Microorganisms in caves are typically oligotrophic, slow growing, and chemoautotrophic or chemoheterotrophic because of the low nutrient and no light environment. Regardless of the metabolism, microorganisms use nutrients, carbon, nitrogen, and phosphorus, for processes such as growth, maintenance and enzyme production. Nutrient allocation to these processes varies based on substrate availability, temperature, moisture and human influences, with important consequences for nutrient cycling. The objective of this work was to assess the function of the resident cave microbial community and assess any changes by season and human activity at the Natural Bridge Caves throughout the year. We assessed microbial function through the use of enzyme assays. The activity for eight enzymes involved in carbon, nitrogen, and phosphorus was assayed monthly at locations within the cave system that were visited by humans on a regular basis and locations with few to no human visitors.
Results/Conclusions
Changes in extracellular enzyme activity were minimal based on human activity; only enzymes involved in organic phosphorus hydrolysis were greater in areas exposed to humans compared to non-human areas. Based on enzymatic stoichiometry all areas of the cave exhibited higher nitrogen and phosphorus demand relative to carbon demand. However, the ratio of carbon acquiring to nitrogen acquiring enzymes, indicating nitrogen demand, was influenced by human activity. Areas not exposed to humans had a significantly greater nitrogen demand than human exposed areas. Additionally the nitrogen demand was significantly greater than phosphorus demand in areas not exposed to humans compared to human exposed areas. These results indicate that human activity may possibly be satisfying part of the microbial nitrogen need through unintentional additions of organic matter (detritus, leaves, skin cells) resulting in a larger microbial community. Indeed, seasons with higher influxes of humans into the cave, spring and summer, had twice the activity of nitrogen acquiring enzymes compared to winter and fall with low visitor numbers. Whether these changes are driven by shifts in substrate utilization profiles or shifts in microbial community composition is current ongoing work.