Caves are a unique microbial ecosystem due to the lack of light, nutrients, and typically a lack of human influence. Microorganisms in caves are usually oligotrophic, slow growing, and chemoautotrophic or chemoheterotrophic because of the low nutrient and no light environment. Regardless of the metabolism, microorganisms use nutrients, such as carbon, nitrogen, and phosphorus, for processes such as growth, maintenance and enzyme production. Carbon allocation varies based on substrate availability, temperature, moisture and human influences, with important consequences for nutrient cycling and microbial community composition. The purpose of this work was to assess the function of the resident cave microbial community and assess any changes to the microbial community by human activity at the Natural Bridge Cavern. We assessed microbial function using two techniques, enzyme assays and carbon utilization profiles. The activity for eight enzymes involved in carbon, nitrogen, and phosphorus was assayed at locations within the cave system that were visited by humans on a regular basis and locations with few to no human visitors. The bacterial community abundance was determined by plate counts and the presence/absence of human indicator bacteria.
Results/Conclusions
Changes in extracellular enzyme activity impacted what substrates were broken down and the overall rate of decomposition and nutrient cycling. Regardless of human impact there was no difference in overall enzyme activity rates, but the highest activity was for nitrogen and phosphorus acquiring enzymes. Enzymatic stoichiometry indicated that all microorganisms in the cave had an increased demand for nitrogen and phosphorus relative to carbon (values below 1), however only carbon:nitrogen (C:N) enzyme acquisition was significantly different between human impacted and non-human impacted environments. In non-human impacted zones the enzyme C:N was lower meaning that the microorganisms had increased N acquiring enzyme activity compared to human impacted zones. This difference may be due to a combination of factors: (1) humans are bringing in additional organic matter and nutrients on their shoes, satisfying part of the N demand of the microorganisms in those zones and/or (2) human activity is altering the microbes present towards a community that overall has lower N demands. Current work is assessing the differences in nitrogen content and community composition between human impacted and non-impacted cave areas.