Chronic toxicity of an environmentally relevant and equitoxic ratio of five metals to two Antarctic marine microalgae shows complex mixture interactivity

Abstract

Metal contaminants are rarely present in the environment individually, yet environmental quality guidelines are derived from single-metal toxicity data. Few metal mixture studies have investigated more than binary mixtures and many are at unrealistically high effect concentrations to freshwater organisms. This study investigates the toxicity of five metals (Cd, Cu, Ni, Pb, and Zn) to the Antarctic marine microalgae Phaeocystis antarctica and Cryothecomonas armigera. Two mixtures were tested: (i) an equitoxic mixture of contaminants present at their single-metal EC10 concentrations, and (ii) an environmental mixture based on the ratio metal concentrations in a contaminated Antarctic marine bay. Observed toxicity, as chronic population growth rate inhibition, was compared to Independent Action (IA) and Concentration Addition (CA) predictions parameterised to use EC10 values. This allowed for the inclusion of metals with low toxicities. The biomarkers chlorophyll a fluorescence, cell size and complexity, and intracellular lipid concentrations were assessed to investigate possible mechanisms behind metal-mixture interactions. Both microalgae had similar responses to the equitoxic mixture: non-interactive by IA and antagonistic by CA. Toxicity from the environmental mixture was antagonistic by IA to P. antarctica; however, to C. armigera it was concentration-dependent with antagonism at low toxicities and synergism at high toxicities by both IA and CA. Differences in dissolved organic carbon production and detoxification mechanisms may be responsible for these responses and warrants further investigation. This study shows that mixture toxicity interactions can be ratio, species, and concentration dependent. The responses of the microalgae to different mixture ratios highlight the need to assess toxicity at environmentally realistic metal ratios. Parameterising IA and CA reference models to use EC10s allowed for the inclusion of metals at low effect concentrations, which may otherwise be ignored. Reference mixture models are generally suitable for predicting chronic toxicity of metals to these marine microalgae at environmentally realistic ratios and concentrations.

Correlated Ni, Cu, and Zn Sensitivities of 8 Freshwater Algal Species and Consequences for Low‐Level Metal Mixture Effects

“…The overall large variations in metal sensitivities and mixture interactions among the species suggest that conclusions on mixture effects cannot be generalized across species, even for those coming from the same trophic level. Similar conclusions were drawn by Koppel et al (2018), who tested environmentally relevant metal mixtures of Cd, Cu, Ni, Pb, and Zn on 2 marine algal species.…”

Effects of copper on the dinoflagellate Alexandrium minutum and its allelochemical potency

“…However, in areas with high Cu concentrations, Cu is unlikely to be the only metal or contaminant at high concentrations. Future studies should therefore focus on other metals alone but also in mixtures with relevant ratio metal concentrations (Koppel et al, 2018). Overall, different ecotypes (Kobiyama et al, 2010) may have different sensitivities to Cu and different responses.…”

Assessing the Risk of Metals and Their Mixtures in the Antarctic Nearshore Marine Environment with Diffusive Gradients in Thin-Films

“…EC10i of single-metal log-logistic concentration-response curves was used to parameterize the independent action (IA, Equation 3) and concentration addition (CA, Equation 4) toxicity mixture models 32 , where xi is the DGT-labile metal concentration and y is the predicted population growth rate inhibition for metal i. The parameters used in these models for each species are provided in Supplementary Table 4.…”