Urban streams receive Cu from both treated wastewater and stormwater runoff sources. Both sources have significant quantities of organic matter (OM) which is known to control Cu speciation and bioavailability. The purpose of this study was to experimentally assess and model Cu biouptake to the freshwater algae Selenastrum capricornutum, including both internalization and surface attachment, in the presence of different source and size fractions of OM. Dissolved OM was either left intact or separated into small (0-650 Da), medium (650 Da-50 kDa), or large (50 kDa-0.45 μm) size fractions using tangential flow filtration (TFF). Conditional stability constants of the OM were determined using competitive ligand exchange-solid phase extraction. Metal ligand concentrations were determined from Cu titrations. Conditional stability constants of the whole effluent OM were 0.5 to 1 log unit higher than those of stormwater OM for both hydrophilic and hydrophobic OM (Hydrophilic: Log KCu-eff-OMf = 8.2±0.2, Log KCu-storm-OM = 7.5±0.3; hydrophobic OM: Log KCu-eff-OM = 8.9±0.4, Log KCu-storm-OM = 7.9±0.1). The stability constants of the source OM were all higher than those of algae (Log KM-total = 6.3 and Log KM-intra = 6.8). Based on short-term uptake experiments, the results show there was 46% less total Cu uptake in the presence of effluent OM than stormwater OM, while intracellular Cu was about 25% less with effluent. This difference corresponded to the higher conditional stability constants measured for effluent OM than stormwater OM. We also found Cu mainly bound hydrophilic OM (80%) in effluent while it mainly bound hydrophobic OM (77%) in stormwater. Incorporating the conditional stability constants and metal ligand concentrations into an algal uptake model, the Cu uptake could be predicted. As for attachment to algal surfaces, stormwater OM promoted more accumulation of Cu on the algal surface, but this accounted for a small percentage of the total algal Cu content. Furthermore, there was a difference in Cu biouptake and stability constants between effluent and storm size fractions. For effluent OM, the Cu uptake and stability constants across size fractions were similar to the whole effluent. For stormwater OM the Cu stability constants were the lowest for the small size stormwater OM, resulting in more Cu uptake as stormwater OM size decreased. Differentiation between OM sources could lead to better predictions of metal uptake.

Organic matter size and source in urban systems controls Cu biouptake by Selenastrum capricornutum

255th National Meeting (2018)

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