Surface chemical reactivity and metal adsorptive properties of natural cyanobacterial mats from an alkaline hydrothermal spring, Yellowstone National Park

Abstract

The propensity for microbes to adsorb dissolved metals onto their surfaces has been well documented, to the point where predictive surface complexation models can accurately account for these reactions under experimental conditions. However, critical surface chemical parameters, such as surface functional group concentrations and proton stability constants, have only been evaluated using laboratory cultures. Whether or not natural microbes are comparable in surface chemical reactivity to laboratory cultures, and whether they display variations across diverse populations, remains untested. To resolve this, we examined natural cyanobacterial mats of various morphologies (i.e., streamers, vertical spires, horizontally laminated structures), sampled from a single hydrothermal system in Yellowstone National Park, in terms of surface chemical parameters and acid-leachable metal contents. Potentiometric titration data of samples that were acid-washed to remove sorbed metals and reveal underlying organic surfaces indicated functional group concentrations of 0.98 ± 0.28 to 2.84 ± 0.41 mmol g − 1 (dry weight) summed over a pK a range of 4 to 10, which is comparable to previously reported experimental values. In contrast, samples that were not acid-washed, but merely rinsed in titration electrolyte adjusted to stream pH, had functional group concentrations ranging from 6.12 ± 1.39 to 19.23 ± 3.14 mmol g − 1 . They were also largely dominated by a single functional group of pK a ∼ 7 that may be explained by the presence of aqueous or solid phase metal carbonate species that are removed from the mats by acid-washing. Analysis of the acid-wash solutions indicate that different metals were concentrated to varying extents, and that metals with low metal-carbonate solubility products, such as Ba, Ca, Fe, Mg, Mn, Ni, Sr, and Zn, were preferentially concentrated by the mats, perhaps as the result of precipitation as, or complexation with, mat-hosted carbonate species. These results highlight the complexity of metal partitioning in natural microbial communities, where a variety of processes other than surface adsorption, such as metabolism, authigenic mineral precipitation, and the physical entrapment of detrital material, may contribute to metal sequestration.