Pathogenic bacteria associated with lesions and thallus bleaching symptomsin the Japanese kelp Laminaria religiosa Miyabe (Laminariales, Phaeophyceae)

During early Spring (April-May) when the seawater salinity drops suddenly a nd the seawater temperature increases drastically, severe lesions and thall us bleaching were observed in the Laminaria religiosa population at Oshoro Bay, Otaru, Hokkaido, Japan. The healthy and diseased kelp blades were coll ected and subjected to enumeration of total number of culturable bacteria a nd bacterial species. Bacterial enumerations were done using 3 different me dia formulations; high-nutrient media (Media 1), low-nutrient media (Media 2) and modified low-nutrient media with 5% kelp extract (Media 3). Seven ba cterial species were isolated from the healthy kelp. These were Alcaligenes aquamarinas, Alteromonas sp., Azomonas agilis, Azotobacter beijerinckii, E scherichia coli, Halobacterium sp. and Halococcus sp. All 7 bacterial speci es were isolated on Media 2 and Media 3, but only 5 species were isolated u sing Media 1 with the absence of Halobacterium sp. and Halococcus sp. Highe st total number of culturable bacteria was 2050 CFU/cm(2) on Media 3. Eight species of bacteria were isolated from the diseased kelp thallus with the addition of Erwinia amylovora. All 8 bacteria grew on Media 2 and Media 3, but only 6 species were isolated using Media 1 with the absence of Halobact erium sp. and Halococcus sp. Highest total number of culturable bacteria wa s 5830 CFU/cm(2) on Media 3. However, only 3 species were isolated from the lesioned area. The most abundant species was Alteromonas sp. followed by H alococcus sp. and Alcaligenes aquamarinas. The surface bacteria showed best growth on Media 3. Scanning Electron Microscopic images of the healthy and diseased thallus gave distinctive evidence of the severity of the lesions as well as the relative abundance in the bacterial population. In an effort to identify the symptoms causative organism, the isolated bacterial specie s were cultured and used to test Koch's postulates. Out of the 8 species, o nly Alteromonas sp. induced lesions on the axenic kelp blades. The inoculat ed bacteria were also re-isolated without any significant contamination. He nce, Alteromonas sp. is suggested as the possible disease causing organism.