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.