Effect of Suspended Particulate Matter on the Accumulation of Dissolved Diarrhetic Shellfish Toxins by Mussels (Mytilus galloprovincialis) under Laboratory Conditions

Abstract

In recent years, detection of trace amounts of dissolved lipophilic phycotoxins in coastal waters has been possible using solid phase adsorption toxin tracking (SPATT) samplers. To explore the contribution of dissolved diarrhetic shellfish toxins (DST) to the accumulation of toxins by cultivated bivalves, mussels (Mytilus galloprovincialis) were exposed to different concentrations of purified okadaic acid (OA) and dinophysistoxin-1 (DTX1) in filtered (0.45 µm) seawater for 96 h. Accumulation and esterification of DST by mussels under different experimental conditions, including with and without the addition of the food microalga Isochrysis galbana, and with the addition of different size-fractions of suspended particulate matter (SPM) (<75 µm, 75–150 µm, 150–250 µm) were compared. Results showed that mussels accumulated similar amounts of OA and DTX1 from seawater with or without food microalgae present, and slightly lower amounts when SPM particles were added. Mussels preferentially accumulated OA over DTX1 in all treatments. The efficiency of the mussel’s accumulation of OA and DTX1 from seawater spiked with low concentrations of toxins was higher than that in seawater with high toxin levels. A large proportion of OA (86–94%) and DTX1 (65–82%) was esterified to DTX3 by mussels in all treatments. The proportion of I. galbana cells cleared by mussels was markedly inhibited by dissolved OA and DTX1 (OA 9.2 µg L−1, DTX1 13.2 µg L−1) in seawater. Distribution of total OA and DTX1 accumulated in the mussel tissues ranked in all treatments as follows: digestive gland > gills > mantle > residual tissues. However, the percentage of total DST in the digestive gland of mussels in filtered seawater (67%) was higher than with the addition of SPM particles (75–150 µm) (51%), whereas the gills showed the opposite trend in filtered seawater with (27%) and without (14.4%) SPM particles. Results presented here will improve our understanding of the mechanisms of DST accumulation by bivalves in marine aquaculture environments.

Accumulation of Dinophysis Toxins in Bivalve Molluscs

“…The uptake of okadaic acid by bivalves from the dissolved phase has been demonstrated [ 63 , 64 ]. The capability of this toxin to pass through lipid bilayers by forming aggregates [ 65 ] or dimers [ 66 ] had been previously shown.…”

“…Other lipophilic toxins—the azaspiracids—for which this capability has not been demonstrated, could also be acquired by mussels in a similar way [ 67 ]. Li et al [ 63 ], observed that the concentration of OA in bivalves exposed to dissolved toxin exceeded the levels that could be expected in view of concentrations in the water, suggesting that an active uptake mechanism could exist, at least in the digestive gland.…”

“…Two species of scallops, Patinopecten yessoensis and Mimachlamys nobilis , were shown to be affected by cultures of a PTX2-producer, Dinophysis acuta , but to a degree that was not dependent on the toxin quota of the cultured cells [ 85 ]. Another okadaic acid-producing species, Prorocentrum lima , has been shown to reduce the filtration rate of mussels [ 86 ], and recently, Li et al [ 63 ] also found a reduced clearance rate in mussels exposed to okadaic acid isolated from the same species. In none of these cases can the possible contribution of other biologically active substances be ruled out.…”

“…Rossignoli [ 64 ] found that OA was taken up by fragments of digestive gland much faster when supplied in dissolved form than in an emulsion of oil droplets. Moreover, it was recently found that dissolved OA and DTX1 can be taken out by different tissues of the mussel Mytilus galloprovincialis [ 63 ], but especially by the digestive gland. Even when, at the pH levels found in the digestive gland, the main toxins of the OA group are ionized, in view of their pKa [ 116 ], they were shown to be able to self-assemble [ 66 ] or to form aggregates of several molecules [ 65 ] in a way that hides the charged parts of the molecule, allowing them to pass through the lipid bilayers (as cell membranes).…”

Simple Diffusion as the Mechanism of Okadaic Acid Uptake by the Mussel Digestive Gland

“…This is true even when an emulsifier was added in order to avoid the aggregation of the oil droplets. The persistent low OA levels found in mussels from China have been attributed to the uptake of dissolved OA from the environment [36], which can be possible by the high stability of okadaic acid in seawater [37]. The observed subcellular distribution of OA in cells of the digestive gland of naturally contaminated mussels is also consistent with the preferential uptake of dissolved OA, because nearly 90% of the OA was in the cytosol [38] and not in lysosomes, as could be expected if the absorption would have taken place by phagocytosis or other endocytic process [30].…”