Speleothems from Sahastradhara Caves in Siwalik Himalaya, India: Possible Biogenic Inputs

Abstract

Stalactites and moonmilk from Sahastradhara caves in Siwalik Himalayas were studied to understand the role of microbes in their genesis. Fourier spectroscopy in the moonmilk indicates a complex milieu of organic compounds that is unusual for inorganic formations. Stable C and O isotopes show trends in the moonmilk and stalactite, which suggest biogenic input; the geochemical inference is consistent with evidence from microscopy and laboratory-based microbial cultures. Light microscopy of moonmilk samples show the presence of a number of microbial forms similar to Cyanobacteria, and scanning electron microscope (SEM) images show microbial structures similar to Spirulina. The total number of microbial cells using SYBR Gold is 6.5 × 10⁵ cells, g sed⁻¹in moonmilk and 3.2 × 10⁵ cells, g sed⁻¹ in stalactites. FISH indicates approximately 3.5 × 10⁵ cells, g sed⁻¹ in moonmilk and 2 × 10⁵ cells, g sed⁻¹ in stalactites. SEM images of the moonmilk indicate a large network of microbial filaments along with minerals, which are identified as calcite based on their x-ray diffraction pattern. In vitro laboratory cultures with pure monogenic strains isolated from the moonmilk and stalactites raise pH in the medium, which facilitate calcite precipitation. The mineral precipitating isolates were identified as: Bacillus pumilis, B. cereus, B. anthracis, B. lentus, B. sphaericus, B. circulans and Actinomycetes. The Sahastradhara moonmilk and statactites are colonized by a diverse microbial community and the isolated bacterial strains induce biomineralization on different nutrient media, supporting their biogenic origin.

Keywords:

• bacteria
• biogenic deposits
• calcite
• speleothem

Acknowledgments

SB and RB thank the Center for Geobiology, University of Bergen, Norway and Department of Geological Sciences, Stockholm University, Sweden for access to laboratory facilities. Ingunn Thorseth and Lise Øvreås, CGB, Norway are thanked for helping with the SEM, DNA and FISH assays. Egil Erichsson at University of Bergen helped in SEM sample preparations. Marianne Ahlbom, Anna Neubeck and Paula Lindgren, Stockholm University helped with the ESEM-EDAX analyses. XRD, Laser-Raman and ICP-MS data was generated at the Wadia Institute of Himalayan Geology, Dehradun. Institute for Microbial Technology, Chandigarh is acknowledged for the identification of cultures. The reviewers are thanked for their careful editing and constructive suggestions.

Additional information

Funding

SB thanks the Norwegian Government Scholarship Pool – International Section, Norway and the Swedish Institute for a postdoctoral fellowship. RB thanks UGC, New Delhi for financial assistance. A part of the study was also funded by SASNET.