## 4 Discussion
In fact, methanolic extracts from both studied strains exhibited interesting antibacterial activities against six Gram negative pathogenic bacteria strains. These results seem to be in perfect agreement with the data of biochemical compositions since a significant relationship between total antioxidant activity and total phenolic compounds has been reported [57; 58]. It is worth noting that microalgae methanolic extracts seem to be particularly rich in flavonoids, and that terpen and carbohydrate were present in moderate amount and alkaloids were also found at trace levels in [59; 60]. Previously, a correlation between metabolites like flavonoids, terpen and carbohydrates presence and antibacterial activities was established [59; 60]. Additionally, important antimicrobial activities of both microalgae extracts were obtained against tested Gram- bacteria pathogen for vertebrates and invertebrate marine animals, especially fish, bivalves, and crustaceans. Those activities could be due to the presence of bioactive compounds that may include isoprenoids, polyktetides, polyunsaturated fatty acids, and alkaloids which may affect the growth and metabolism of bacteria [61; 62]. It has been reported that antibacterial activities of microalgae extracts, against both Gram+ and Gram- bacteria, were attributed to eicosapentaenoic acid and palmitoleic acid, particularly produced by diatoms [63; 64]. Interestingly, no antibacterial activities of _Chlamydomonas_ or _Navicula_ species against aquatic animal pathogen have been previously reported. However, antibacterial activities were described for _Chlamydomonas reinhardtii_ and _Navicula_ sp. against human pathogenic bacteria such us _Staphylococcus aureus_, _Escherichia coli_, _Salmonella typhi_, and _Bacillus_ sp. [65; 66]. [PERSON] and [PERSON] [67] reported the mechanisms of action involved in the bacterial killing process. Among them are the interactions of the antimicrobial compounds with the cell membrane and intracellular accumulation through the cell wall and membrane permeability [68]. To ensure algae biomass safety for potential feed supplement applications, the toxicity of microalgae extracts was studied against fish cell lines. Hepatocytes, such as PLHC1 cells,are good models for studying toxicity since the liver is the primary site for drug metabolism and biotransformation [69]. According to [PERSON] et al. [70], treated concentrations of _Chlamydomonas_ sp. MA\({}_{\text{a1}}\) and _Navicula_ sp. MA\({}_{\text{n1}}\) methanol extracts are considered non-toxic since the cell viability is greater than 70%. In recent years, many studies have reported the in vivo immunostimulant effect of different microalgae such as _Nannochloropsis gaditana_, _Tetraselmis chuii_, and _Phaeodactylum tricontrum_ on fish production [71; 72]. Microalgae have been gaining attention in the treatment of wastewater, especially those contaminated by heavy metal. Interestingly, the newly isolated strains showed large cadmium removal capacities, with values ranging from 89.8% to 99.38%. Previously, rapid cadmium biocomoval by diatom cells has been observed in several other studies using cadmium concentration lower than that tested in the present study. In fact, _Navicula incerta_ showed linear removal of cadmium using initial ion metal concentrations of 5 mg/L [73]. [PERSON] et al. [74] also reported a cadmium removal capacity of _Navicula subminuscula_ in aqueous solution containing 8 mg/L cadmium. _Chlamydomonas_ genus has a great potential in metal remediation [75]. [PERSON] et al. [76] reported that _Chlamydomonas moevusii_ remove up to 8 mg/L of cadmium. Also, _Chlamydomonas reinhardtii_ remove up to 42.6 mg/L 67.4 mg/L of cadmium using fresh cells and lyophilized biomass, respectively [77; 78]. Several other microalgae have shown an important ability to remove cadmium ions, especially _Spirulina platensis and Chlorella vulgaris_[79; 80; 81].