / Published online: 29 December 2010 # Springer Science+Business Media, LLC 2010 Abstract The Atacama Desert is one of the driest places on Earth, with an arid core highly adverse to the development of hypolithic cyanobacteria. Previous work has shown that when rain levels fall below ~1 mm per year, colonization of suitable quartz stones falls to virtually zero. Here, we report that along the coast in these arid regions, complex associations of cyanobacteria, archaea, and heterotrophic bacteria inhabit the undersides of translucent quartz stones. Colonization rates in these areas, which receive virtually no rain but mainly fog, are significantly higher than those reported inland in the hyperarid zone at the same latitude. Here, hypolithic colonization rates can be up to 80%, with all quartz rocks over 20 g being colonized. This finding strongly suggests that hypolithic microbial communities thriving in the seaward face of the Coastal Range can survive with fog as the main regular source of moisture. A model is advanced where the development of the hypolithic communities under quartz stones relies on a positive feedback between fog availability and the higher thermal conductivity of the quartz rocks, which results in lower daytime temperatures at the quartz–soil interface microenvironment. Introduction The Atacama Desert, located between 17° and 27°S latitude in northern Chile, is bordered on the east by the Andes Mountains and on the west by the Coastal Range. It is the driest and probably the oldest desert on Earth [21, 23]. To survive in these conditions, life must adapt to very low water availability due to the almost complete absence of rain, highly saline soils, and high solar radiation [3, 10, 36]. The hyperarid core region of the Atacama is located near 24°S in the central depression east of the Coastal Range. Climate studies of this region [7, 36, 50] have shown that rain is extremely low, averaging less that 1 mm/year and years can pass without any measurable rain. The occurrence of fog is also reduced in the area due to coastal mountains reaching over 2,000 m that block the penetration of the marine fog [42]. The antiquity and severity of the aridity in this central depression is evidenced by the accumulation of significant nitrate deposits in the basins of this area [12]. Several researchers have noted that the microbial soil concentrations in the hyperarid zone are one or two orders Electronic supplementary material The online version of this article (doi:10.1007/s00248-010-9784-5) contains supplementary material, which is available to authorized users. A. Azúa-Bustos (*) : R. A. Mancilla : L. Salas : R. Vicuña Departamento de Genética Molecular y Microbiología, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Alameda 340, Santiago, Chile e-mail: ajazua@uc.cl C. González-Silva Centro de investigación del Medio Ambiente (CENIMA), Universidad Arturo Prat, Iquique, Chile C. P. McKay NASA-Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA A. Azúa-Bustos : R. Vicuña Millennium Institute of Fundamental an Applied Biology, MIFAB, Santiago, Chile B. Gómez-Silva Departamento Biomédico, Facultad de Ciencias de la Salud, Universidad de Antofagasta, Antofagasta, Chile Microb Ecol (2011) 61:568–581 DOI 10.1007/s00248-010-9784-5