RATES OF MICROBIAL DEGRADATION OF NATIVE AND FOREIGN ORGANIC CARBON SOURCES IN KARST STREAMBED SEDIMENTS

Metabolic rates of heterotrophic microorganisms living in karst streambed sediments depend on the type of organic matter input into the system. To better understand how microbes consume organic carbon native to their environment, the metabolic rates of microorganisms within the sediment of a karst surface stream reach at Rock Bridge State Park were compared to an underground cave stream reach at Tumbling Creek Cave (TCC). Organic carbon input to Rock Bridge State Park is primarily dead leaves and decaying plant matter, whereas bat guano is the main source of nourishment for the microbial community in the cave system. Since metabolic rates in the cave system are dependent on a single source of organic carbon, bat guano was used as the additional source of organic carbon for the surface and subsurface sediment systems. In the laboratory, four microcosm experiments were constructed each using 18 one liter Erlenmeyer flasks containing streambed sediments and water from Rock Bridge State Park and Tumbling Creek Cave to simulate in-situ metabolic rates in each location. The control for the metabolic rates consisted of streambed sediment and water, whereas the experimental microcosm for each location was composed by homogenizing an additional source of organic carbon, 25 ml of bat guano from TCC, into the streambed sediment. Pore water samples were taken from these sediments weekly, for six weeks, to measure changes in the concentrations of total iron, ferrous iron, total manganese, and alkalinity as the microbial biomass responded to an additional source of organic carbon. Adding guano to the Rock Bridge and TCC sediments resulted in increases of total dissolved manganese concentrations and alkalinity. Microbial manganese reduction (Mn(IV) to Mn(II)) occurs in suboxic environments and results in an increase in dissolved manganese concentrations. The addition of the bat guano to Rock Bridge sediments resulted in a rate of dissolved manganese production of 1.8 nM/d over the first three weeks, while the control sediments produced dissolved manganese at a rate of 10.2 nM/day over the same time period. The experimental microcosm had a smaller rate of increase in total dissolved manganese suggesting a lag time for the microorganisms before they will incorporate a new source of organic carbon.