Growth, grazing and bioluminescence of marine heterotrophic dinoflagellates

Heterotrophic dinoflagellates are a diverse group of microorganisms and an important component of the plankton community in the ocean. Yet their biology is not well understood due mainly to the difficulties in culturing these dinoflagellates. Most of heterotrophic dinoflagellates in the genus Protoperidinium are bioluminescent. They can make an important contribution to the total near-surface bioluminescence in the ocean. The knowledge of which factors affect the bioluminescence in these dinoflagellates is necessary in modelling the distribution of bioluminescence in the sea as well as understanding the role of bioluminescence in these organisms. Light is one of the factors affecting the bioluminescence in dinoflagellates. Mechanical stimulable bioluminescence (MSL) in three heterotrophic dinoflagellates: Protoperidinium brochi, Protoperidinium pellucidum and Protoperidinium oceanicum were all photoinhibitable. Their bioluminescence capacity (BCAP) did not change while their MSL was photoinhibited. The time course of the photoinhibition of MSL in P. brochi and P. oceanicum were similar to those for autotrophic dinoflagellates. The light intensities required for the photoinhibition of MSL in P. brochi and P. oceanicum suggested that in the mixed layer, MSL of these two species are photoinhibited during the day. The relationship between the growth and photoinhibition in P. brochi indicated that the dinoflagellate\u27s sensitivity to light changed with the growth rate. The biochemical mechanisms causing the photoinhibition of bioluminescence in dinoflagellates are not well understood. No primary photoreceptor has been identified as responsible for this effect. The first action spectrum for the photoinhibition of MSL in heterotrophic dinoflagellates was obtained using Protoperidinium depressum. This action spectrum, and other observations, suggested that carotenoids are not the primary photoreceptor for this effect. Heterotrophic dinoflagellates have long been recognized as grazers in the ocean. However, only few laboratory studies showed the food spectrum of these organisms, and only few quantitative data on the growth and grazing of these organisms are available. This scarcity of data makes it difficult to assess the quantitative role of these organisms in the tropho-dynamics of the sea. A heterotrophic dinoflagellate, Gyrodinium sp. was isolated from Narragansett Bay. The growth and grazing of this dinoflagellate was investigated in the laboratory. This locally undocumented dinoflagellate was able to consume non-chain forming diatoms, single diatom cells dropped from chains, medium sized dinoflagellates and some of the nano-flagellates. The grazing results suggested that this small heterotrophic dinoflagellate can be an important grazer of phytoplankton in the Narragansett Bay, especially during summer when dinoflagellates and small flagellates become important in phytoplankton community