Diarrhetic shellfish toxins in Tasmanian coastal waters:causative dinoflagellate organisms, dissolved toxins andshellfish depuration

The Diarrhetic Shellfish Toxins (DST), okadaic acid (OA) + dinophysistoxin-1 (DTX-1), were detected above the regulatory limit of 0.20 μg/g of digestive gland (DG) in (non-commercial) blue mussels (Mytilus edulis) from Sullivans Cove, Tasmania. Pectenotoxin-2 (PTX-2), PTX-2 seco acids and 7-epi-PTX-2 SA were also detected in mussels. This was associated with the occurrence of the toxic dinoflagellates, Dinophysis acuminata and D. fortii, which were seasonally prevalent at high cell densities (up to 7,380 cells/L for D. acuminata, 500 cells/L for D. fortii). A high density of D. truncata (1,850 cells/L) did not result in increased DST levels in M. edulis at Parsons Bay, Tasmania, suggesting that this may be a non- or weakly toxic dinoflagellate. Subtle variations among Dinophysis morphotypes can pose problems for rapid and accurate identification. Tasmanian sequences of the D1-D3 region of the large subunit rDNA of D. fortii were indistinguishable from those of D. fortii from France and D. acuta from the North Atlantic, while Tasmanian D. acuminata was indistinguishable from European and New Zealand D. acuminata. Genetic sequencing of New Zealand D. acuta failed to discriminate between Tasmanian D. fortii and New Zealand D. acuta and neither did sequencing discriminate between European D. fortii and D. acuta. A field depuration experiment was conducted in the Derwent River by placing M. edulis in 38 μm mesh size cages to screen out Dinophysis plankton cells. Mussels displayed biphasic depuration kinetics with a faster rate of PTX loss over the first 30 days followed by an increase of OA + DTX-1 depuration once there was no further change in PTX levels. The slow rate of depuration of OA + DTX-1 from day 15 to 30 followed by an increase in depuration may be attributed to mussels using lipid storage during a period of reduced food availability leading to a release of toxins in bound fractions. Solid Phase Adsorption Toxin Tracking (SPATT) detected dissolved DST in the Derwent River seawater medium at levels as high as 0.34 OA + DTX-1 μg/SPATT bag. Cellular and exuded toxicity of Prorocentrum lima varied between two culture strains isolated from different locations in Tasmania, Australia. Cellular OA was greater in the Little Swanport (PLLSP) strain (36 pg/cell) compared to the Louisville Point (PLLV) strain (3.8 pg/cell), which was the only strain producing DTX-1. PTX-2 was produced by both strains at small concentrations up to 1.2 pg/cell. This is the first reported occurrence of PTX-2 produced by P. lima. The Louisville strain excreted higher concentrations of OA (reaching 18 μg/SPATT bag) in the first 20 days compared to the Little Swanport strain (11 μg OA/SPATT bag). For both strains this declined to 4 μg/SPATT bag on day 40. Both strains exuded higher dissolved toxin levels at low cell abundance of 1,200 cells/L (PLLV strain reaching 1.6 μg OA + DTX-1/SPATT bag) compared to at 2,400 cells/L (0.4 μg OA + DTX-1/SPATT bag). Tasmanian strains of P. lima were more toxic than other global strains and poses a potential DSP risk to Tasmanian shellfish farms. In-vitro experiments with Prorocentrum lima suggest that dissolved toxins are exuded from DST producing dinoflagellates as well as from depurating mussels. Most of the DST was present dissolved in the seawater (94 %) when SPATT bags were exposed to P. lima cultures (6 % of DST in cells). Only a small amount of DST (1 %) was detected in the seawater medium when SPATT bags were exposed to contaminated mussels (99 % of DST in mussels). OA displayed an increase by more than 0.11 μg/g DG in mussels immersed in dissolved DST for 48 hrs indicating that mussels can accumulate DST in in-vitro conditions. Dissolved DST can pose an additional threat to shellfish farms and can extend harvest closure periods after toxic dinoflagellate blooms. Toxicity differences among dinoflagellate species and strains can pose problems for shellfish monitoring programs and may require phytoplankton regulatory limits to be varied according to locality.