## 4 Discussion
Indeed, under the same (0.1 g/L) loading of MgAC, there was 22.59% algicidal activity of M.S, compared with 57.38% of A.S, in the case of the mono-cultures. The same phenomenon was observed for co-cultures: 27.42 and 88.67% algicidal activity of M.S. and A.S., respectively. However, when the ACs loading was increased, the algicidal activity reached 100% in both strains and all three ACs. These results of growth inhibition by CaAC treatment were consistent with those for MgAC and AlAC. By contrast, CaAC, for comparison with the least positively chargedsurface, showed an inferior level of algicidal selectivity and lower inhibition effects on HABs in the co-culture (Figure 2B,E). ROS-producing capability is another factor affecting the impact levels of AC types on microalgal strains. Accordingly, AIAC exhibited an ability to create a highly oxidative stress environment, followed by MgAC and CaAC (Figure 3). This was noticeable, and confirmed, when comparing the growth-inhibiting effects of each AC on the mono-cultures or co-cultures (Sections 3.1, 3.2 and 3.5). The production of ROS contributes significantly to the level of cell disruption as well as the inhibition effects of the material employed [43]. As for the zeta potentials among the three ACs, they differed, showing the various capabilities of the respective approaches and their levels of electrostatic interaction. A.S. was most highly effected by MgAC, followed by M.S. According to ROS production and levels of interaction, every species behaved differently. These results are in accordance with our other data, which showed that M.S. behaved similarly and actively with MgAC, and less sensitively than A.S. It was noticed that when the level of interaction between ACs and microalgae attained a certain level, the effect was shifted, from the stimulation to the inhibition, for each microalgae strain. The speed of this shift is different for each strain. The amine functional groups of MgAC acted as a biocidal agent: via the electrostatic interaction between the negatively charged cell wall and the positively charged MgAC, they affected cell-wall instability [19]. The AC-treatment conditions strongly influenced the allelopathic interactions between the _Microystis_ and _Anabaena_ strains. The _Anabaena_ strain appeared to be negatively affected even at low doses of AC. Inhibition of biomass strongly occurred under exposure to the \"mixture\" of AC and _Microystis_, while the reverse (by strain) occurred under high AC dosages. These observations are indications that the allelopathic effect of the _Microystis_ strain is very high under the conditions of \"mixture\" treatment. They also point to the effects of AC on the interactions between treated cells: improved cell-wall stability, inhibition/stimulation/metabolization of secondary metabolites, and greater susceptibility of cyanobacteria cells to allelochemicals (Section 3) [44]. Therefore, under high dosages of AC, the _Microystis_ strain appeared to be less inhibited than the _Anabaena_ strain. This showed that the allelopathic effect was lower in the latter strain, or that the resistance was higher in the former strain, whose phenomenon is possibly related to the N-fication capability of _Anabaena_ under AC-treatment conditions. In other words, filamentous N-fixing cyanobacteria, such as _Anabaena_ spp., are more strongly distinguished by _Microystis_. Additionally, _Anabaena_ was more sensitive to single-AC treatment, and more strongly affected under the \"mixture\"-treated conditions. This complicates the issue of the relationship between available substances in the environment such as nutrients, released metabolites, or algal culture motion/kinematic [38]. After MgAC is internalized, it can be seen that AC spills and takes over the entire cell as the consequence of the positively charged AC's effect on the plasma membrane. Specifically, the double-lipid layer breaks down, and cell damages subsequently follows, sometimes releasing intracellular compounds (Figure 6, Figures S2 and S3).