Zackry Stevenson (MICRO PhD ’24) recently published a paper describing a field study that took place right on the ISU campus. Zak measured water quality parameters in a stretch of the stream that runs through Pammel Woods over a couple of years to try to figure out what conditions led to the formation of rusty iron deposits. He focused in on mineral films, which look like oil spilled on water except they break up rather than swirl when touched. He used electron microscopy to show these films are made up of iron oxides and are just tens of nanometers thick. Zak sequenced DNA from the different iron deposits to get a sense of which putative iron cycling bacteria were involved in their formation. To assess whether the iron could be entering the stream from shallow groundwater, Zak installed shallow wells called piezometers into the streambank. This infrastructure is now in use by ISU classes. After this study, we have a better sense of why iron deposits form in this particular area – because of the shallow water table and naturally iron-rich soils, and ponding in the streams from debris, which lead to slowed streamflow and the development of pools that receive discharge of iron-rich groundwater. Microbes then cycle iron between dissolved/reduced and particle/oxidized forms to generate the deposits we observe.
You can find these deposits for yourself if you walk the creek bed downstream from the pedestrian footbridge when water is present in the stream, but predominantly in stagnant pools. If the creek is full of flowing water, the deposits get washed away.
Image is of a floating iron surface film.
Link to article: https://www.cambridge.org/core/journals/geo-bio-interfaces/article/hydr…