Abstract
Restricted contact with the external environment has allowed the development of microbial communities adapted to the oligotrophy of caves. However, nutrients can be transported to caves by drip water and affect the microbial communities inside the cave. To evaluate the influence of aromatic compounds carried by drip water on the microbial community, two limestone caves were selected in Brazil. Drip-water-saturated and unsaturated sediment, and dripping water itself, were collected from each cave and bacterial 16S rDNA amplicon sequencing and denaturing gradient gel electrophoresis (DGGE) of naphthalene dioxygenase (ndo) genes were performed. Energy-dispersive X-ray spectroscopy (EDX) and atomic absorption spectroscopy (AAS) were performed to evaluate inorganic nutrients, and GC was performed to estimate aromatic compounds in the samples. The high frequency of Sphingomonadaceae in drip water samples indicates the presence of aromatic hydrocarbon-degrading bacteria. This finding was consistent with the detection of naphthalene and acenaphthene and the presence of ndo genes in drip-water-related samples. The aromatic compounds, aromatic hydrocarbon-degrading bacteria and 16S rDNA sequencing indicate that aromatic compounds may be one of the sources of energy and carbon to the system and the drip-water-associated bacterial community contains several potentially aromatic hydrocarbon-degrading bacteria. To the best of our knowledge, this is the first work to present compelling evidence for the presence of aromatic hydrocarbon-degrading bacteria in cave drip water.
Highlights
Caves are aphotic environments isolated to some degree from the surface and lacking photosynthetic activity [1,2,3]
This paper aims to analyze the occurrence of potential bacterial degradation of aromatic compounds in the bacterial communities of dripping water, and drip-water-saturated and unsaturated sediment samples from two different carbonate caves
Variance in the dominant bacterial community observed in denaturing gradient gel electrophoresis (DGGE) showed an influence of drip water on bacterial sediment profiles (Text S1, Figure S5) confirmed by next-generation sequencing
Summary
Caves are aphotic environments isolated to some degree from the surface and lacking photosynthetic activity [1,2,3]. A cave environment commonly has low levels of organic matter and some essential micronutrients change according to the mineralogical composition associated with carbonate rocks [4]. Other factors such as runoff, bats, tourism, invertebrates’ feces, and drip water could exert a significant influence on the organic nutrients available in caves [5] and, on the cave microbial communities. Dripping water could transport organic and inorganic material from the surface [15,16,17] This material carried to caves could change the availability of nutrients and modify microbial communities in favor of a chemoheterotrophic-based community
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