Free monounsaturated oleic fatty acid, (OA) was detected in forest soils and sediment samples of the Ula River basin by GC-FID analysis of sample extracts fractionated by Florisil adsorption chromatography and after mild esterification of appropriate extractants to methyl esters. The toxicity of OA in environmental samples was expressed in Toxic Units calculated using the amount of OA in 1 g of dry weight and its EC50 and was found to vary from 42.7 to 48.6 TU in sediment and from 87.7 to 96.6 TU in soil samples of different locations of the Ula River basin. Data of the model studies showed that OA enhanced the respiration rate in Vibrio fischeri cells as do other classical uncouplers. OA also inhibits the enzyme luciferase competitively in whole cells of V. fischeri, as the natural luciferase substrate decanal protects bioluminescence against the toxic action of OA at equivalent concentrations. This study confirms that oleic acid as a compound of natural origin is toxic for bioluminescence at least by two mechanisms of action: 1) uncoupling activity (respiration enhancement) and 2) competitive inhibition.

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