Toxin accumulation, detoxification and oxidative stress in bivalve (Anomalocardia flexuosa) exposed to the dinoflagellate Prorocentrum lima

Abstract

Prorocentrum lima is a cosmopolitan benthic dinoflagellate capable of producing the diarrhetic shellfish toxins (DSTs) okadaic acid (OA) and dinophysistoxin (DTX). These compounds may cause oxidative stress and accumulate in bivalve tissues, which become vectors of intoxication to human consumers. We investigated DST accumulation, detoxification and oxidative stress biomarkers in clams (Anomalocardia flexuosa) experimentally exposed to P. lima cells or their compounds. Experimental diets consisted of 6000 cells mL^-1 of the non-toxic chlorophyte Tetraselmis sp. (C; control condition), and combinations of C with 10 P. lima cells mL^-1 (T10), 100 P. lima cells mL^-1 (T100), or to a toxin concentration of ∼4 μg OA L^-1 and ∼0.65 μg DTX-1 L^-1 (T100d). Clams were exposed to these diets for 7 days (uptake phase), followed by a 7-day depuration period. No DSTs were detected in clams exposed to treatments C (control) nor to T100d (dissolved compounds) during either uptake or detoxification phase. Conversely, clams exposed to T10 or T100 accumulated, on average, up to 2.5 and 35 μg DST kg^-1 in their whole bodies at the end of the uptake phase. These concentrations are ∼64 and ∼4.5 times lower than the regulatory level of 160 μg OA kg^-1, respectively. Accumulated OA quotas were 12-22 times higher in the digestive gland (DG) than in remaining tissues over the uptake phase. Quick toxin transformation was indicated by the early detection of conjugated compounds - DTX-1 and OA esters - in the DG after 6 h of exposure, with OA-ester representing the main compound (30 - 100 %) in that tissue over the experiment. During the depuration period, detoxification rates represented 0.024 h^-1, 0.04 h^-1 and 0.052 h^-1 for OA, DTX-1 and OA-ester, respectively. The activities of catalase, glutathione S-transferase, glutathione peroxidase and the levels of oxidative stress by lipoperoxidation varied similarly in the DG of A. flexuosa individuals subjected to T100, T100d and the control condition. However, contrasting antioxidant responses were measured in those exposed to T10. These findings indicate that no oxidative stress was primarily induced by DST-producing dinoflagellates in this clam species under laboratory conditions representative of toxic bloom situations. Even though, possible interactions should be considered under multistressor scenarios.

Keywords: Bivalve mollusk; Harmful algae; Lipophilic toxins; Oxidative stress; Toxic dinoflagellates; Toxin kinetics.