Bacterial diversity in vadose cave pools: evidence for isolated ecosystems

Microbial diversity of cave pools, especially vadose pools, has received relatively little attention. To help fill this gap, this study reports on the bacterial diversity of 17 pools in three New Mexican arid land caves: Carlsbad Cavern, Lechuguilla Cave, and Hell Below Cave. These pools are spread throughout the caves and, with two exceptions, are not connected. The pools share a basic water chemistry, with fresh water of the calcium-magnesium-bicarbonate type. These 17 pools have Chao1 values between 40 and 1738; the Shannon diversity averages 4.6 ± 1.1, ranging from 2.6 to 6.4; and the Simpson averages 0.881 ± 0.099, ranging from 0.622 to 0.981. No two pools had the same communities, even at the phylum level. Nitrospirae, Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria were found >5% abundance in nine or more cave pools. Actinobacteria, Chloroflexi, Fibrobacteres, Firmicutes and Plantomycetes were at >5 % in four to six pools. Of the top ten widespread bacterial genera, Nitrospira was found in all pools, with >5 % in eleven pools. Other common genera include Polyclorovans, Propionibacterium, Polaromonas, Haliangium, Bacillus, Subgroup 6 uncultured Acidobacteria, Candidatus Omnitrophica, and uncultured Nitrosomonadaceae. Presence of several potential nitrogen cycling bacteria (e.g., Nitrospira) in the study pools suggests that nitrogen cycling may be an important bacterial role. There is some evidence of human contamination, particularly in the heavily visited Big Room, Carlsbad Cavern, but it is not the dominant control. Rather than a single stable cave pool community, adapted to the cave pool ecosystem, the data show 17 different communities, despite relatively similar conditions. The data support the hypothesis that each pool is a unique, isolated ecosystem, with differences likely caused more by the isolation of each pool than by variable chemistry. Thus, the common habit of grouping samples, while useful for some questions, may not capture the diversity present in cave ecosystems.