Seasonal variation of bacterial community and their functional diversity in drip water from a karst cave

Caves are natural laboratories to study the subsurface deep biosphere, where drip water connect the caves with outside environments. Drip water may bring microorganisms and DOC from the outside environments into caves which subsequently play a fundamental role in sustaining the oligotrophic subsurface ecosystem. To understand the seasonal dynamics of bacterial communities, their potential metabolic functions, and their association with environmental factors such as precipitation, temperature, drip rate, pH, NO3−, SO42−, Ca2+, Mg2+, TOC and FI, drip water samples from Heshang Cave were collected across a two years’ sampling period and subject to 16S rRNA Illumina sequencing and the Biolog-ECO plates analysis. Our results show that the richness and diversity of bacterial communities in the drip water revealed no significant seasonal difference, whereas the compositions of bacterial communities showed a clear seasonal variation, especially Acinetobacter and Pseudomonas , the dominant groups. In details, the relative abundance of Acinetobacter was higher in spring and summer seasons, while lower in autumn and winter seasons. In contrast, the relative abundance of Pseudomonas showed an opposite variation to that of Acinetobacter . Precipitation, temperature inside and outside the cave, drip rate and conductivity showed strong seasonal variations, whereas other environmental factors exhibited moderate variation within the two years’ period. Among all the environmental factors investigated, precipitation significantly shaped the bacterial community structure, explaining about 27.6% of their seasonal variation. Pseudomonas significantly negatively correlated with multiple environmental factors such as precipitation, drip rate and temperature, whereas Acinetobacter showed a positive correlation only with the drip rate. Furthermore, potential metabolic functions of bacterial communities in drip water also show clear seasonal variation as indicated both by 16S rRNA PICRUSt and Biolog-ECO plate analysis. The redundancy analysis (RDA) results showed that the relative abundance of Acinetobacter demonstrated a significant relationship with the seasonal dynamics of metabolic ability. In addition, Acinetobacter revealed a remarkable positive correlation with amino acid metabolism, lipid metabolism, metabolism of terpenoids and polyketid, xenobiotics biodegradation and metabolism, and carboxylic acids metabolism. Pseudomonas showed a significant negative correlation with lipid metabolism, metabolism of terpenoids and polyketid, xenobiotics biodegradation and metabolism, and amine metabiolism. Overall, our results showed that precipitation significantly controls the seasonal variation of bacterial communities in drip water and Acinetobacter plays an important role in the variation of bacterial functions. This the first report about the bacterial communities and their carbon utilization variation over a 2 years’ period which greatly enhances our understanding about the seasonal dynamics of microbial communities and their correlation with environmental conditions.