Spatial and temporal distribution and characteristics of eukaryote-dominated microbial biofilms in an acid mine drainage environment: implications for development of iron-rich stromatolites

Spatial and temporal distribution and characteristics of three eukaryotic biofilms were monitored for an 18-month period in an acid mine drainage environment at the Green Valley coalmine in Indiana, USA. Each biofilm is dominated (> 90 %) by a single eukaryotic microorganism based on enumeration: Euglena mutabilis, the diatom species Nitzschia tubicola, and a filamentous alga belonging to the genus Klebsormidium sp. The E. mutabilis-dominated biofilm occurs year round, covering up to 100 % of the channel bottom in spring and fall. The N. tubicola-dominated biofilm is less abundant, exists as small patches in spring and fall, expands from these patches to cover up to 50 % of the channel bottom in June, and is absent in winter. The Klebsormidium-dominated biofilm is restricted to small patches covering < 5 % of the channel bottom from spring through fall and is absent in winter. Also present are floating microbial scum layers. The eukaryotic biofilms and scum layers contribute to the attenuation of precipitates and to the formation of organosedimentary structures, or stromatolites, by trapping and binding chemical precipitates via aerotaxis and phototaxis and by serving as a medium for passive accumulation of precipitates. Each stromatolite layer represents the morphological characteristics of each eukaryotic biofilm that served as the architect of the layer and the time of year the biofilm populated the channel. Processes involved in stromatolite formation also attenuate chemical sediments by binding them to the channel bottom and prior stromatolite surface rather than allowing them to be carried to the adjacent drainage system where they may become bioavailable to other forms of life.