Detailed profiles of 7-O-acyl esters in plankton and shellfish from the Portuguese coast

In bivalve mollusks from the Portuguese coast contaminated by diarrhetic shellfish poisoning (DSP), most of the parent toxins, okadaic acid (OA) or dinophysistoxin-2 (DTX2), are found esterified, and toxicity assessment is only performed after an alkaline hydrolysis step to recover the parent molecules in their free form. The presence of 7-O-acyl esters with fatty acids (FAs) has already been confirmed previously in Mytilus galloprovincialis and Donax trunculus samples. This paper reports the presence of acyl esters in a wider range of estuarine and offshore bivalve species found by direct analysis in LC-MS. The total of acyl esters found in each species represented the percentages commonly found by hydrolysis in those species in previous years, justifying the majority of the esters commonly found in shellfish. This implies that any diol esters remaining after digestion of toxic microalgae would represent only a minor contribution to the ester's contents. Esters with C14:0, C16:0, C16:1, C20:5 and C22:6 FAs were the most abundant, followed by esters with C18:0, C18:1, C18:2, C18:3 and C18:4. This is the first report of OA and DTX2 esters with odd FAs: C15:0, C17:0, C17:1, and probably a branched FA: iso-C16:0. Esters with iso-C16:0 where found in high percentages particularly in two species of estuarine clams, where they represented 13-34% of total esters found. Esters were also found in plankton, predominantly with C16:0. Total esters in plankton were not higher than 10%, not enough to justify per se the high levels found in bivalves.