Human urine in Lechuguilla Cave: the microbiological impact and potential for bioremediation

During extended exploration trips in caves, it is sometimes not possible to remove all excreted urine due to its volume and weight. Excess urine can be particularly problematic in dry caves where, without dilution, urine introduces a significant source of nitrogen into these otherwise nitrogen-limited environments. It was the aim of this study to determine the impact that human urine could have on cave microbiota over an extended period of time. To do this, we examined the microbial community structure of a heavily impacted site in Lechuguilla Cave, USA. Using a molecular phylogenetic approach we generated a 136-member 16S rRNA clone library that demonstrated representatives of the Alpha- Beta- and Gammaproteobacteria, the Bacteroidetes, Firmicutes, Actinobacteria and Deinococcus-Thermus group at this site. The structure of the microbial community at the impacted site suggests that it is colonized by endemic cave species rather than human commensal organisms, while metabolic inference suggests that these organisms are taking advantage of both the nitrogenous and organic compounds in urine for growth. The intrinsic nature of such metabolic activity in the cave environment was confirmed by examining non-impacted sites using cultivation, which demonstrated that endemic species express both the capacity to degrade urine and to reduce urea to nitrogen gas. Our results differ from those of previous studies by implying a more resilient nature of the microbial ecosystem in caves to invasion by exogenous (commensal) species, while suggesting that endemic microbial species may be able to mitigate the impact of excess nitrogen in the cave through bioremediation.