Microbial biomass and community structure of a stromatolite from an acid mine drainage system as determined by lipid analysis

Lipids were extracted to determine the microbial biomass and community structure of a Fe-rich stromatolite from acid mine drainage (AMD) at the Green Valley coal mine site (GVS) in western Indiana. The distribution of biomarkers correlated well with layers in the laminated stromatolite. Our results show that the biomass of the top layer of the stromatolite was dominated by phototrophic organisms which constituted 83% of the total biomass. Biomass of the lower layers was dominated by prokaryotic microorganisms. The presence of terminal methyl-branched fatty acids and mid methyl-branched fatty acids suggests the presence of Gram-positive and sulfate-reducing bacteria, respectively. Fungi appear to also be an important part of the AMD microbial communities as suggested by sterol profiles and the presence of polyunsaturated fatty acids. Hydroxy fatty acids and C19 cyclopropane fatty acids were also detected and likely originated from acid-producing, acidophilic bacteria. The presence of Archaea is indicated by abundant phospholipid ether-linked isoprenoid hydrocarbons (phytane and phytadienes). The AMD Fe-rich stromatolites at GVS thus appear to be formed by interactions of microbial communities composed of all three domains of life; Archaea, Bacteria, and Eukarya. The identification of microeukaryote-dominated stromatolites implicates the prominent role of these organisms in the formation and preservation of these structures. In addition, the production of oxygen through photosynthesis by these organisms in AMD systems may be important for retrodicting the interaction of microbial communities in Precambrian environments in the production of microbially-mediated sedimentary structures and oxygenation of Earthʹs early atmosphere.