## 4 Discussion  
Recent findings of growth-limiting bacteria densely living in a biofilm of seagrass and macroalgae have offered an insight into possible future HAB control by harnessing ecosystem services [51]. Meanwhile, the loss of seagrass and macroalgal beds due to human impacts have been accelerated over the last half-century [52; 53; 54]. Such deterioration of coastal vegetation coincided with the increasing occurrences of HAB [51; 55]. The increasing toxic blooms of the dinoflagellate _A. minutum_ accompanied by the large-scale decline of seagrass beds around the Mediterranean coast is one of the specific cases [56]. [PERSON] et al. [37] documented that _H. akashiwo_ cysts were found only from the sediments where eelgrass had disappeared and studied sites where the abundant growth-limiting bacteria detected rarely experienced blooms of paralytic shellfish toxin-producing dinoflagellate _Alexandrium_ in Puget Sound, WA, USA. On the positive side, the efforts to protect and restore seagrass and macroalgal beds are underway worldwide [57; 58]; for instance, vigorous restoration activities of seagrass bed in Hinase, Okayama, Japan increased the natural bed more than twenty times since 1985 [57]. The efforts will enhance the resistance of coastal areas against HAB events in the long run. However, this holistic approach to prevent HABs may not be as applicable as direct controls, such as the use of flocculants [59]. Thus, it is crucial to develop an environmentally friendly measure that can be widely applicable and cost-effective. _Ulva_ can be attractive candidate organisms revealed to harbor GLB [34; 37], growing fast compared to other macroalgae or seagrass [60; 61], relatively easy to culture [62], restoring water quality through the uptake of excess nutrients [63; 64], releasing allelochemicals negatively affecting the growth of HAB species [65; 66], and having commercial importance in that they are frequently consumed under the name \"Aonori\" in Asia [67; 68]. Several _Ulva_ species have already been tested to be organisms composing large scale integrated multi-trophic aquaculture (IMTA) systems [67; 69]. In the present study, harmful algal growth-limiting bacteria (GLB) were found in the order of \(10^{6}\) CFU per 1 g of _U. pertusa_ grown on the pumice stone and the mobile floating cage at Stn. 1, showing the equivalent value to the natural growing macroalgal species at Stn. 2 as well as previous studies estimating the densities of GLB on _Ulva_ sp. and _Gelidium_ sp. in Osaka Bay, Japan [34] and on _U. lactuca_ in Puget Sound, USA [37]. To our best knowledge, this is the first report to reveal that algal growth-limiting bacteria can be harbored within a few months on the newly introduced _Ulva_ thallus, suggesting the possible future field application to control HABs by introducing _Ulva_ beds as a source of GLB. Previous studies have shown that a higher proportion of GLB was often associated with particles [28; 44]. In the study in Puget Sound, most isolated bacteria from seawater having more extensive activity ranges were in the particle-associated form [37]. In the present study, the density of PAB (Particle-associated bacteria) as GLB was higher than FLB (Free-living bacteria) at the natural macroalgal bed (Stn. 2), killing most of the tested HAB species except _A. cattenella_ (Figure 5b). Moreover, 88% of GLB detected from seawater at the natural macroalgal bed was the same bacterial genera as GLB detected from macroalgal biofilms (Figure 6). It is known that macroalgal beds are one of the essential sources releasing particulate and dissolved organic matter (POM and DOM) to the surrounding environments [70; 71; 72]. The POM derived from the macroalgae at Stn. 2 may have increased substrate availability for specific bacterial communities with high exoenzymatic activities [73; 74; 75] that resulted in frequent detection of GLB as PAB forms in the water.