Effects of perchlorate at different trophic levels and characterization of reducing bacteria of this pollutant from hypersaline soils of the Colombian Caribbean.

Abstract

Perchlorate is an emerging contaminant that affects the functioning of the thyroid gland in humans and alters the normal development of living beings. This compound can be formed naturally and by different anthropogenic activities, which are responsible for its wide diffusion in all environmental matrices. This chemical compound is persistent in ecosystems; so, it is necessary to use different types of treatments for its degradation. For the reduction of perchlorate, biological treatment with aerobic, facultative and anaerobic bacteria degradation and/or reduction of this pollutant is preferably used because the chemical methods, such as ion exchange, used for this purpose, are neither very efficient, nor very selective and incomplete, thus generating contaminated resins that must be treated later, which, in some cases, becomes more toxic than the original compound. Although there are currently adequate bacterial methods for the degradation of perchlorate, there is, however, no evidence that these have a universal application and such application is not standardized in tropical environments; that is why it is necessary to isolate native bacteria from hypersaline soils from the Colombian Caribbean, as a new economic and effective alternative to reduce perchlorate, thus generating a lesser impact on ecosystems. Perchlorate-reducing bacteria are a promising strategy for the bioremediation of contaminated sites. In this study, ten strains were isolated that grew in media containing up to 30.0% NaCl, pH variations (6.5-12.0) and moderately high concentrations of KClO4 (up to 10000 mg/kg), suggesting that these are halotolerant organisms. According to the sequencing of the 16S ribosomal gene, such strains belong to Vibrio, Bacillus, Salinovibrio, Staphylococcus and Nesiotobacter genus. These bacteria could reduce perchlorate levels from 10 to 25%. The main objective of this study was to evaluate the reduction of perchlorate in native bacteria isolated from hypersaline soils of Galerazamba (Bolivar), Parque Salamanca (Magdalena) and Manaure (Guajira) in the Colombian Caribbean; as well as to determine the toxic effects of this pollutant in in-vitro and in-vivo models. For this purpose, the ecotoxicological evaluation of perchlorate exposure was carried out in HEK, N2a, and 3T3 cell lines, as well as in the biological models Vibrio fischeri, Pseudokirchneriella subcapitata, Daphnia magna and Eisenia fetida, and the evaluation of several endpoints and the multivariate analysis between the chemical concentration of the pollutant and the toxicological responses of the models was evaluated. The ecotoxicological evaluation this contaminant’s toxic effects in different biological models exhibited a similar toxicity against the cell lines analyzed, with LC50 values of 19, 15 and 19 mM for HEK, N2a and 3T3 respectively; while in V. fischeri, the toxicity examined was considerably lower (EC50=715mM) with reduction of bioluminescence, while the survival of freshwater algae P. subcapitata was significantly affected by perchlorate (LC50=72 mM), and its effect on lethality in crustacean D. magna was significant (LC50=5mM). For the E. fetida earthworm, the LC50 was 56 mM in the soil. In the latter organism, perchlorate induced avoidance behavior, weight loss, decreased the production of eggs and offspring, as well as morphological and histopathological effects, such as malformations, dwarfism and necrosis. In conclusion, the environmental risk assessment by exposure varies according to the species, which presents a high environmental risk to the biota and the health of the ecosystems. This study evaluated the ecotoxicological impacts at different trophic levels; the results could be used as basic information to make decisions about the regulation of perchlorate in the environment. E. fetida is proposed as a sensitive model to generate information on the toxicological impact of KClO4 in soils. It is concluded that native bacteria isolated from hypersaline environments of the Colombian Caribbean are a promising tool for the reduction of this contaminant; this will improve the environmental quality of soils in tropical environments contaminated with perchlorate.