Abstract
The mineralogical composition of caves makes the environment ideal for inhabitation by microbes. However, the bacterial diversity in the cave ecosystem remains largely unexplored. In this paper, we described the bacterial community in an oxic chamber of the Sopradeira cave, an iron-rich limestone cave, in the semiarid region of Northeast Brazil. The microbial population in the cave samples was studied by 16S rDNA next-generation sequencing. A type of purple sulfur bacteria (PSB), Chromatiales, was found to be the most abundant in the sediment (57%), gravel-like (73%), and rock samples (96%). The predominant PSB detected were Ectothiorhodospiraceae, Chromatiaceae, and Woeseiaceae. We identified the PSB in a permanently aphotic zone, with no sulfur detected by energy-dispersive X-ray (EDX) spectroscopy. The absence of light prompted us to investigate for possible nitrogen fixing (nifH) and ammonia oxidizing (amoA) genes in the microbial samples. The nifH gene was found to be present in higher copy numbers than the bacterial-amoA and archaeal-amoA genes, and archaeal-amoA dominated the ammonia-oxidizing community. Although PSB dominated the bacterial community in the samples and may be related to both nitrogen-fixing and ammonia oxidizing bacteria, nitrogen-fixing associated gene was the most detected in those samples, especially in the rock. The present work demonstrates that this cave is an interesting hotspot for the study of ammonia-oxidizing archaea and aphotic PSB.
Highlights
Caves offer a unique environment for microbial life, which is an aphotic, non-photosynthetic, and commonly oligotrophic environment, isolated to some degree from the surface
We focused on the bacterial community in Sopradeira cave, a limestone cave in the semiarid Northeast region of Brazil
To the best of our knowledge, our group is the first to demonstrate the predominance of purple sulfur bacteria in dark zones of cave samples
Summary
Caves offer a unique environment for microbial life, which is an aphotic, non-photosynthetic, and commonly oligotrophic environment, isolated to some degree from the surface. They present a distinct mineralogical composition, associated with rocks [1,2,3] that provide an opportunity for colonization by different microbial communities. There was an increase in the number of studies conducted on microbial communities in tropical caves, especially those close to the Equator line (such as Brazilian caves) [4,5,6,7,8]. Culture-independent techniques provide more detailed information regarding the microbial communities inhabiting the caves, and reveal the various diverse and unexpected [6] metabolic activities that exist within the cave ecosystem [4,9,10]
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