Role of thermophilic microorganisms in the precipitation of heavy metals from subaerial hot springs of the Taupo Volcanic Zone, North Island, New Zealand : a microcosm study
The active precipitation of precious and heavy metals at the surface of hot springs in the Taupo Volcanic Zone (TVZ), North Island, New Zealand, provides a modern analogue for fossilised epithermal metal deposits found throughout the globe. This project explores the relationship, if any, between the microbial populations in these springs and the elevated concentration of metal(loid)s in their precipitates by attempting to replicate the geochemical environment present at the surface of the springs in the laboratory through a series of microcosm experiments. Molecular analysis of microbial mat samples from Waimangu found diverse microbial communities present in the microbial mats, each with a unique community structure. A wide variation in the number and type of microbial phylotypes was found in the spring waters with both bacterial and archaeal DNA amplified. A rod shaped, Gram-positive bacterium was isolated from sediment samples from Echo Crater. Sequence analysis revealed this bacterium has 99% identity with the 16S rRNA gene sequence of Alicyclobacillus herbarius, an acidophilic Gram-positive bacterium, which grows aerobically at an optimum temperature of 55-60°C and an optimum pH of 4.5-5.00. The colonies of the A. herbarius-like isolate appear to be surrounded by a mucous-like material, which may be an exopolysacchride (EPS). Some EPS are known to precipitate metals from dilute solutions and the ability of an EPS layer from the isolate to precipitate metal(loid)s would be of particular interest. A microcosm experiment was constructed to attempt to replicate the geochemistry of the surface hydrothermal environment. Initially, abiotic experiments were conducted to determine the effect of pH on the solubility of metal(loid)s, commonly found in the precipitates of these systems, in the presence of iron and sulphur in the form of sulphate. Though many surface hydrothermal fluids contain sulphur as sulphide, sulphide was not included due to experimental difficulties in preventing loss to oxidation prior to the fluid entering the microcosms. Samples of the precipitates, suspended material and the solution were gathered and analysed using a variety of techniques to determine their chemical and mineralogical composition. It was found that while the precipitates in the AMEs partially resembled precipitates from the natural systems, the lack of sulphide in the fluid prevented the formation of iron pyrite and other metallic sulphides, which are abundant in the natural systems. Interestingly, particles of gold between one to ten microns in width were precipitated in one of the AMEs. The precipitation of hexagonal gold microparticles within the microcosms could represent a new environmentally friendly method for producing these platelets for use in micro- and nanoparticle technology along with other elements of interest.