The hot springs in Yellowstone National Park, USA, provide concentrated microbial biomass and associated mineral crusts from which surface functional group (FG) concentrations and pKa distributions can be determined. To evaluate the importance of substratum surface reactivity for solute adsorption in a natural setting, samples of iron-rich sediment were collected from three different springs; two of the springs were acid-sulfate-chloride (ASC) in composition, while the third was neutral-chloride (NC). At one of the ASC springs, mats of Sº-rich Hydrogenobaculum-like streamers and green Cyanidia algae were also collected for comparison to the sediment. All samples were then titrated over a pH range of 3–11, and comparisons were made between the overall FG availability and the concentration of solutes bound to the samples under natural conditions. Sediments from ASC springs were composed of hydrous ferric oxides (HFO) that displayed surface FGs typical of synthetic HFO, while sediments from the NC spring were characterized by a lower functional group density, reflected by decreased excess charge over the titration range (i.e., lower surface reactivity). The latter also showed a lower apparent point of zero charge (PZC), likely due the presence of silica (up to 78 wt. %) in association with HFO. Variations in the overall HFO surface charge are manifest in the quantities and types of solutes complexed; the NC sediments bound more cations, while the ASC sediments retained significantly more arsenic, presumably in the form of arsenate (H2AsO4−). When the microbial biomass samples were analyzed, FG concentrations summed over the titratable range were found to be an order of magnitude lower for the Sº-rich mats, relative to the algal and HFO samples that displayed similar FG concentrations on a dry weight basis. A diffuse-layer surface complexation model was employed to further illustrate the importance of surface chemical parameters on adsorption reactions in complex natural systems.