Zinc phosphate dissolution by bacteria isolated from an oligotrophic karst cave in central China

Abstract

Biogeochemical processes are fundamental to sustain the ecosystem in subsurface caves, but to date they are still far from well understood. To investigate microbially mediated phosphorus and zinc cycles, we isolated three bacterial strains from the dripping water in Heshang cave, central China, identified as Exiguobacterium aurantiacum E11, Pseudomonas fluorescens P35, and Pseudomonas poae P41, respectively. Microbial capabilities in the dissolution of phosphorus-containing minerals were tested with zinc phosphate (Zn₃(PO₄)₂) in batch culture at 30°C. A spectrophotometer, atomic absorption spectrum, and scanning electronic microscopy were used to measure the microbial growth, soluble Zn(II) concentration, and to observe the morphology of Zn₃(PO₄)₂ before and after microbial dissolution. P. fluorescens and P. poae, the well-known phosphorus solubilizing bacteria (PSB), are observed to solubilize Zn₃(PO₄)₂ with an efficiency of 16.7% and 17.6%, respectively. To our knowledge, E. aurantiacum is firstly reported in this study to dissolve phosphorous-containing minerals with a higher efficiency of 39.7%, expanding our understanding about the ubiquitous occurrence of PSB in natural environments. Aqueous Zn(II) concentration positively correlates with H⁺ activity, confirming the presence of acidification mechanisms widely exploited by PSB. Few itching pits were observed on the surface of Zn₃(PO₄)₂ after microbial dissolution, inferring that microbial dissolution is not always associated with the direct contact with minerals. Even though the soluble Zn(II) concentration reached up to 370 mg/L in the system inoculated with E. aurantiacum E11, inhibition of microbial growth was not detected by spectrophotometer. Our laboratory data revealed the importance of microbially-mediated P and Zn cycles in the subsurface ecosystem.

Keywords

karst cave / phosphate solubilizing bacteria (PSB) / zinc toxicity / biogeochemical process / subsurface biosphere

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