Lasse Tor Nielsen:
Toxin production in Dinophysis and the fate of these toxins in marine mussels

Date: 30-11-2012    Supervisor: Per Juel Hansen

Diarrhetic shellfish poisoning (DSP) poses a considerable threat to food safety and to the economy of shellfish fishers and farmers in many parts of the world. Thousands of DSP intoxications have been reported, and bivalve harvesting can sometimes be closed down several months in a row. The toxins are primarily produced by the marine mixotrophic dinoflagellates Dinophysis spp., known to occur in most parts of the world. Dinophysis can, along with other planktonic organisms, be consumed by filter-feeding bivalves, and thus the toxins can accumulate. Dinophysis can produce the three toxin groups, okadaic acid (OA), dinophysistoxins (DTX) and pectenotoxins (PTX) – the latter two in a few different isomers. Toxin profiles, as well as cellular toxin quotas, vary tremendously between Dinophysis populations in situ, both within and between species, and the toxicity of a Dinophysis population is obviously decisive for its potential to cause toxin accumulation in bivalves.

One major aim of this thesis was to unravel the factors that determine the toxicity of Dinophysis populations, and for that purpose, I studied the biology, ecology and toxicology of the two widespread Dinophysis species, D. acuminata and D. acuta. I grew the two species in laboratory cultures at different irradiances (7-130 μmol photons m-2 s-1) and with different food availability. The results showed that irradiance had no effects on toxin profiles, and only limited effects of the cellular toxin contents. Rather, toxin production rates followed growth rates, thus giving stable toxin contents. Food availability also did not change the toxin profiles of either species, but starvation did increase the cellular contents of each of the toxins present. The observation that toxin production continued for several weeks after the ciliate food (Mesodinium rubrum) was depleted, demonstrates that toxin production is not directly dependent upon food uptake.

In D. acuta, we found a novel isomer of DTX-1, tentatively termed DTX-1b, and we also studied toxin excretion, finding large extracellular proportions (<90%) of especially OA and DTX-1b. The ecological roles of OA/DTX and PTX are currently unknown, but our results accentuate the potential for extracellular functions.

I also took advantage of the recent insights into Dinophysis culturing to produce the first study on accumulation of DSP toxins from Dinophysis in a bivalve species. Our results showed that the bivalve (Mytilus edulis) retained the toxins very efficiently, and exceeded the regulatory limit for OA equivalents within only a few hours of feeding. Accumulation continued linearly with time for the entire one-week study period, eventually causing mussels to be highly toxic. The mussels transformed all three toxins (OA, DTX-1b and PTX-2) to acyl esters and/or seco acids, and half-life retention time of each toxin was determined.

Finally, a review of cellular organisation and plastid retention in Mesodinium spp. and Dinophysis spp. is provided. In addition to the interest in relation to harmful algal blooms, the two species are also of great evolutionary interest due to their ability to sequester and utilize the plastids from their ingested prey. The phenomenon is especially interesting in Dinophysis spp., since the chloroplasts undergo remarkable changes in ultrastructure during the process of acquisition.