SEASONAL VARIATIONS IN DEGRADATION OF DISSOLVED ORGANIC MATTER BY GROUNDWATER MICROBIAL COMMUNITIES IN A KARST SYSTEM

Dissolved organic matter (DOM) in water affects geological processes, such as carbonate dissolution, and biological processes like ecosystem function, water quality, and nutrient sources for groundwater microbial communities in cave and karst ecosystems. Microbially active water samples collected along recharge and discharge sites from the Cascade Cave system, Kentucky, during February and July 2013 were enriched with one of three DOM materials: a mixed tree leachate, mixed grass leachate, or tryptone. Enrichments were examined using fluorescence spectroscopy techniques to determine the role seasonal variations in groundwater microbial communities, as well as DOM source, have on DOM degradation rates and microbial growth. Each DOM material displayed a different contribution of autochthonous DOM from the initial time of enrichment, although the primary contributor of DOM was from allochthonous sources. Over time, all of the enriched water samples showed an increase in the relative contribution of autochthonous DOM, with tree leachate increasing at the slowest rate, grass leachate at an intermediate rate, and tryptone increasing at the fastest rate. Additionally, enriched samples of summer water displayed more rapid degradation of DOM than winter water samples. Degradation rates also varied by the location of the microbial communities along the flow path. Degradation of the leachates was associated with an overall decrease in diversity and a general increase in the relative abundances of members of the Proteobacteria, based on 16S rRNA gene pyrosequencing. These results suggest that the ability of groundwater microbial communities to degrade DOM, and cause change to geological and biological processes, varies by season, DOM source, and location along the flow path.