Viability, growth and domoic acid toxicity of the diatom Nitzschia bizertensis following filtration by the mussel Mytilus sp.

The diatom Nitzschia bizertensis was recently discovered in Bizerte Lagoon (Tunisia, SW Mediterranean Sea), where it was shown to produce the amnesic shellfish poisoning toxin, domoic acid (DA). This diatom was previously found to be widespread in both time and space throughout the lagoon. The present study was carried out to test the hypothesis that N. bizertensis is able to survive after being filtered and ejected as biodeposits (faeces plus pseudofaeces) by mussels. N. bizertensis cultures (strains BD4 and BD8), at natural bloom concentrations, were fed to mussels (Mytilus sp.). The stomach contents (after 1 h) and biodeposits (after 48 h) were observed for the presence and viability of N. bizertensis cells. Light microscopy revealed intact and partial cells of N. bizertensis in both the stomach contents and biodeposits of the mussels. Incubation of the biodeposits resulted in the regrowth of N. bizertensis. This provides evidence that transplanted mussels have the potential to serve as vectors for the transport of N. bizertensis. Furthermore, cells regrown from the biodeposits contained DA, but at levels lower than those in pre-filtered cells (up to 11.9 fg cell−1). Thus, regrown cells remain toxigenic. The results suggest that one must guard against spreading N. bizertensis and other DA producers via the transfer of shellfish from contaminated to pristine areas.