Setsuko Sakamoto

Production of paralytic shellfish toxins by a bacterium Moraxella Sp. isolated from Protogonyaulax tamarensis

A bacterium Moraxella sp. isolated from Protogonyaulax tamarensis was cultured in various conditions. Changes of toxicity and toxin components of the cells during culture were analyzed by bioassay and HPLC-fluorometric analysis. Toxin productivity of Moraxella sp. increased when it was cultured in nutrition-deficient environments. The main toxins produced by Moraxella sp. in these conditions were gonyautoxins (GTXs), mainly GTX 1 and 4 which are major toxins of P. tamarensis.

Occurrence of tetrodotoxin in Alexandrium tamarense, a causative dinoflagellate of paralytic shellfish poisoning

A significant amount of tetrodotoxin was identified in the cultured cells of Alexandrium tamarense, a causative dinoflagellate of paralytic shellfish poisoning. This result supports the hypothesis that both tetrodotoxin and paralytic shellfish toxins can be detected in organisms which possess either of them, although their proportion is different individually. The present result also supports our previous finding that A. tamarense is the origin of tetrodotoxin detected in the highly toxic scallop Patinopecten yessoensis during the bloom of A. tamarense.

Distribution of three algicidal Alteromonas sp. strains in seagrass beds and surrounding areas in the Seto Inland Sea, Japan

Harmful algal blooms (HABs) often cause great damage to the fish aquaculture industry in the western part of Japan. Bacteria that effectively kill such phytoplankton are found in coastal seawater and are considered to influence the occurrence of HABs. Algicidal bacteria are found in abundance in seagrass beds; however, the distribution of particular bacterial strains has not yet been clarified. In this study, we determined the abundance of three algicidal Alteromonas sp. strains that were isolated as Chattonella antiqua-killing bacteria in seagrass beds and their surrounding areas in summers of 2013–2015. The strains were detected using a quantitative polymerase chain reaction (qPCR) amplification method. Two of the three algicidal bacterial strains were remarkably abundant in the seawater of an enclosed seagrass bed compared to the surrounding areas. In addition, the abundance of the three algicidal bacteria decreased when the eelgrass withered in late summer. These results suggest that growth of these algicidal bacteria was stimulated by eelgrass. The bacterial abundance estimated by qPCR was much greater than that determined by the culture-dependent method in June 2015 when the eelgrass grew thickly. This implies that the qPCR assay could be a sensitive tool to evaluate algicidal bacteria in natural environments.

Identification of overwintering vegetative cells of the bivalve-killing dinoflagellate Heterocapsa circularisquama in Uranouchi Inlet, Kochi Prefecture, Japan

Red tides of Heterocapsa circularisquama have led to serious damage of bivalve aquacultures in western coastal areas of Japan. To understand the whole picture regarding the ecology of this species, it is essential to clarify its overwintering mechanisms. In this study, the population dynamics of H. circularisquama were investigated from February 2004 to November 2005, and overwintering cells were identified for the first time in water columns of Uranouchi Inlet, Kochi Prefecture, Japan. Heterocapsa circularisquama cells were detected by the indirect fluorescent antibody technique using monoclonal antibodies that specifically recognize and react to this species. Vegetative cells were almost always detected from the first observation in February 2004 to November 2005 with temperatures of 10.5–30.6°C. During the period from winter to spring, this species survived in areas with a temperature higher than 10°C. The overwintering cells of H. circularisquama were isolated in March 2004, and identification was made via observation of the morphology and body scales of the cultured cells. These overwintering cells were identified as H. circularisquama and reacted to the monoclonal antibody. These results indicate that H. circularisquama can overwinter and survive throughout the year in a vegetative cell state in Uranouchi Inlet.

Occurrence of Karenia papilionacea (Dinophyceae) and its novel sister phylotype in Japanese coastal waters

Several species of the genus Karenia (Dinophyceae) form blooms and often cause the mortality of cultured and wild fish. In Japan, blooms caused by two species - namely Karenia mikimotoi and Karenia brevis - have been reported so far. On the basis of morphological and molecular-phylogenic examinations, the present investigation found Karenia papilionacea and its novel sister phylotype for the first time in the coastal waters of the various regions of Japan. Of 34 strains isolated from the coastal waters, 27 strains displayed the typical morphological characteristics of K. papilionacea and further showed consensus DNA sequences corresponding to those of the originally described K. papilionacea. The other 7 strains displayed the same morphological characteristics of K. papilionacea, but showed divergent DNA sequences, at a genetic distance of over 0.04 (Internal Transcribed Spacer regions) from those of the original phylotype of K. papilionacea. These divergent strains were characterized as a novel sister phylotype (phylotype-I) of K. papilionacea. In the coastal waters of Japan, K. papilionacea-like (K. papilionacea and/or its phylotype-I) formed blooms at 20.3-30.4°C and salinity 30.1-33.9. No K. brevis was identified in Japanese coastal waters during this study. These findings demonstrated that K. papilionacea occurs along the coasts of western Japan and possibly shares several coastal regions with K. mikimotoi and K. papilionacea phylotype-I. In order to assess the risks of Karenia blooms to aquaculture, it is essential that the growth physiology and ichthyotoxicity of K. papilionacea and its novel phylotype should be characterized.