Toxicity of mine dump effluent containing five hexachiorocyclohexane (alpha, beta, gamma, delta and epsilon-HCH, sum 159.4 mu g/L) and two trichlorobenzene (TCB, sum 65.2 mu g/L) isomers to two microalgae (Scenedesmus quadricauda and Coccomyxa subellipsoidea) was studied over 24 h exposure and also with 2- and 10-fold diluted stock solution (i.e. 1 x, 0.5x and 0.1 x strength). Individual isomers revealed rather dose dependent accumulation typically higher in Scenedesmus than in Coccomyxa (max. sum of HCH 14.99 mu g/g DW with bioaccumulation factor 94) and delta-HCH was dominant isomer. TCB isomers showed low accumulation in algae. 0.1 x dose elevated chlorophylls and carotenoids in Coccomyxa while enzymatic activities (SOD, CAT, and APX), thiols (glutathione and phytochelatin 2) and ascorbic acid were rather elevated by 1 x dose in both species. Malic acid, rather than citric acid, increased in response to 0.5x and 1 x concentration. Sum of fatty acids was higher in Coccomyxa than in Scenedesmus with palmitic, oleic, linoleic and alpha-linolenic acids being dominant compounds in both species. Detailed profiling revealed that saturated and monounsaturated fatty acids increased in Coccomyxa while polyunsaturated fatty acids in Scenedesmus in response to increasing dose of organochlorines. Accumulation of organochlorines and metabolic responses in algae are reported here for the first time. (C) 2017 Elsevier B.V. All rights reserved.