Distribution of Bacteria Influential on the Development and the Decay of Gymnodinium nagasakiense Red Tide and Their Effects on Algal Growth

Abstract

Distribution of bacteria influential on the algal growth during the process of development and decay of Gymnodinium nagasakiense red tide and their effects on the growth of G. nagasakiense and Skeletonema costatum were investigated in Uranouchi Inlet, Kochi. Seawater samples collected around red tide period of G. nagasakiense were filtered through either 0.8μm or 0.2μm filters. Into these filtrates were inoculated axenic cultures of G. nagasakiense or S. costatum. After in-cubation, cell numbers of G. nagasakiense or S. costatum in 0.8μm-filtrates were counted and were compared with those in 0.2μm-filtrates to estimate the bacterial effect on the growth of the two algae. During the time of the development of G. nagasakiense red tide when diatoms were still predominated, natual bacteria were stimulative for both G. nagasakiense and S. costatum. The stimulative effect for S. costatum, however, solely decreased just before the peak of G. nagasakiense red tide, while that for G. nagasakiense was still kept. On the contrary, bacterial effects for both two algae were inhibitory in the decline process of G. nagasakiense bloom. These results indicate that natual communities of bacteria in 0.8μm-filtrates had significant effects on the growth of specific algal species and suggested that they play an important role on the development and decay of algal bloom, in addition to the succession of species predominating.