ASSESSING THE TRANSFER AND TRANSFORMATION OF COLLOIDAL AND PARTICULATE ORGANIC MATTER IN KARST ECOSYSTEMS

Aquatic natural organic matter (NOM) contains classes of lipids and organic acids that reflect microbial community structure and environmental conditions, and serve as biomarkers of bioremediation and other degradation processes. In karst areas, sinking streams carry allochthonous (terrigenous) NOM rapidly into the phreatic zone, often with minimal filtration or retardation. Despite the lack of photodegradation, significant biotic and abiotic transformations occur along subsurface flowpaths that result in structural changes in allochthonous NOM and the production of autochthonous (microbially-derived) NOM.

To track these changes, we extracted particulate organic matter from sinking stream and cave stream sediments, and colloidal organic matter from the corresponding waters, at several locations along a 2 km flowpath in the Cascade Cave system in Kentucky. Lipid profiles of the extracts indicated that poly-unsaturated fatty acids associated with higher plants, algae, fungi and protozoa decreased by 23% along the flowpath. Mono-unsaturated fatty acids representative of Gram-negative bacteria increased by 75%, and mid-branched saturated fatty acids indicative of Actinomycetes, metal-, and sulfate-reducers increased by 200%. The infrared absorbance of saturated aliphatic hydrocarbons, amides, carboxyls, and organo-silicate complexes increased in cave stream particulate NOM with distance from the surface. Further analyses of these dynamic fractions of colloidal and particulate NOM may highlight important differences between attached and free-living microbial communities, reveal strategies for survival in low-energy cave environments, and inform conceptual models of contaminant transport. Additionally, the inclusion of cellular biomarker analysis in hydrogeological investigations will add new dimensions to our current understanding of biogeochemical cycling and the long-term impacts of climate change on karst aquifers.