Agar-digesting bacteria associated with ‘rotten thallus syndrome’ of Gracilaria sp.

Abstract

A condition of the tetrasporophyte stage Gracilaria spp., characterized by white to pinkish discoloration and gradual disintegration of the thallus, has been observed in tank-held stocks. Microscopic observation revealed no fungal or protozoan parasites. Appropriate dilutions of homogenates plated on nutrient agar and bromthymol blue teepol agar showed the presence of bacteria, all of which were agar-digesting, at the rate of 1.42 × 10⁷ cells per g of affected thalli. Colonies on bromthymol blue teepol agar were round and yellow, while those on nutrient agar appeared creamy and round with entire edges, and were rapid agar digesters. The bacteria were Gram negative, fermentative and motile rods. Based on biochemical characteristics, the isolates were classified as belonging to the genus Vibrio. Microscopic observations of thalli cross-sections showed erosion of the pericarp, thus revealing the cortical and the medullary cells. Scanning electron microscopy revealed rod-shaped bacteria, including dividing cells, in affected tissues. Antibiotic sensitivity tests indicated that the bacteria were sensitive to Polymyxin B, nalidixic acid, nitrofurazone and oxytetracycline.