MICROBE-MINERAL INTERACTION: HOW MICROBIAL COMMUNITIES IN DEEP KARST SINKHOLES RESPOND TO MINERALS THAT ARE LACKING FROM THESE SYSTEMS

The rampant population growth in Yucatan Peninsula, Mexico, has added a threat to the groundwater from sewage treatment systems that cannot keep pace with the growing population. One of the current treatment options is to inject secondary treated wastewater effluent into deep saline groundwater with the expectations on the native microbial communities to degrade the organic carbon load. One objective of this study was to evaluate whether the addition of various minerals into the virtually metal free groundwater would have effect on diversity of the native microbes by providing them with selected lacking elements. The other objective was to observe the effects of the microbial activity and geochemistry of the water on the mineral surfaces. Traps of eleven minerals (apatite, calcite, chalcocite, chalcopyrite, dolomite, hematite, magnetite, pyrite, pyrrhotite, siderite, sphalerite) were placed into deep karst sinkholes that offer access to groundwater, and left in place in three zones: fresh, saline, and fresh-saline water interface. After four months the biofilm DNA was extracted from the minerals in the traps. The microbial diversity was assessed as DNA fingerprint from randomly amplified polymorphic DNA analysis, and the fingerprints were evaluated for diversity and similarity. The results show that the microbial communities on different minerals vary most in the aerobic zone, where the microbial diversity was greatest on the Fe²⁺ containing minerals (chalcocite, pyrite, pyrrhotite, siderite, sphalerite). However, the microbial communities were also most different on the Fe²⁺ minerals in the freshwater zone and very similar in the interface and saline zones. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to evaluate the changes on the mineral surfaces. Most changes to the mineral surface were observed both in the oxidative (freshwater) and reducing (saline) layers to the iron containing minerals. Framboidal pyrite formation was seen on several iron-containing minerals under reducing conditions, but only in the dark conditions, suggesting biogenic origin of the framboids.