Occurrence of potentially toxic microalgae and diarrhetic shellfish toxins in the digestive tracts of green sea turtles (Chelonia mydas) from southern Brazil

Algal toxins are involved in the mortality and/or illness of marine organisms via consumption of contaminated prey, or upon direct exposure to toxic cells. In this study, the presence of potentially toxic microalgal cells was investigated within the digestive tract contents of a threatened species of green turtle (Chelonia mydas). Additionally, lipophilic toxins were determined by LC-MS/MS in tissue samples (liver, stomach and/or intestine) of selected animals (n = 39 individuals) found dead-stranded in southern Brazil, from winter/2015 to autumn/2016. Thirteen potentially toxic species of microalgae (both benthic and planktonic), including seven dinoflagellates, six cyanobacteria and one diatom, were found in the digestive tract contents of green turtles. Among them, dinoflagellates belonging to the Dinophysis acuminata species complex were the most frequent (36%) and abundant (maximum average abundance of 566 cells g-1 in spring/2015). Moreover, 23% of the examined sea turtles exhibited detectable levels of the diarrhetic shellfish toxin okadaic acid (OA) in washed digestive tissues. Seven individuals accumulated OA in their intestines (max. 24.1 ng g-1) and two in the stomachs (max. 7.4 ng g-1). Toxin levels in the tissues were directly and significantly (r = 0.70, p < 0.025) associated with the cell abundance of OA-producing D. acuminata and Prorocentrum lima species complexes within the digestive contents of green turtles. Although OA concentrations were relatively low, possible chronic exposure might deteriorate general health conditions of exposed sea turtles, increasing the risk for diseases. Okadaic acid has been regarded as a tumor-promoting compound and an environmental co-factor in the incidence of fibropapillomatosis, a frequent disease in juvenile green turtles inhabiting this geographic region. Even though, only one green turtle containing OA in the digestive tissues (out of six examined) also presented fibropapillomatosis in this study. Notwithstanding, sea turtles are sentinels of ocean health. Monitoring the accumulation of algal toxins and their negative effects on these organisms contributes to conserving biodiversity and marine habitats.