Biomediated Precipitation of Calcium Carbonate and Sulfur in a Faintly Acidic Hot Spring

Abstract:

A faintly acidic hot spring named “female Tower” (T=73.5 ℃, pH=6.64 ) is located in the Jifei Geothermal Field，Yunnan province, Southwest China. The precipitates in the hot spring are composed of large amounts of calcite and sulfur, as reveals by XRD analysis. Scanning electron microscopy (SEM) analysis show the microbial mats are formed of various coccoid, rod and filamentous microbes. Transmission electron microscopy (TEM) analysis show that intracellular sulfur granules are commonly associated with these microbes. Energy dispersive X-ray spectrometer (EDS) analysis shows that the surface of microbes are mainly composed of Ca, C, O and S. A culture-independent molecular phylogenetic analysis demonstrates the majority of bacteria in the spring are sulfur-oxidizing bacteria. In the spring water, H₂S concentration was up to 60 ppm, while SO₄^2- concentration was only about 10 ppm. We suggest that H₂S might be utilized by sulfur-oxidizing bacteria in this hot spring water, leading to the formation of sulfur granules intracellularly and extracellularly. In the meantime, this reaction increases the pH in ambient environments, which fosters the precipitation of calcium carbonate precipitation in the microbial mats. This study suggests that the sulfur-oxidizing bacteria could play an important role in calcium carbonate precipitation in faintly acidic hot spring environments.