Cyanobacterium Microcystis aeruginosa response to pentachlorophenol and comparison with that of the microalga Chlorella vulgaris Paulo de Morais a,b , Teodor Stoichev b, *, M. Clara P. Basto a,b , V. Ramos b , V.M. Vasconcelos a,b , M. Teresa S.D. Vasconcelos b a CIIMAR/CIMAR and Faculdade de Cie ˆncias, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal b Centro Interdisciplinar de Investigac ¸a ˜o Marinha e Ambiental (CIIMAR/CIMAR), Universidade do Porto, Rua dos Bragas, 289, 4050-123 Porto, Portugal article info Article history: Received 11 September 2013 Received in revised form 26 December 2013 Accepted 27 December 2013 Available online 8 January 2014 Keywords: Pentachlorophenol Cyanobacteria Microalgae Toxicity Removal abstract Pentachlorophenol (PCP) effects on a strain of the cyanobacterium Microcystis aeruginosa were investigated at laboratory scale. This is the first systematic ecotoxicity study of the effects of PCP on an aquatic cyanobacterium. The microalga Chlorella vulgaris was studied in the same conditions as the cyanobacterium, in order to compare the PCP toxicity and its removal by the species. The cells were exposed to environmental levels of PCP during 10 days, in Fraquil culture medium, at nominal concentrations from 0.01 to 1000 mgL 1 , to the cyanobacterium, and 0.01 to 5000 mgL 1 , to the microalga. Growth was assessed by area under growth curve (AUC, optical density vs time) and chlorophyll a content (chl a ). The toxicity profiles of the two species were very different. The calculated effective concen- trations EC 20 and EC 50 were much lower to M. aeruginosa, and its growth inhibition expressed by chl a was concentration-dependent while by AUC was not concentration- dependent. The cells might continue to divide even with lower levels of chl a . The num- ber of C. vulgaris cells decreased with the PCP concentration without major impact on the chl a . The effect of PCP on M. aeruginosa is hormetic: every concentration studied was toxic except 1 mgL 1 , which promoted its growth. The legal limit of PCP set by the European Union for surface waters (1 mgL 1 ) should be reconsidered since a toxic cyanobacteria bloom might occur. The study of the removal of PCP from the culture medium by the two species is an additional novelty of this work. M. aeruginosa could remove part of the PCP from the medium, at concentrations where toxic effects were observed, while C. vulgaris stabilized it. ª 2014 Elsevier Ltd. All rights reserved. 1. Introduction Pentachlorophenol (PCP), a broad-spectrum pesticide, is the most toxic of the chlorophenols (CPs) due to its higher number of chlorine atoms, hydrophobicity and acidity. It is considered a priority pollutant since it has a very long half-life in the environment and is harmful at very low concentrations (Tao et al., 2012; Xing et al., 2012). It can be an endocrine * Corresponding author. Tel.: þ351 22 340 1805; fax: þ351 22 339 0608. E-mail address: tstoichevbg@yahoo.com (T. Stoichev). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research 52 (2014) 63 e72 0043-1354/$ e see front matter ª 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.12.036