The subterranean world of caves supports a diverse community of organisms, despite the lack of solar radiation and often low availability of nutrients. Lava tubes, one type of type of caves, are common in volcanic terrain worldwide, yet are understudied compared to their more famous limestone cave counterparts. The walls and ceiling of lava tubes are often covered in colorful microbial mats, however little is known about which organisms are present and what ecological roles they fulfill. One such ecological role could be cycling nitrogen within the cave systems. The oxidation of nitrogen provides energy for some types of microorganisms, and the microbial fixation of nitrogen makes nitrogen bioavailable. Study of the organisms responsible for N transformation in soil and aquatic habitats, continue to yield new discoveries. These analyses have not been conducted in subterranean habitats where N limitation is a major constraint on biological production. Understanding how the bacteria in lava tubes are able to obtain this scarce, critical element from a nitrogen-poor environment would provide greater insight into the ecological dynamics of lava tubes. Molecular methods were used to determine the diversity of species present in 11 Azorean lava tubes, and to determine if these bacteria are capable of participating in nitrogen cycling. Samples represented a range of colors of microbial mats, different precipitation and land use regimes above the lava tubes, and different aged lava flows.
Twelve phyla have been identified using 16S rDNA analyses from the lava tubes, including Nitrospirae (nitrite oxidizers), Actinobacteria, Proteobacteria (four divisions), Bacteriodetes, Planctomycetes, Gemmatimonadetes, Verrucomicrobia, Chloroflexi, Firmicutes, OP10, and TM7. Ammonia oxidation is suggested to occur in 8 of the 11 caves study, base on the presence of the amoA gene. Nitrogen fixation is also suggested to occur in several caves. These results indicate microbial participation in nitrogen cycling in the lava tube ecosystem. We observed small differences in community composition among differently colored microbial mats, but no strong trend in community composition by color was found. Overall, lava tube microbial communities were revealed to contain significant diversity. These results will be used as the basis of a community analyses to compare the community structure in terms of abiotic factors such as soil and water chemistry as well as land use above the lava tubes.
