## 4 Discussion  
HAB-controlling methods are required to be environmentally friendly and safe. Ammonium compounds and their derivatives show some promising potential in specific concentrations, though there are issues with respect to toxicity. In the current study, the _Microcytsis_ and _Anabuena_ strains were influenced by different growth inhibition or stimulation levels in terms of the effects of AC on the mono-culture. In the case of the co-culture, meanwhile, _Microcytsis_ strongly inhibited _Anubuena_ growth, especially under AC treatment. Moreover, the change in AC dosage was associated with the interaction between the algal strains. Two strains of cyanobacteria were co-cultured in this study. Of these two types of algae, _Microcytsis_ often showed dominance in the culture population over the _Anabuena_ strain when comparing the differences between the mono-culture and co-culture in the presence and absence of AC. These observations are consistent with previous studies on _Microcytsis_ sp. 's ability to inhibit or stimulate the growth of cyanobacteria or microalgae [32; 35; 36; 37]. The significant reduction in photosynthetic efficiency also partially inhibited the growth of _Anabaena_ strains, thereby affecting the biomass distribution and production. It has been reported that _Microcytsis_ cell-free media inhibit photosynthesis and interfere with the carbonic anhydrase function of dinoflagellate _Peridinium gatunense_[34]. [PERSON] et al. (2011) also reported that the glutathione-S-transferase (GST) activity of _Anabaena_ sp. is promoted in the presence of _Microcytsis aeruginosa_. These studies affirm, moreover, that allelochemicals can indirectly mediate interactions between microalgal strains. This phenomenon was not clearly shown in the present experimental results, but notably, it was in fact shown when comparing differences, more specifically, in the increase in the growth inhibition effect across cultures (from mono-culture to co-culture) at the same AC concentration. In terms of the impact of AC on cultures, the growth inhibition between the studied strains was likely due to allelopathic interactions. The demonstrated ability to significantly inhibit the growth of the _Anabaena_ strain in a _Microcytsis_ cell-free medium further confirms this hypothesis. The release of secondary metabolites and biologically active substances into the growth medium by _Microcytsis_ likely inhibits the _Anabaena_ strain, and it is consistent with observations of their impact on plankton [31; 32; 36; 37; 38; 39]. Activities such as catalase, peroxidase, or superoxide dismutase are likely to be enhanced by the increase in the production of oxidative stress species due to allelochemicals from _Microcytsis_, thereby inhibiting the growth of _Anabuena_[32]. Therefore, an increase in the production of ROS, due either to the _Anabaena_ strain itself or the presence of _Microcytsis_, is likely related to the disruption of nitrogenase activity as well as the suppression of N2-fixation of _Anabaena_ strains [32; 40; 41]. In this study, algicides were chosen as an approach using types of AC (MgAC, CaAC, and AlAC). These types of AC, having no apparent harmful effects on plankton, also provide the desired functions of ammonium. These ACs, including the organic parts of the ammonium sites, attaches to the cell surface. This causes mucus release that increases adhesion on the cell surface, and cell adhesion and coagulation in turn [42]. Consequently, the surface interactions of AC at the ammonium sites likely disrupt the internal environment of the cell, leading to cell lysis. It can be assumed that MgAC, CaAC, and AlAC use the same HAB-control mechanism. All three ACs have relatively high algicidal activity, with a focus on the harmful toxin genera and somewhat lesser one on the non-N-fixing genera, in the order AlAC > MgAC > CaAC. _Anabaena_ is more sensitive and highly disrupted.