## 4 Discussion
Sensitivity to algicides is associated with cell morphology, cell wall thickness, and cell size among various algae [14; 18; 19; 50; 51]. During this study, algae with colonial morphology and large cell size showed a lower sensitivity to juglone than did algae with a unicellular morphology and small cell size (Figure 2; Figure 4; Table 3). Cyanobacterial cell walls are approximately 10 nm thick in algae with unicellular morphology, such as _Synechococcus_; 15-35 nm thick in algae with small chain morphology, such as _Phormidium_; and more than 700 nm thick in algae with large chain morphology, such as _Oscillatoria_[52]. Prokaryotic cells lack membrane-bound organelles, unlike eukaryotic cells, which also influences alleochemical sensitivity. The cell walls and sizes of most prokaryotes, including cyanobacteria, are considerably thinner and smaller than those of eukaryotic cells, such as green algae and diatoms. During our laboratory bioassays, the green algae formed colonial cells, unlike the diatom _S. hantzschii_ and cyanobacterium _M. aeruginosa. S. hantzschii_ and _M. aeruginosa_ exhibited unicellular morphologies and were more sensitive to juglone than were other algae. However, the diatoms _F. crotonensis_ and _A. formosa_ exhibited chain and colonial morphologies, respectively. Filamentous chain morphologies were more resistant to low concentrations of juglone (0.1 mg L\({}^{-1}\)) than was the unicellular _S. hantzschii_. To conclude, juglone produced differential alleopathic effects and even hormesis in various kinds of algae and, in particular, it inhibited some bloom-forming harmful algae, suggesting that it can selectively control harmful nuisance algal species such as _S. hantzschii_ and _M. aeruginosa_. Our study is the first to our knowledge focused on the allelopathy of juglone in diatoms, reporting interesting results on both growth inhibition and hormesis, which will improve the understanding of algal allelopathy. The hormetic responses of diatoms observed in this study suggest that low levels of juglone applied to natural ecosystems would not affect non-target diatom abundance and composition to the extent of blocking its control effects. Based on our results and those from the literature [24; 25; 26; 27], juglone is an effective candidate for the control of harmful algal blooms. However, further studies evaluating the extent and magnitude of the synergistic effects of allelochemical combinations as well as the allelochemical effects of more specified (particularly lower) concentrations will be a necessary step in advancing the field of allelochemical control of harmful algal blooms. Writing of the original draft, M.-H.P. Conceptualization, supervision, and revision, S.-J.H. Statistical analyses, K.K. All authors have read and agreed to the published version of the manuscript. This work was supported by the National Research Foundation of Korea (313-2008-2-D00572). The authors are grateful to two anonymous reviewers for their constructive comments. We would like to thank Editage (www.editage.co.kr) for English language editing. The authors declare no conflict of interest. \begin{table}
\begin{tabular}{c c} \hline \hline Algal group or types & Allelopathic sensitivity to juglone (0.1 mg L\({}^{-1}\)) \\ \hline Cyanobacteria & _Microystis_(u) * _Anabaena_ and _Phormidium \textgreater{} Oscillatoria \textgreater{} Microcystis_(c) ** \\ \hline Taxonomic groups & cyanobacteria \textgreater{} diatoms \textgreater{} green algae \\ \hline Morphology & unicellular type \textgreater{} small chain type \textgreater{} large chain type \textgreater{} colonial type \\ \hline \hline \end{tabular} (Remarks) *: unicellular type, **: colonial type.  
\end{table} 
Table 3: Differential allelopathic sensitivity of the tested algae to juglone at 0.1 mg L\({}^{-1}\). The degree of sensitivity was determined according to algal taxa, groups, and cellular morphology.