Association of the gonyaulacoid dinoflagellate Alexandrium ostenfeldii with spirolide toxins in size-fractionated plankton

Abstract

Spirolides are pharmacologically active macrocyclic imines discovered in shellfish and plankton sizefractions collected from near-surface waters along the southeastern coast of Nova Scotia (NS), Canada. These compounds are most prevalent in the water column within plankton size-fractions from 21 to 56 µm during late spring. Analysis of bulk plankton material by liquid chromatography combined with ion-spray mass spectrometry indicated that gonyaulacoid dinoflagellates were dominant members of the plankton assemblage in these fractions when spirolides were abundant at Graves Shoal, NS, but that spirolides disappeared from the water column after a shift to diatom dominance. Spirolide composition in the plankton was rather consistent over time and among different sizefractions within a site, but the profile was radically different between sites in Nova Scotia. At Ship Harbour, NS, spirolides A, B, C and desmethyl-C were the major constituents, whereas at Graves Shoal, the primary spirolide derivatives were B, D and an isomer D2. Correlation analysis of spirolide abundance and the occurrence of particular taxa in size-fractionated plankton showed that the highest correlations were with Alexandrium species (r 2 = 0.93) and GB42 cells (r 2 = 0.83). These latter cells are now recognized as athecate forms of gonyaulacoid dinoflagellates, primarily of Alexandrium ostenfeldii. By analysis of size-fractionated plankton and comparison with cultured plankton isolates from spirolide-rich areas, we have substantiated the primary role of A. ostenfeldii as the source of spirolides in the water column.