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Toxicity to aquatic algae and cyanobacteria

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The 72-hour toxicity evaluations for D-glucono-1,5-lactone in aquatic algae (Desmodesmus subspicatus) were partially read-across from that of sodium D-gluconate.  The results of the 72-hour toxicity evaluations for sodium D-gluconate in aquatic algae, indicated that no inhibition of cell growth occurred at 100 mg/l; however, at 1000 mg/l, cell growth was inhibited by 70%.  Furthermore, no inhibition of average specific growth rate was reported to occur at 100 mg/l.  At 1000 mg/l the specific growth rate was inhibited by 42%.  The 72-hour EC50 for D-glucono-1,5-lactone was reported to be greater than 100 mg/L, based on stimulation of Desmodesmus subspicatus biomass production.

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The toxicity of D-glucono-1,5-lactone in aquatic algae was partially read across from study data generated for sodium D-gluconate. One study was performed to assess the toxicity of sodium D-gluconate to freshwater algae species Desmodesmus subspicatus (Stiene, 2001). The study was conducted according to OECD Guideline 201 with the following deviations: the use of 0.08 mg/L FeCl3·6H20, illumination between 8900 and 9300 lux, and the initial biomass concentration was 10 x 10E4 cells/mL. This study also was conducted according to EU Method C.3 with the same exceptions noted above for OECD Guideline 201. The 72-hour results indicated that no inhibition of cell growth occurred at 100 mg sodium D-gluconate/L; however, at 1000 mg sodium D-gluconate/L, the cell growth was inhibited by 70%. No inhibition of average specific growth rate was observed at 100 mg sodium D-gluconate/l; however, a 42% inhibition of specific growth rate was observed at 1000 mg/L.

Rübelt (1997) conducted an acute toxicity study in Desmodesmus subspicatus (reported as Scenedesmus subspicatus) using D-glucono-1,5-lactone. The study was not conducted according to GLP; however, it followed OECD Guideline 201 with exceptions. The 72-hour EC50 for D-glucono-1,5-lactone was reported to be greater than 100 mg/L, based on stimulation of algae biomass production. Based on the deviations from OECD protocols and the lack of compliance with GLP, this study was rated ‘3’ (unreliable) according to Klimisch standards.

D-glucono-δ-lactone is a cyclic ester of gluconic acid which, in aqueous solution, forms an equilibrium mixture of the lactone and gluconic acid. Gluconic acid is a somewhat weak carboxylic acid with a dissociation constant of pKa = 3.6. The dissociation of an acid into a proton and an anion is an equilibrium, the reverse of which is the re-association of that same anion with a proton to reform the original acid. The pKa of 3.6 means that, when the ambient pH = 3.6, half the gluconic acid molecules will exist in the form of the uncharged acid, and half as the anion. At pH < 3.6, the undissociated form will predominate, and pH > 3.6 the anion will predominate. Sodium gluconate and potassium gluconate are both 1:1 salts of gluconic acid, which will each dissolve in water to generate separate sodium or potassium cations and gluconate anions. Sodium and potassium are both strong bases, and are therefore expected to remain ionized at essentially any pH, but the gluconate anions deriving from the salts will be subject to the same equilibrium as those deriving from the free acid. To be in equilibrium, both the forward and the backward reaction must possess the same pKa value, so the gluconate anion is predicted to posses the same pKa of 3.6 as the free acid. In this way, gluconic acid in aqueous solution is in equilibrium with its cyclic esters and its anion, according to the pH of the system, and in any system with sufficient buffering capacity, the effects of introducing equimolar amounts of gluconic acid, D-glucono-δ-lactone, sodium gluconate or potassium gluconate would be indistinguishable. Hence these four substances are considered to be appropriate surrogates for each other in sufficiently buffered aqueous systems, such as environmental waters, flora and fauna.