Redox speciation and biogeochemical gradients: Assessing spatial niches and monitoring dynamics in natural systems with voltammetric microelectrodes

Biogeochemical gradients may be described by the spatial distribution of redox species distributed in water, where overlap of electron donors and acceptors out of equilibrium defines available sources of potential energy and essentially determines possible microbial metabolisms. Observed changes in redox speciation along a gradient associated with microbial biofilms may additionally provide some environmental basis for assessing physiology of sampled microorganisms. Voltammetric microelectrodes have been used in a variety of environments to describe the links between ecology and geochemistry (Luther et al., 2001). Recent work in Yellowstone National Park hydrothermal waters, the Frassassi caves in central Italy (a sulfidic cave system), and Green Lake in New York (a meromictic lake) have expanded our abilities to use microelectrodes for assessing As(III) concentrations and uncovering more details of sulfur speciation in a wide range of natural waters. We are using these data to design redox-specific culture media, make inferences about microbial physiology, constrain biogeochemical gradients over very fine scales, and observe dynamics in biogeochemical systems. Describing microbial communities and the geochemical environments that surround them at appropriate scales is of importance to begin assessing the links between microbial activity and geochemical cycling. Diversity in an environment may be better assessed if we first know how many different geochemical environments there are in that environment and if the microbial ecology in those environments is essentially independent from environments neighboring it. Because microelectrodes measure multiple redox species simultaneously and do so in matter of seconds, they are also useful in monitoring the dynamics of a biogeochemical system, which will be of use in studying the response of communities to perturbation. We will present results showing the characterization of lateral and vertical gradients over different scales, the application of voltammetry to infer physiology of as-yet uncultured organisms, and the monitoring of geochemical dynamics affecting systems on variable time scales. Luther, G.W., Rozan, T.F., Taillefert, M., Nuzzio, D.B., Di Meo, C., Shank, T.M., Lutz, R.A., and Cary, S.C., 2001: Chemical Speciation Drives hydrothermal vent ecology. Nature, V. 410, Iss. 6830, p. 813-816