## 4 Discussion  
The present study provides an evaluation of the toxic effects of Cd, Cr, and Pb on _C. dorsiventrale_, highlighting distinct physiological and molecular responses, as well as their bioremediation potential. Cd and Cr were more toxic to _C. dorsiventrale_ than Pb, particularly during the lag and exponential phases. This finding is in agreement with a previous study showing the negative impact of Cd and Cr on the growth of the green marine microcalga _Dunaliella salina_[16]. Other studies have indicated a tolerance to these two metals in plants, green microalgae and diatoms [52; 53]. In this sense, we infer that _C. dorsiventrale_ faced metal stress by exploiting the nutrients released by decomposing dead cells [54]. Regarding exposition to Pb, _C. dorsiventrale_ growth did not seem significantly affected, which may be explained by the importance of Pb as an inducer of peroxidase enzyme activity, which induces the Indole Acetic Acid (IAA) hormone known to stimulate growth and cell division in microalgae [55]. _Chlorocoecum aquaticum_ exhibited significant tolerance to Pb\({}^{2+}\), evidenced by an LC\({}_{50}\) value of 100 mg L\({}^{-1}\), which underscores its potential as an effective bioremediation agent for lead-contaminated water [23]. In regards to algal metabolic activity, we observed a reduction in total chlorophyll concentration in _C. dorsiventrale_ after Cr and Pb exposures. The decrease in chlorophyll content has been documented as a common stress response in microalgae cells [25; 56]. [PERSON] et al. [57] reported that _Chlorocoecum_ sp. treated with 10 \(\upmu\)M As(III) exhibited a minimal reduction of 21% in chlorophyll concentration. In concordance, _Chlorella vulgaris_, exposure to Zn and Hg resulted in a dose-dependent reduction in photosynthetic pigments, with decreases ranging from 32% to 100% as metal concentrations increased [58]. This decrease may be attributable to the substitution of magnesium within the chlorophyll molecule by trace metals which obviously threatens photosynthesis [49; 59; 60]. Metal toxicity is also reflected by the fluctuations of Chl-a/Chl-b ratios according to metal concentrations with increased levels after Pb exposures while a decrease was registered after Cr exposure. We infer that this shift between these two pigments may be explained by their complementary defense mechanisms against reactive oxygen species due to metallic stress [61]. Despite pigment adjustment to ensure the removal of reactive oxygen species, we observed a decrease in lipid content in _C. dorsiventrale_ under Cd and Cr treatments along with that of polysaccharides after exposure to the highest concentrations of all tested metals. Clearly, this persistent oxidative stress will affect the integrity of cell macromolecules such as lipids, proteins, polysaccharides and nucleic acids. Indeed, FTIR analysis suggested an interaction of Pb and Cr metal ions with polysaccharides and phospholipids. Previous studies showed a decrease in carbohydrate content in _Monoraphidium_ sp. exposed to 40 \(\upmu\)M Cd [62]. It is worth noting that Pb did not affect _C. dorsiventrale_ lipid content, inducing only a slightly lower reduction in its carbohydrates which is illustrated through FTIR analysis by similar spectra for untreated and treated cells by Pb and Cr. Moreover, soluble-protein content seemed to be non-affected in _C. dorsiventrale_ cells treated with Cd or Cr. _Senedesmus_ sp. exposure to Pb induced a rise in protein amount by 11.6% at 5 mg/L of metal, while a significant inhibition of soluble-protein content was observed in _Chlorella sorokiniana_ treated with 52 mg/L of Pb [63]. At a molecular level, CAT and SOD encoded genes were overexpressed after Pb and Cr exposures, representing an up-regulation triggered by metals as previously reported in the literature [16; 64; 65; 66]. Briefly, SOD constitutes the first barrier of defense against O\({}_{2}\)\({}^{\bullet-}\) by quickly converting it to O\({}_{2}\) and H\({}_{2}\)O\({}_{2}\).