The discovery of mixotrophy in photosynthetic species of Dinophysis (Dinophyceae): light and electron microscopical observations of food vacuoles in Dinophysis acuminata, D. norvegica and two heterotrophic dinophysoid dinoflagellates

Abstract

Dinophysis acuminata C1aparède et Lachmann and Dinophysis norvegica Ehrenberg (two toxic species responsible for diarrhoeic shellfish poisoning) from West Boothbay Harbor were both found to contain food vacuoles. Cells with food vacuoles constituted up to 6 and 36% of the D. norvegica and D. acuminata populations respectively. Food vacuoles were detected whenever these species were present except for a period in August/September when they were missing from the D. norvegica population. Cells containing food vacuoles were filledwith transparent globules, matching the appearance of two related, heterotrophic species which feed upon ciliates, Dinophysis rotundata Claparède et Lachmann and Oxyphysis oxytoxoides Kofoid. Cells were often megacytic and displayed a more intense orange-red plastidic pigmentation than did cells lacking food vacuoles, although no food vacuoles themselves were red. Feeding behaviour in D. acuminata and D. norvegica was not seen.The ultrastructure of food vacuoles was examined in all four species. Food vacuoles in D. acuminata contained membranous vesicles, lipid-like globules, cylindrical ejectile-like organelles, abundant paracrystalline vesicles and, in three cells, abundant concave starch-like grains. The food vacuoles of D. norvegica match those of some D. acuminata cells. In D. rotundata food vacuoles contained plastids (of the simple type with internal pyrenoid) and rare prasinophyte scales. Some O. oxytoxoides food vacuoles resembled certain D. acuminata food vacuoles. Others contained well-preserved trichocysts, virus particles and (in one instance) prasinophyte scales. Details of D. acuminata food vacuoles, including the high density of starch-like grains, the characteristic extrusomes, the paracrystalline vesicles and the close resemblance of some of these food vacuoles to those of O. oxytoxoides, suggest that D. acuminata preys upon ciliates.