## 4 Discussion  
The present study showed that juglone inhibited the growth of various algae, particularly harmful, bloom-forming species such as _S. hantzschii_, a diatom that flourishes in cold seasons, and _M. aeruginosa_, a cyanobacterium abundant in warm seasons. This study also demonstrated that juglone produced positive stimulatory effects as well as hormetic effects on the growth of some green algae and diatoms, suggesting its differential allelopathic effects on different algal assemblages. According to the broad definition, allelopathy includes both inhibitory and stimulatory effects of one plant upon another, including microorganisms [37]. During this study, the observed allelopathic effects of juglone differed among diverse algal assemblages, including diatoms, green algae, and cyanobacteria (Table 2). Notably, a low level (0.1 mg L\({}^{-1}\)) of juglone selectively inhibited the growth of _S. hantzschii_ by 66.2 \(\pm\) 8.6%. However, the growth response of the other three diatoms (_A. formosa_, _F. crotonensis_, and _S. acus_) varied with different concentrations of juglone, showing stimulatory hormetic responses at 0.1 mg L\({}^{-1}\) but inhibitory effects at 1 mg L\({}^{-1}\). Hormesis, the beneficial or stimulatory effect of a toxicant at a dose lower than that causing the first detectable negative effects, also was observed in some green algae and cyanobacteria treated with low concentrations of juglone (see Table 2) [24; 25]. During a study of the co-culture of two green algae, [PERSON] et al. [38] observed similar differential allelopathic effects; they observed that the growth of _Chlorella pyrenoidosis_ was inhibited by _Hydrodichyton reticulatum_, with a notable hormetic effect at low concentrations. [PERSON] et al. [39] also reported hormesis in the growth of macroalga _Ulva lactuca_ zoospores induced by low concentrations of a biocide (triphenylborane pyridine). These studies demonstrated that hormetic responses are related to low concentrations of allelochemicals in different algal species. Positive allelopathic effects (growth stimulation) in addition to hormesis are more likely to occur in green algae in contrast with the effects observed for cyanobacteria (Table 2). During our study, all tested green algae showed growth stimulation from all concentrations of juglone in both the laboratory and field bioassays (Figure 2; Figure 4). Similar results were reported in allelopathic studies of several green algae [38; 40; 41]. Additionally, previous studies demonstrated that allelochemicals (e.g., \(\beta\)-sitosterol-\(\beta\)-D-glucose, dicyclohexanyl orzane, L-2-azetidinescarboxylic acid, lignin and hydrolyzable polyphenol) obtained from several aquatic and terrestrial plants had lower inhibitory effects on the growth of green algae than on cyanobacteria [19; 22; 42; 43]. These results conclusively suggest that green algae have a higher resistance to allelochemicals than do cyanobacteria. Some green algae change morphology in response to physical, chemical, and biological stressors in their environments [19]. We observed that colony formation of _S. quadricauda_ and _C. vulgaris_ was enhanced by juglone in the laboratory experiments. This may be a defense strategy against allelochemicals. Additionally, previous studies suggested that phenolic compounds could be metabolized by some green algal cells [44; 45; 46]. Such biodegradation abilities are another survival strategy against allelochemicals. These survival strategies stimulate the growth of green algae if allelochemical concentrations are below a certain threshold. Additionally, some organic solvents were reported to affect algal growth in both inhibiting and stimulating manners depending on different concentrations and algae [47]. Therefore, there might be a potential effect of methanol on our results. Conversely, negative allebotatic effects (growth inhibition) of juglone were common for cyanobacteria (Figure 1b; Figure 4; Table 2). The effects varied depending on juglone levels, cyanobacteria taxa, and cell type.