The effects of light intensity on the growth of Japanese Gambierdiscus spp. (Dinophyceae).

Marine toxic dinoflagellates of the genus Gambierdiscus are the causative agents of ciguatera fish poisoning (CFP), a form of seafood poisoning that is widespread in tropical, subtropical and temperate regions worldwide. The distributions of Gambierdiscus australes, Gambierdiscus scabrosus and two phylotypes of Gambierdiscus spp. type 2 and type 3 have been reported for the waters surrounding the main island of Japan. To explore the bloom dynamics and the vertical distribution of these Japanese species and phylotypes of Gambierdiscus, the effects of light intensity on their growth were tested, using a photoirradiation-culture system. The relationship between the observed growth rates and light intensity conditions for the four species/phylotypes were formulated at R>0.92 (p<0.01) using regression analysis and photosynthesis-light intensity (P-L) model. Based on this equation, the optimum light intensity (Lmax ) and the semi-optimum light intensity range (Ls-opt ) that resulted in the maximum growth rate (μmax ) and ≥80% μmax values of the four species/phylotypes, respectively, were as follows: (1) the Lmax and Ls-opt of G. australes were 208μmol photons m-2 s-1 and 91-422μmol photons m-2 s-1 , respectively; (2) those of G. scabrosus were 252 and 120-421μmol photons m-2 s-1 , respectively; (3) those of Gambierdiscus sp. type 2 were 192 and 75-430μmol photons m-2 s-1 , respectively; and (4) those of Gambierdiscus sp. type 3 were ≥427 and 73-427μmol photons m-2 s-1 , respectively. All four Gambierdiscus species/phylotypes required approximately 10μmol photons m-2 s-1 to maintain growth. The light intensities in coastal waters at a site in Tosa Bay were measured vertically at 1m intervals once per season. The relationships between the observed light intensity and depth were formulated using Beer's Law. Based on these equations, the range of the attenuation coefficients at Tosa Bay site was determined to be 0.058-0.119m-1 . The values 1700μmol photons m-2 s-1 , 500μmol photons m-2 s-1 , and 200μmol photons m-2 s-1 were substituted into the equations to estimate the vertical profiles of light intensity at sunny midday, cloudy midday and rainy midday, respectively. Based on the regression equations coupled with the empirically determined attenuation coefficients for each of the four seasons, the ranges of the projected depths of Lmax and Ls-opt for the four Gambierdiscus species/phylotypes under sunny midday conditions, cloudy midday conditions, and rainy midday conditions were 12-38m and 12-54m, 1-16m and 1-33m, and 0m and 0-16m, respectively. These results suggest that light intensity plays an important role in the bloom dynamics and vertical distribution of Gambierdiscus species/phylotypes in Japanese coastal waters.