## 4 Discussion
### Distribution of Growth-Inhibiting Bacteria against _A. catenella_ (Group I)  
Bacteria that inhibit the growth of _A. catenella_ (Group I) were detected from the surface of the seagrass (_Z. marina_) leaves of Akkeshi-ko Estuary (Stn. 1) with a maximum density of 4.7 \(\times\) 10\({}^{6}\) CFU g\({}^{-1}\) wet leaf (Figure 10a). It is reported that growth-inhibiting bacteria against _A. catenella_ attached to seagrass, and the maximum density was 7.5 \(\times\) 10\({}^{7}\) CFU g\({}^{-1}\) wet leaf collected from the biofilm of _Z. marina_ at the center of Padilla Bay in Puget Sound, Washington, U.S.A. [10]. The density of the growth-inhibiting bacteria was 1.8 \(\times\) 10\({}^{8}\) CFU g\({}^{-1}\) wet leaf from the surface of _Z. japonica_ leaves at the same location, and 6.9 \(\times\) 10\({}^{5}\) CFU g\({}^{-1}\) wet thallus from the biofilm on the surface of _Ulva lactuca_ collected at Cattle Point in Puget Sound. Growth-inhibiting bacteria against _A. catenella_ were also detected at densities of 4 \(\times\) 10\({}^{4}\)-1 \(\times\) 10\({}^{6}\) g\({}^{-1}\) wet weight from the seaweeds _Lamiaria japonica_, _Corallina pililifera_, and _Ulva pertusa_ at Shinori Coast, Hakodate, Japan [16]. The existence of growth-inhibiting bacteria on seagrasses and seaweeds thus appears to be common in coastal seas. Herein, PAB isolated from seawater in seagrass beds (Stn. 1) were detected at the maximum density of 6.0 \(\times\) 10\({}^{2}\) CFU mL\({}^{-1}\) and FLB were detected at the maximum density of 7.9 \(\times\) 10\({}^{2}\) CFU mL\({}^{-1}\) as growth-inhibiting bacteria (Figure 10b). There have been very few investigations [10,16] of the distribution and abundance of growth-inhibiting bacteria against _A. catenella_ in nature. The density of 4.1 \(\times\) 10\({}^{3}\) CFU mL\({}^{-1}\) of growth-inhibiting bacteria against _A. catenella_ was obtained in the seawater of Holmes Harbor, Puget Sound [10]. Approximately 1 \(\times\) 10\({}^{2}\)-2 \(\times\) 10\({}^{3}\) cells mL\({}^{-1}\) of growth-inhibiting bacteria against _A. catenella_ were detected in the seawater of the seaweed bed of Shinori Coast in Hakodate, Hokkaido, Japan [16]. The densities of growth-inhibiting bacteria obtained from Akkeshi-ko Estuary and Akkeshi Bay are slightly lower than those studied in Puget Sound [10], but they are similar to the values reported from Shinori Coast, Japan [16]. It should be noted that the existence of growth-inhibiting bacteria with high frequency was confirmed in the present study. Our comparison of FLB and PAB as growth-inhibiting bacteria in seawater revealed that the densities of PAB tended to exceed those of FLB at Stn. 3, located offshore (Figure 10d). PAB were defined as bacteria that attach to particles and use the particles as a nutrient source, and they defined FLB as bacteria that use dissolved organic matter as a nutrient source [35]. The activity of PAB in terms of thymidine incorporation rate was revealed to be 10-100 times higher than that of FLB [35]. It is reported that there are larger numbers of algicidal bacteria against red tide flagellates in the fraction of PAB than that of the FLB category in seawater samples collected at approximately 20 km off Himeji in the Seto Inland Sea of Japan [42]. Considering the previous reports and this study's findings, we speculate that the greater part of growth-inhibiting bacteria are in the PAB fraction. We also speculate that when PAB are detached from the particles they had attached themselves to, they would become FLB and presumably reach an inactivated state within a short time. Gene sequence analyses of 16S rRNA were carried out herein for the obtained strains of growth-inhibiting bacteria. Some of these bacteria from seawater were found to be the same types of bacteria isolated from the surface of the seagrass _Z. marina_. For example, the strains AK28 (Stn. 1, leaf), AK41 (Stn.1, leaf), and AK45 (Stn. 1, FLB) were closely related to _Sulfibobacter_ sp.