Nutrient limitation and microbially mediated chemistry: studies using tuff inoculum obtained from the Exploratory Studies Facility, Yucca Mountain

Flow-through bioreactors are used to investigate the relationship between the supply (and limitation) of major nutrients required by microorganisms (C, N, P, S) and effluent chemistry to obtain data that can be useful to develop models of microbially mediated aqueous chemistry. The bioreactors were inoculated with crushed tuff from Yucca Mountain. Six of the 14 bioreactor experiments currently in operation have shown growth, which occurred in as few as 5 days and as much as a few months after initiation of the experiment. All of the bioreactors exhibiting growth contained glucose as a carbon source, but other nutritional components varied. Chemical signatures of each bioreactor were compared to each other and selected results were compared to computer simulations of the equivalent abiotic chemical reactions. At 21 C, the richest medium formulation produced a microbial community that lowered the effluent pH from 6.4 to as low as 3.9. The same medium formulation at 50 C produced no significant change in pH but caused a significant increase in Cl after a period of 200 days. Variations in concentrations of other elements, some of which appear to be periodic (Ca, Mg, etc.) also occur. Bioreactors fed with low C, N, P, S media showed growth, but had stabilized at lower cell densities. The room temperature bioreactor in this group exhibited a phospholipid fatty acid (PLFA) signature of sulfur- or iron-reducing bacteria, which produced a significant chemical signature in the effluent from that bioreactor. Growth had not been observed yet in the alkaline bioreactors, even in those containing glucose. The value of combining detailed chemical and community (e.g., ester-linked PLFA) analyses, long-duration experiments, and abiotic chemical models to distinguish chemical patterns is evident. Although all of the bioreactors contain the same initial microorganisms and mineral constituents, PLFA analysis demonstrates that both input chemistry and temperature determine the character of the long-term population of microorganisms. Where microbial growth occurs, that community can impact the chemistry of water significantly. These principles are well known, but we note their relevance to modeling microbially mediated chemistry. We recognize, in addition to microbial growth, three categories of chemical effects, each of which will require a different approach and constitutive equation(s): (1) unidirectional bacterial modification of the chemistry (i.e., pH) that is directly related to the dominance of particular species, (2) secondary impact of direct microbial modifications (i.e., increased dissolution of solids as a result of reduced pH), and (3) cyclical effects that may be attributed to internal regulation (e.g., osmoregulation or internal pH regulation) or evolution of the microbial community