Effect of Fe (III) on Pseudokirchneriella subcapitata at circumneutral pH in standard laboratory tests is explained by nutrient sequestration

Abstract

The complex chemistry of iron (Fe) at circumneutral pH in oxygenated waters and the poor correlation between ecotoxicity results in laboratory and natural waters have led to regulatory approaches for iron based on field studies (US Environmental Protection Agency Water Quality Criteria and European Union Water Framework Directive proposal for Fe). The results of the present study account for the observed differences between laboratory and field observations for Fe toxicity to algae (Pseudokirchneriella subcapitata). Results from standard 72-h assays with Fe at pH 6.3 and pH 8 resulted in similar toxicity values measured as algal biomass, with 50% effect concentrations (EC50) of 3.28 mg/L and 4.95 mg/L total Fe(III), respectively. At the end of the 72-h exposure, however, dissolved Fe concentrations were lower than 30 μg/L for all test concentrations, making a direct toxic effect of dissolved iron on algae unlikely. Analysis of nutrient concentrations in the artificial test media detected phosphorus depletion in a dose-dependent manner that correlated well with algal toxicity. Subsequent experiments adding excess phosphorus after Fe precipitation eliminated the toxicity. These results strongly suggest that observed Fe(III) toxicity on algae in laboratory conditions is a secondary effect of phosphorous depletion. Environ Toxicol Chem 2017;36:952-958. © 2016 SETAC.

Toxicity of dissolved and precipitated forms of barium to a freshwater alga (Chlorella sp. 12) and water flea (Ceriodaphnia dubia)

“…This finding suggests either that precipitated barium sulfate was more toxic than dissolved barium or that other, indirect, mechanisms of toxicity were at play in the presence of the precipitate that contributed to the overall algal response. Such indirect mechanisms of toxicity have been observed for iron(III) precipitation in freshwater algal tests in which phosphorus becomes limiting in the test media with increased iron hydroxide precipitation (Arbildua et al ). In the algal tests of the present study conducted in SSW in which precipitation of barium sulfate occurred, phosphorus concentrations also decreased with increasing barium concentrations but did not decrease significantly ( p < 0.05) until concentrations of 100 mg Ba/L or more, at which point only 9 to 25% of the initial phosphorus remained (data not shown).…”

Improving the Measurement of Iron(III) Bioavailability in Freshwater Samples: Methods and Performance

“…Lastly, operationally defined dissolved iron has been found to correlate strongly with DOC levels in natural waters (Allard et al, 2011 ; Fujii et al, 2008 ), but the complexation of iron by DOC is associated with lower bioavailability (Arbildua et al, 2017 ). Complexed soluble iron can be recovered by other methods such as the acid‐soluble method applied in these experiments.…”

“…However, it is interesting to understand the potential ramifications of the findings of the present study given that toxicity data from the literature comes from a wide range of colloidal iron ages (in many cases not specified). For example, Biesinger & Christensen ( 1972 ) derived chronic 16% effect concentration values for Daphnia magna after exposure to 4‐day‐old iron suspensions, whereas Arbildua et al ( 2017 ) and Sykora et al ( 1972 ) used 3‐ and ~2‐h‐old suspensions in their experiments, respectively. Yoshida et al ( 2006 ) demonstrated how iron(III) uptake in diatoms substantially decreased when comparing treatments with direct addition of Fe 3+ versus iron colloids aged for 1 and 3 days, and up to 3 weeks.…”

“…High levels of iron in the aquatic environment can lead to negative impacts, affecting habitat conditions and directly or indirectly exerting chemical or physical toxicity on organisms and communities (e.g., Kotalik et al, 2019 ; and references therein). Laboratory ecotoxicological assessments have detected toxicity at concentrations above the solubility limit of iron(III) and therefore it is suspected that precipitates contribute to the observed toxicity response (e.g., Arbildua et al, 2017 ). However, from a toxicodynamic perspective, it is not clear what fractions and mineral phases are bioavailable and responsible for toxicity to freshwater organisms (Australian and New Zealand Governments and Australian State and Territory Governments [ANZG], 2020 ).…”

Chronic Toxicity of Iron to Aquatic Organisms under Variable pH, Hardness, and Dissolved Organic Carbon Conditions

“…In the majority of the algal tests, the pH was highest in the controls and decreased as Fe concentrations increased. Tests performed by Arbildua et al (2017) at CIMM with algae exposed to Fe achieved significantly better pH control through the use of MES and HEPES buffers. In the tests with C. dubia and P. promelas using buffers, average pH remained relatively stable over the course of the exposures at ±0.2 SU of target pHs.…”