The Antioxidant Response Plays a Role in the Acclimation of Chlamydomonas reinhardtii to Stress Induced by Cobalt Ions

Cobalt (Co) is an essential micronutrient for many organisms, but, at higher concentrations, it be-comes harmful primarily due to competitive interactions with other metal ions. Compared to other heavy metals, such as Cd, Cu, Cr, Pb, or Ni, this element has been less studied in algae with respect to its toxicity and tolerance. Enzyme inhibition and disruption of nutrient homeostasis may lead to oxidative stress in Co-exposed cells. The aim of this study was to assess the impact of CoCl2 on growth, photosynthetic pigment content, maximum quantum yield of photosystem II (Fv/Fm), effi-ciency of nonphotochemical quenching of chlorophyll fluorescence (NPQ), oxidative stress markers, and antioxidant defence mechanisms in the model green microalga Chlamydomonas reinhardtii. The measured antioxidants included soluble thiols, ascorbate (Asc), proline (Pro), α-tocopherol (α-Toc), and plastoquinol (PQH₂-9). Superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) activities were also determined. Exposure to CoCl2 resulted in increased levels of thiols, Asc, α-Toc, PQH₂-9, and CAT activity. At lower concentrations of CoCl2, no increase in oxidative stress markers was observed, suggesting efficient antioxidant protection. In contrast, exposure to higher concentrations of CoCl2 caused the inhibition of growth and chlorophyll (Chl) synthesis, a reduction in the Chl a/Chl b ratio, Fv/Fm parameter, NPQ induction efficiency, and levels of lipophilic anti-oxidants, along with an increase in lipid hydroperoxides.