Photosynthetic Diel Cycling Influences Inverse Zinc and Copper Solubility Dynamics within a Constructed Wetland.

Shallow unit process open water (UPOW) wetlands host a photosynthetic microbial biomat rather than emergent plants. Sunlight-associated maxima during diel cycles in water column pH (∼7-10) and dissolved oxygen (∼5-20 mg/L) within this nature-based construct have implications for biogeochemical processes and contaminant attenuation. In field applications, an inverse solubility dynamic was observed with decreases in zinc and increases in copper, in association with photosynthesis. Laboratory flow-through photosynthetic bioreactors amended with field-derived biomat reproduced key geochemical diel variables and harbored similar microbial structure and function. Sequential extractions of solid phases within flow-through bioreactors and isotope-enriched microcosms revealed that Zn and Cu primarily accumulated in the chemically labile phases of the surficial water-biomat interface layer. Equilibrium modeling suggested that diel changes in Zn were influenced by calcite sorption/coprecipitation. However, rather than analogous precipitation at higher pH, Cu experienced prominent influences from organic associations with sediments and dissolved organic carbon ligands. Retrospective analysis of diel cycling in other published field and laboratory studies indicates a potential concentration-related diel cycling pattern for Cu but not for Zn. These findings collectively inform a more mechanistic understanding of photosynthetic influences on diel aqueous geochemical metal cycling in analogous systems such as wetlands and periphyton-influenced streams.