Copper and zinc isotope fractionation during phototrophic biofilm growth

Copper (Cu) and zinc (Zn) are two trace metals that exhibit both limiting and toxic effects on aquatic microorganisms. However, in contrast to good knowledge of these metal interactions with individual microbial cultures, the biofilm, complex natural consortium of microorganisms, remains poorly understood with respect to its control on Cu and Zn in the aquatic environments. Towards constraining the magnitude and mechanisms of Cu and Zn isotope fractionation in the presence of phototrophic biofilms composed of different proportion of diatoms, green algae and cyanobacteria, we studied long-term growth in a rotating annular bioreactor and quantified the uptake of metals and their isotope fractionation at environmentally-relevant Cu and Zn concentrations. An enrichment of the biofilm in heavy Cu isotope at the beginning of growth suggests the dominance of adsorption processes, followed by intracellular uptake leading to progressive enrichment in light isotope and an excretion of heavy isotope, likely linked to Cu(II) reduction. In the case of Zn, we evidenced only weak isotope fractionation which implies the presence of heavier isotope adsorption (notably in the case of cyanobacteria-dominated biofilm) followed by intracellular incorporation of lighter isotopes. The microbial community plays important role in overall magnitude and even direction of fractionation, suggesting sizable complexity of the processes controlling metal isotope fractionation during phototrophic biofilm growth. However, Cu and Zn isotopes during long-term metal accumulation in riverine biofilm can be used for monitoring the source of environmental pollution in aquatic systems, provided that variations within different sources exceed the natural isotopic fractionation between the biofilm and aqueous solution.