Responses to sublethal copper exposure in two strains of Chlamydomonas reinhardtii (Volvocales, Chlorophyceae) in autotrophic and mixotrophic conditions

Abstract

The aim of this work was to determine the responses to sublethal copper exposure (20 and 40 mM CuCl) in two Chlamydomonas reinhardtii strains: the wild type 4A+and the pleiotropic mutant cur9, originally selected as a copper-tolerant phenotype in mixotrophic conditions. Bioassays were performed in both autotrophic and mixotrophic media, under low irradiation (50 mol -2-1). At the onset, Cu2+ concentration in culture media was estimated by MINEQL software. Oxidative stress indicators, such as changes in photosynthetic pigment ratios, catalase activity, concentration of thiobartituric acid reactive substances (TBARS) and growth rate demonstrated a better performance of the wild type and the mutant strain in autotrophic and mixotrophic conditions, respectively. Moderate oxidative stress was accompanied by ultrastructural changes, such as accumulation of starch and vacuoles with electronic-dense deposits and membranous structures together with mild thylakoidal and mitochondrial crest disorganization. In mixotrophic conditions isocitrate lyase (ICL) activity peaked after 72-hour incubation. An inverse relation between catalase and ICL activities was verified and proved to be a good indicator of oxidative stress correlating consistently with other characteristic features such as, depression in growth rate or increase in TBARS dosage.