Accounting for dissociation and photolysis: A review of the algal toxicity of triclosan

Abstract

Triclosan, an antimicrobial agent commonly used in down-the-drain consumer products, is toxic to freshwater microalgae. However, the rapid photolysis and pH-dependent dissociation of this compound may give rise to uncertainty in growth inhibition tests with freshwater microalgae, if these are not well characterized. Methods are presented to minimize these uncertainties by stabilizing pH with an organic buffering agent (Bis-Tris) and by the application of ultraviolet (UV) covers to remove UV wavelengths. Toxicity tests with these methods were in compliance with the validity criteria of the Organisation for Economic Co-operation and Development test 201, and no negative effects were seen in controls relative to the unmodified method. The methods were used for toxicity tests with triclosan at pH levels of 7.0, 8.0, and 8.5, yielding effective concentration, 10% values of 0.5 µg/L, 0.6 µg/L, and 12.1 µg/L, respectively. The observed change in toxicity with pH was proportional to the change in bioconcentration factor (BCF) as calculated using the cell model (a dynamic flux model based on the Fick-Nernst-Planck equations, in this case parameterized for an algal cell). Effect concentrations produced with the methods presented in the present study offer robust data on which to base risk assessment, and it is suggested that similar approaches be used to minimize uncertainty when other compounds that dissociate and photolyse are tested.

Effects of triclosan on aquatic invertebrates in tropics and the influence of pH on its toxicity on microalgae

“…The neutral form of TCS, at lower pH, has been reported to be more toxic than its ionized form (Rendal et al 2011 ). The effect of test media pH on TCS toxicity for microalgae has previously been reported for Scenedesmus subspicatus in which toxicity increases with decreasing pH until all TCS is present in an unionized state, then further decrease in pH has no influence as TCS becomes fully neutral at pH 6 and fully ionized at pH 10 (Roberts et al 2014 ). Therefore, there is still a need to investigate these effects for other algal species.…”

“…Two experiments were conducted in accordance with OECD guideline 201 (OECD 2006 ). One followed the guideline without modifications, but a second was conducted with two modifications adapted from the experiment of Roberts et al ( 2014 ), including (1) pH control using 4.5-mM Tris buffer at target pH level of 7.5 and (2) an acclimatization period of 2 × 72 h in the algal medium contained Tris buffer prior to the test. Tris buffer, or Tris(hydroxymethyl)methylamine, NH 2 C(CH 2 OH) 3 , ≥99.8 % Purity, was purchased from UNIVAR.…”

“…However, we found that the algae growth in the control cultures was a bit less than the OECD recommendation (<16-fold) within the 72-h test period. Therefore, the initial cell density of the algal test with Tris buffer was adjusted to 5 × 10 4 cell/mL following the OECD guidelines and Roberts et al ( 2014 ) that resulted in algae growth in the control cultures > 16-fold. Each test consisted of six TCS concentrations (0.5, 1.2, 3.1, 7.8, 19.5, and 48.8 μg/L) in three replicates.…”

“…Many historic toxicity studies have not adequately considered the bioavailability of TCS in test media. TCS is a weak acid with p K a of 8.1 (Roberts et al 2014 ) and can rapidly undergo photolysis (degradation half-life <30 min under sunlight irradiation, Tixier et al 2002 ). The neutral form of TCS, at lower pH, has been reported to be more toxic than its ionized form (Rendal et al 2011 ).…”

Effects of triclosan and triclocarban on the growth inhibition, cell viability, genotoxicity and multixenobiotic resistance responses of Tetrahymena thermophila

“…(Brausch and Rand, 2011;Bedoux et al, 2012;Dann and Hontela, 2011;Hwang et al, 2014). It was found that TCS is highly toxic to algae (Roberts et al, 2014). For example, the 96 h growth inhibition EC 50 (median effective concentration) of TCS for Pseudokirchneriella subcapitata was 0.53 lg L À1 (Yang et al, 2008), which is even an order of magnitude below the aforecited maximum concentration of TCS in surface water.…”