Carbonate Disequilibrium in the External Boundary Layer of Freshwater Chrysophytes: Implications for Contaminant...

The interplay between biological and chemical reactions in the freshwater phytoplankton phycosphere and the resulting modulations of contaminant speciation and uptake is poorly characterized. Here we modeled the effect of algal C and N uptake on carbonate cycling and speciation of selected contaminants in the phycosphere (external boundary layer) of chrysophytes, a key phytoplankton group in oligotrophic systems. We calculated an enrichment in H⁺ concentration relative to that in the bulk solution (pH 7.0) of approximately 40% or a depletion of approximately 30% for NH₄⁺ or NO₃^--grown cells, respectively, at the algal membrane surface of a 5-μm radius cell. Such changes are mainly due to direct H⁺ uptake or release at the plasmalemma if NO₃^- or NH₄⁺ is the N source, respectively. Due to these pH changes in the external boundary layer, competition between H⁺ and metals for uptake is enhanced, for NH₄⁺-grown cells which contributes to a decrease in potential metal uptake. Our model suggests that the uptake of protonated weakly acidic organic acids (HA) is greater in NH₄⁺-grown cells compared to that in NO₃^--grown cells. The account of chemical reactions in the algal external boundary layer could improve ecological risk assessments for a wide range of contaminants.