Epipelic diatoms from an extreme acid environment: Beowulf Spring, Yellowstone National Park, USA

A floristic study is presented of the diatom community from Beowulf Spring, Yellowstone National Park (YNP), U.S.A., a strongly acidic (pH<3) and thermophilous (temperature >50 degrees C) environment with potentially toxic concentrations of dissolved metals. A sampling transect between regions of active hydrous ferric oxide (HFO) precipitation and mats of periphytic red algae (Cyandiaceae) revealed that diatoms are present mainly in the regions of the spring characterized by the highest temperatures and brown arsenic-rich HFO mats. The diatom flora is dominated by a deme within the Nitzschia thermalis complex, in association with N. ovalis, Pinnularia acoricola, and Eunotia exigua. Abiotic silica precipitates were observed on the surfaces of most diatom cells in uncleaned material. Cells of Cyanidiaceae also participate in active silica deposition. These observations not only highlight the tolerance of hot-spring diatoms to extreme environments with respect to temperature, pH, and arsenic (As) concentration, but also illustrate the clear zonation between epipelic algal communities within a hot spring.