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

Abstract

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 NH or NO-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 NO or NH 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 NH-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 NH-grown cells compared to that in NO-grown cells. The account of chemical reactions in the algal external boundary layer could improve ecological risk assessments for a wide range of contaminants.

Unravelling Metal Speciation in the Microenvironment Surrounding Phytoplankton Cells to Improve Predictions of Metal Bioavailability

“… 57 However, when DIC is more limiting than inorganic nitrogen, we found that nitrogen sources can be the dominant factor in influencing extracellular pH ( Figure S3 ). Similarly, the recent modeling data by Lavoie et al 19 on freshwater chrysophytes suggested that nitrogen uptake influenced phycosphere pH more than DIC uptake. At present, the precise molecular pathways involved in modulating phycosphere pH remain speculative.…”

“… 32 , 33 An example of the calculation of Cu complexation kinetic by DOC in the phycosphere is given in the SI ( Note S3 ), and the results suggested that Cu equilibrated with DOC in this thin layer. The phycosphere thickness was simply assumed to be equivalent to one cell radius; 19 , 34 the actual thickness might be altered by factors such as algal shape/size and cell movement relative to water. 35 …”

“…Note, we calculated the influence of pH on metal speciation with the assumption of thermodynamic equilibrium being reached in the phycosphere, because complexation of most metals by natural ligands including exudates and DOC is relatively fast. , An example of the calculation of Cu complexation kinetic by DOC in the phycosphere is given in the SI (Note S3), and the results suggested that Cu equilibrated with DOC in this thin layer. The phycosphere thickness was simply assumed to be equivalent to one cell radius; , the actual thickness might be altered by factors such as algal shape/size and cell movement relative to water …”

“… 16 In contrast, Eichner et al 17 reported that the utilization of Fe minerals by Trichodesmium colonies was little influenced by changes in pH and O 2 in and around the colonies. Moreover, two modeling studies showed that speciation of Si 18 and Cd 19 in the phycosphere differed from that in the bulk media. Here, we hypothesize that metal bioavailability can be better predicted by taking phycosphere metal speciation into account, rather than that in bulk waters.…”

Comparative Plastid Genomics of Non-Photosynthetic Chrysophytes: Genome Reduction and Compaction

“…Photosynthetic chrysophytes are generally mixotrophs that rely on the ingestion of organic nutrients, with the exception of the phototrophic synuralean lineage (Graupner et al, 2018;Lie et al, 2018). The chrysophyte Hydrurus foetidus forms filamentous branches within a soft polysaccharide coat and is a putatively facultative phototrophic species (Klaveness and Lindstrøm, 2011;Lavoie et al, 2018). Genome size and cell size differ significantly among species exhibiting different nutritional modes (Olefeld et al, 2018).…”