Characterising the flux of carbon between calcium carbonate substrata, aqueous fluids, bacteria and a biofilm matrix

Quantification of the flux of elements between minerals, biofilms and aqueous solutions is essential in order to fully elucidate the role of microorganisms in mass transfer processes. Bacteria play a fundamental role in nearly all life and environmental processes and are by far the most abundant organisms on the planet. Their main mode of growth is in the form of biofilms growing on surfaces and although extensive study has been carried out into the problems biofilms cause to humans, the effects of biofilms in the environment are still poorly understood. Much of the published research describing biofilm growth on geological substrata emphasises the species present rather than their effect on the substratum. Although previous studies have shown that the production of organic or mineral acids by microbial biofilms can cause dissolution of mineral substrata, there is little quantitative data on the specific flux of elements between minerals, microbial cells, biofilm matrix and aqueous solutions. With growing evidence that microbial life occurs in abundance in the subsurface, biofilm activity within buried rocks and sediments may have important implications for global geochemical cycling of specific elements. Results will be presented as part of an ongoing laboratory analog experiment which has been designed to quantify the flux of carbon between all matrices in a laboratory substratum-biofilm-aqueous system. Water collected from a limestone cave system (pH 7.5-8; viable count 4.7×10^3 - 2.0×10^4 cfu.ml^{-1{) is used as innoculum and nutrient source for growing mixed consortium biofilms on a natural calcite (CaCO_3) substratum in a flow-through reaction cell (24 days; 25^oC; 1ml.min^{-1}). Portions of the biofilm are periodically removed for microbiological analysis and the mineral surface microscopically examined for extent of alteration. A parallel experiment utilises an isotopically labelled synthetic ¹³C-CaCO_3 substratum. This enables carbon to be traced from the substratum, through the biofilm and into the aqueous and gas phase by analysing ¹³C/¹²C isotopic ratios in all system components. Through this mass-balance approach it is possible to quantify, for the first time, the carbon fluxes into the biofilm and the aqueous solution from the carbonate mineral substrate.