Mechanisms and effects of light-mediated zinc uptake by photosynthetic biofilm: Implications for diel metal cycling in mining-impacted streams

Concentrations of metals cycle daily in the water column of some mining-impacted streams and rivers in the Rocky Mountains of the western USA. Understanding the processes that drive diel cycling of metals will help guide regulatory decisions and help model metal transport and exposure to biota. I hypothesized that biofilm in High Ore Creek (HOC), Montana, sorbs and releases zinc (Zn) on a diel cycle, and this uptake-and-release controls aqueous total and dissolved (0.45-gm filtered) Zn concentrations in the water column. Based on field monitoring, streamside experiments, and laboratory experiments, I determined that (1) laboratory-cultured photosynthetic biofilm removes Zn from the water under light and releases Zn under darkness, (2) this uptake and release is driven by adsorption of Zn2+ and precipitation of Zn-containing minerals in the biofilm matrix at the high pH (>10) in the matrix caused by photosynthesis during light periods and by the reverse processes at lower pH caused by respiration during dark periods, (3) Zn is removed from the water column during light periods through diffusion of ionic Zn (Zn2+) down a concentration gradient from the water column into the biofilm matrix that is maintained through changes in Zn speciation in the water in the matrix and through adsorption of Zn2+ and (or) precipitation of Zn-containing minerals in the matrix, (4) biofilm growing in HOC is capable of removing enough Zn from the water column to drive the diel Zn cycle, and (5) a black Mn-Zn-oxide crust beneath the biofilm on the substrate in HOC appears to be the second compartment in a two-compartment light-mediated Zn uptake system that is a long-term sink for Zn, thus explaining why Zn is not released into the water column of HOC at night even though Zn is released from the biofilm matrix in darkness.