The effect of deicing salt solutes on Moina macrocopa and Allium cepa in a toxicity test experiment

The effect of deicing salt solutes on Moina macrocopa and Allium cepa in a toxicity test experiment Chloride salts are the most commonly used deicing materials for winter maintenance of roads. Numerous studies indicate a significant increase in the salinity of aquatic ecosystems associated with the long-term use of deicing materials in countries located in cold climates. The functioning of ecosystems largely depends on salinity, since salinity is one of the key factors determining the species composition, the structure of food webs and the productivity of aquatic communities. Given the growing threat of salinization of groundwater and surface waters, it is extremely important to study the effect of deicing materials on the biota and functioning of aquatic ecosystems. The aim of this research is to determine the threshold concentrations of solutions of the deicing salt mixture “Bionord” containing sodium and calcium chlorides, at which negative effects on the development of animal and plant test objects are observed. In this study, we used the salt-containing mixture “Bionord” as a model deicer. Similarly, with the most commonly used ice melting chemicals, the «Bionord» salt mixture contains a large amount of sodium and calcium chlorides (about 85% of the total weight). To evaluate the toxicity of the deicer solutions, we used acute and chronic toxicity tests with cladoceran Moina macrocopa (Straus, 1820) (Cladocera: Moinidae) and standard onion-based test with Allium cepa L. (Liliopsida: Amaryllidaceae) (Allium-test). In acute and chronic toxicity tests with Cladocera, the females on the first day of their life (body size 0.5-0.6 mm) were placed individually in jars with aged (not less than for 72 h) tap water with a volume of 20 ml with the addition of a deicer at a certain concentration. A group of animals that was placed in the medium without the deicer was used as a control. In the acute toxicity tests, we used the following concentration of the deicer: 1.3; 2.5; 4.0, 5.0; 6.0; 8.0; 10.0 g/l. The mortality of animals was recorded 24 and 48 hours after the start of the experiment. The concentration of the deicer (LC₅₀) at which 50% of animals was observed to die, compared to the control, was determined in the acute toxicity test. In a chronic toxicity test, animals were tested in the following range of concentrations of the deicer: 0.3; 0.6; 1.3; 2.5; 5.0; 6.0 and 8.0 g/l. The chronic toxicity test was conducted until the death of all test animals. Based on the data obtained in the chronic toxicity test, the specific growth rate of juvenile females, average fecundity, and average life span of M. macrocopa were calculated for each concentration of the deicer. Bulbs of onions of the Stuttgartenrisen variety with a diameter of 1.8 ± 0.1 cm and a weight of 2.27 ± 0.17 g were used in the onion test. Bulbs with their bottoms were placed in test tubes containing 20 ml of a solute of the deicer or tap water for 48 hours. Three bulbs were tested for each concentration and for the control. The following concentrations of the deicer were used in the onion test: 1.0; 2.5; 5.0; 7.0; 10.0; 15.0; 20.0; 50.0 g/l. The general toxic and cytotoxic effects were evaluated in the onion test. The average root length and the total root length on each bulb were used as indicators of the total toxicity of the solutions of deicer. To evaluate proliferative activity, we calculated the mitotic index as the fraction of dividing cells in the apical root meristem to the total number of cells. Based on the results of the experiments, we determined median effective mixture concentrations (EC₅₀) at which there is a 50% decrease, compared to the control, in the values of root growth indicators: average root length, sum of root lengths on each bulb and mitotic index. Median lethal concentration (LC₅₀) of the deicing salt determined in the 48-hour acute toxicity test with females of M. macrocopa was equal to 5.1 g/l. In the chronic test, we showed that the exposure to the solutions of the deicing salt in the range of concentrations from 0.3 to 5.0 g/l does not affect the life span, specific growth rate of juveniles and fecundity of females ofM. macrocopa. The median effective concentration (EC₅₀) of the deicing salt determined in the Allium-tests were 6.3, 5.2 and 10.4 g/l for the sum of root lengths, average root length on each bulb and proliferative activity at the tips of roots (mitotic index), respectively (See Table 1 and 2). Complete inhibition of onion root growth was observed at the concentration of the decider equal to 20 g/l, while the death of all test animals in the acute toxicity test occurred at the concentration of the deicer equal to 8,0 g/l (See Fig. 1). Thus, we demonstrated that similar concentrations of the deicer induced 50% inhibition of the growth of onion roots and 50% mortality of cladocerans. These values, in general, corresponded to a critical salinity of 5-8 %o above which qualitative changes occur both in the external and internal condition of aquatic animals. The electrical conductivity of the deicer solutions, which had a negative effect on the selected test species, coincides with the previously obtained values of the electrical conductivity of sodium chloride solutions harmful to cladocerans. We can assume that the main mechanism of the effect of the deicing material that we study is associated with the biological effect of its chlorine and sodium salts. Taking this into account, the value of electrical conductivity measured for solutions of deicing salt can be used to assess its negative potential effects. We estimated that in the absence of timely cleaning, regulated by the rules for using the material, the runoff from each square meter of the treated surface can lead to the pollution of 8-13 liters of fresh water. Thus, the basic requirement for the use of deicing salts on roads is the need to comply with the cleaning regime of the treated surfaces. Otherwise, the gradual accumulation of sodium and calcium chlorides in water bodies can cause an increase in salinity which will affect the survival of freshwater aquatic organisms and lead to serious disturbances in the functioning of aquatic ecosystems. The paper contains 1 Figure, 2 Tables and 30 References. The Authors declare no conflict of interest.

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