The effect of ionic solutes on EC₅₀ values measured using the microtox test

Abstract

The Microtox test uses the marine luminescent bacterium, Photobacterium phosphoreum. To provide the organism with osmotic protection, sodium chloride (2.0%) is usually added to freshwater samples before analysis. This procedure is shown to cause a dramatic lowering of measured toxicity where ionic compounds are the major toxicants present. The reason for this is the consequent variation in ionic strength upon addition of NaCl rather than formation of less-toxic chloro complexes. Data are given for the toxicants Zn, Cd, Cu, NH4+, and phenol.It was found that measurements on aqueous samples could be made without adjustment provided that the salinity was in the range of 1.0-7.0%. Freshwaters can also be analyzed without adjustment of ionic strength by adding sucrose (20.4%) to provide the necessary osmotic protection.It is therefore recommended that the ionic strength of aqueous samples should not be altered as part of the test and that receiving waters should be used as the diluent wherever possible.

Separation of pH, dilution, lonic strength and chemical matrix effects for biological monitoring of urines with the Microtox® test using nicotine, cotinine and reference urines

“…The 2.0% sodium chloride concentration is the lowest concentration in the optimum luminescence range. Other investigators have reported the protective effect of increasing saline concentrations within our optimum range on toxicant EC50 values (27,28). This present study justifies the use of 2% saline (22) for sensitivity reasons.…”

Toxicity of nonionic surfactants

“…A higher effective concentration is interpreted as lower toxicity. Hinwood and McCormick [13] have described the details of the experimental procedure. These authors also indicated that log Γ increases with concentration only up to a certain point beyond which no further increase can be observed.…”

Toxicity of cadmium species on luminescent bacteria

“…One of these widely used bioassays is the Microtox R assay, a rapid screening alternative to toxicity tests with higher aquatic organisms, such as fishes and invertebrates, applies the luminiscent bacterium Photobacterium phosphoreum as the test organism [4,5]. Nevertheless, studies concerning metal species toxicity to the luminiscent bacteria are scarce [6][7][8].…”