Recent findings of algicidal bacteria (AB) inhabiting natural macroalgal surfaces suggest that cultivating macroalgae can increase the concentration of AB in neighboring waters, creating an area where harmful algal blooms are less likely to occur. However, the seasonal variation of AB abundance within macroalgal beds remains unknown. In this study, ropes with juvenile sporophyte of kelp (Saccharina japonica var. religiosa) were installed at a fishing port on Shakotan Peninsula, Hokkaido, Japan, and the growth of kelp and the density of AB on kelp surface, seawater, and sediment were determined over a year.
ABs affecting the dinoflagellate Karenia mikimotoi identified through co-culturing experiments were detected only in the sediment in December 2022 at the beginning of the investigation. The abundance of ABs significantly increased on the kelp surfaces and was also detected from surrounding seawater in June 2023, corresponding to kelp distal tissue loss. Interestingly, at almost the end of the growing season (i.e., September 2023), no ABs were found on the kelp but were detected on Ulva australis growing on the rope, sediment, and surrounding seawater. This study provides new insights into the future application of macroalgal cultivation as a biological control of HABs crucial for sustainable aquaculture practices.