Effect of temperature, salinity and nutrients on the growth and toxin content of the dinoflagellate Gymnodinium catenatum from the southwestern Mediterranean.

The dinoflagellate Gymnodinium catenatum is considered the primary cause of recurrent paralytic shellfish toxins (PSTs) in shellfish on the Moroccan Mediterranean coasts. The impacts of key environmental factors on the growth, cell yield, cell size and PST content of G. catenatum were determined. Results indicated that increasing salinity from 32 to 39 and nitrate concentrations from 441 µM to 1764 µM did not significantly (ANOVA, P-value >0.63) modify the growth rate of the studied species. Gymnodinium catenatum exhibited the highest growth rate at 24 °C. Cells arrested their division at 15 °C and at ammonium concentration above 441 µM, suggesting that this nitrogen form is toxic for G. catenatum. Furthermore, G. catenatum was unable to assimilate urea as a nitrogen source. In G. catenatum cells, eight analogues of saxitoxin were detected, belonging to the N-sulfocarbamoyl (C1-4, B1 and B2) and decarbamoyl (dc-GTX2/3) toxins. C-toxins contributed 92 % to 98 % of the molar composition of the PSTs. During the exponential growth, C2 tended to dominate, while C3 prevailed during the stationary phase. Toxin content per cell (ranging from 5.5 pg STXeq.cell-1 to 22.4 pg STXeq.cell-1) increased during the stationary growth phase. Cell toxin content increased with the concentrations of nitrate, ranging from 12.1 pg STXeq.cell-1 at 441 µM to 22.4 pg STXeq.cell-1 at 1764 µM during the stationary growth phase. The toxin content of G. catenatum showed the highest values measured at the highest tested temperatures, especially during the stationary phase, where toxicity reached 17.8 pg STXeq.cell-1 and 16.4 pg STXeq.cell-1 at 24 °C and 29 °C, respectively. The results can help understand the fluctuations in the growth and PST content of G. catenatum in its habitat in response to changing environmental variables in the Mediterranean Sea when exposed to increases in warming pressure and eutrophication.

Bacterial Loads in Moroccan Mussels from Harvest to Sale.

This study concerns the seasonal variation of bacteria in seawater and mussels harvested from a Moroccan coastal area subject to clandestine shellfishing. The changes in the level of bacterial counts of mussels from harvest to sale are also presented. Both seawater and mussels showed regular increases in bacterial loads from fall to summer. Freshly harvested mussels and market mussels were the most contaminated, while freshly shucked mussels, obtained by removing shells after heating shellstock, were the least contaminated. Heating, traditionally used to remove shells, was found to reduce the initial bacterial loads by 72%. However, the storage of shucked mussels for 6 to 8 h at ambient temperatures prior to marketing resulted in an increase in the number of bacteria either due to recontamination or by growth of survivors. Thus, market mussels were 20 to 86 times more contaminated than shellstock and shucked mussels, respectively. No human pathogens were found, but several species of marine vibrios were identified.