USING SEDIMENT-WATER MICROCOSMS AS TEACHING TOOLS FOR UNDERSTANDING LOW-TEMPERATURE GEOCHEMICAL PROCESSES

Microcosms (self-sustaining ecosystems) are a common research tool used to re-create natural environments in the laboratory, so that otherwise inaccessible processes can be examined and studied. They are particularly useful for evaluating microbial activities, but they can also be used as educational tools when teaching topics related to low temperature aqueous geochemistry. Because microorganisms are present in great abundance in nearly every environment on Earth and they often play an intimate role in mediating geochemical reactions. For example, bacteria often act as catalysts for geochemical reactions occurring in natural environments; meaning that, while thermodynamics would predict these reactions to happen abiotically, the bacteria greatly speed up these otherwise imperceptibly slow reactions. This can be a challenging geochemical concept for students to grasp yet it is easily demonstrated with microcosms. Microcosms also give us a “sneak peak” into what is happening in complex natural environments by allowing us to research various geochemical processes, such as reduction/oxidation (redox) reactions, that are mediated by bacteria. With these ideas in mind, microcosms experiments can be easily optimized for observation of a variety of processes. In my undergraduate geochemistry courses students construct (with my assistance) microcosms for semester long evaluation. They design individual experiments, regularly observe and document visual changes, collect fluids for chemical analysis, plot data and determine reaction rates. Eventually they draw conclusions and submit a formal report. The typical microcosm consists of sediment from a natural system saturated with a native water, supplemented with various electron acceptors and donors, and sealed off. The bacteria are then allowed to grow in response to patterns (or zones), reflecting the substrate and aerobic concentrations throughout the microcosm. Experimental design can be simple or examine effects of added pollutants, oxidizing verses reducing conditions, exposure to sunlight, and so on. Microcosms provide students an inquiry-based lab experience tied to fundamental biogeochemical processes, while also helping them gain appreciation for effects of environmental change.