1. ¹Department of Earth and Planetary Science, University of California at Berkeley, Berkeley, CA, USA
2. ²Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
3. ³Chemistry Directorate, Lawrence Livermore National Laboratory, Livermore, CA, USA
4. ⁴Department of Environmental Science, Policy and Management, University of California at Berkeley, Berkeley, CA, USA

Correspondence: P Wilmes, Department of Earth and Planetary Science, 307 McCone Hall, University of California at Berkeley, Berkeley, CA 94720, USA. E-mail: pwilmes@berkeley.edu

Received 12 June 2008; Revised 4 August 2008; Accepted 4 August 2008; Published online 9 October 2008.

Abstract

Pellicle biofilms colonize the air–solution interface of underground acid mine drainage (AMD) streams and pools within the Richmond Mine (Iron Mountain, Redding, CA, USA). They exhibit relatively low species richness and, consequently, represent good model systems to study natural microbial community structure. Fluorescence in situ hybridization combined with epifluorescent microscopy and transmission electron microscopy revealed spatially and temporally defined microbial assemblages. Leptospirillum group II dominates the earliest developmental stages of stream pellicles. With increasing biofilm maturity, the proportion of archaea increases in conjunction with the appearance of eukaryotes. In contrast, mature pool pellicles are stratified with a densely packed bottom layer of Leptospirillum group II, a less dense top layer composed mainly of archaea and no eukarya. Immunohistochemical detection of Leptospirillum group II cytochrome 579 indicates a high abundance of this protein at the interface of the biofilm with the AMD solution. Consequently, community architecture, which most likely develops in response to chemical gradients across the biofilm, is reflected at the functional gene expression level.