MICROCLIMATOLOGY OF AND CONDITIONS FOR HYPOLITHIC CYANO-AND HETEROTORPHIC BACTERIAL COLONIZATION OF GYPSUM PLATES IN THE CHIHUAHUAN DESERT: BIG BEND NATIONAL PARK, TEXAS

Locally, the floor of the Chihuahuan Desert in southwest Texas is littered with gypsum plates. These gypsum plates form along bedding planes in Cretaceous-aged Pen Formation siltstone, weather out of receding outcrops, and are deposited onto the desert floor. Once on the desert floor the undersides are colonized by cyano- and heterotrophic bacteria. Where colonized, the undersides of the natural gypsum plates are highly pitted and embayed. These colonies of cyanobacteria thrive in this environment at unknown conditions. The purpose of this experiment is to collect microclimatological data on the hypolithic environment in order to gain a better understanding of the conditions in which these colonies exist. The experiment is located in Big Bend National Park (N 29^o 19.007', W 103^o 12.390') in a Sotol grassland. The conditions that are being monitored include soil moisture (m³/m³), temperature (^oC), and UV-radiation (W/m²). These data are being monitored using Micro HOBO [trade mark] sensors to measure soil moisture, temperature and photosynthetically active radiation at 10 second intervals. This data are collected via the software BoxCar Pro 4 [trade mark]. Monitoring began on March 31, 2005 and will continue for one year. The sensors are mounted beneath gypsum plates collected in the Christmas Mountians (Terlingua Ranch Property). Data collected through mid May record hypolithic temperatures ranging from 56^oC (133.7^oF) to 7.3^oC (44.56^oF) with diurnal variations for the months April-mid May ranging from dT=39.05^oC (56.08^oC High/17.75^oC Low) to dT=28.37^oC with an average dT=32.3^oC. Photosyntheically active radiation beneath the gypsum plates ranges (daytime highs) from 706.9 W/m² (MAX) to 450W/m² (MIN)with a general seasonal increase through the measurement period. At temperatures in excess of 50^oC the cyano-bacterial colonies will go dormant under dry conditions. Temperature/ radiation/ moisture conditions after daytime rainfall events, however, provide brief (5-8 hour) periods where the cyanobacterial colonies should become active. Laboratory experiments utilizing inoculated and sterile gypsum plates are underway to determine if the colonies depend upon the gypsum simply for protection or if the colonies extract nutrients and or water from the substrate.