| Summary: | The intensity and frequency of harmful algal blooms (HABs) have increased, posing a threat to human seafood resources due to massive kills of cultured fish and toxin contamination of bivalves. In recent years, bacteria that inhibit the growth of HAB species were found to be densely populated on the biofilms of some macroalgal species, indicating the possible biological control of HABs by the artificial introduction of macroalgal beds. In this study, an artificially created <i>Ulva pertusa</i> bed using mobile floating cages and a natural macroalgal bed were studied to elucidate the distribution of algal growth-limiting bacteria (GLB). The density of GLB affecting fish-killing raphidophyte <i>Chattonella antiqua</i>, and two harmful dinoflagellates, were detected between 10<sup>6</sup> and 10<sup>7</sup> CFU g<sup>−1</sup> wet weight on the biofilm of artificially introduced <i>U. pertusa</i> and 10 to 10<sup>2</sup> CFU mL<sup>−1</sup> from adjacent seawater; however, GLB found from natural macroalgal species targeted all tested HAB species (five species), ranging between 10<sup>5</sup> and 10<sup>6</sup> CFU g<sup>−1</sup> wet weight in density. These findings provide new ecological insights of GLB at macroalgal beds, and concurrently demonstrate the possible biological control of HABs by artificially introduced <i>Ulva</i> beds.
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