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Content archived on 2024-05-07

Importance of organic matter from terrestrial sources for the production, community structure and toxicity of phytoplankton; role of micropredators for transmission of toxins to commercial shellfish and fish larvae


Background headlines: Harmful blooms of microalgae, including dinoflagellates and blue-green algae (cyanobacteria) are now a major environmental and economical problem in many European coastal waters. These blooms present acute hazards to animal health (wildlife, farmed fish) and human health. The importance of changes in the ratio of concentrations of dissolved N, P and Siand the increased role of silicon limitation in bringing about harmful bloom development are well documented. In addition, many microalgae can take updissolved organic nutrients. Accordingly, it has been hypothesized that increases in concentration of dissolved organic matter (DOM) from land sources would probably play a role in shifting the species composition of phytoplankton assemblages from diatoms to dinoflagellates. Moreover, some species, including the potentially highly toxic dinoflagellates, are mixotrophic; the diarrhoeic shellfish poisoning species Dinophysis acuminate,for instance, has been hypothesised to feed on cyanobacteria, in addition to aquatic heterotrophic bacteria. On the other hand, bacteria associated with toxic algae have been shown to produce a significant fraction of the toxins. New working hypotheses in the project: (i) The feeding of dinoflagellates ontoxin-producing cyanobacteria by phagotrophy is source of toxins in toxic dinoflagellates. (ii) Phagotrophy by micropredators is a route for the transfer and accumulation of toxins in commercial molluscs (e.g. mussels) and fish larvae.

Objectives of the project and related research strategy:
1. To investigate whether uptake of dissolved organic nutrients from terrestrial sources promote the growth of toxin-producing microalgae, and thus influence the community structure. Analysis of DOM will be made on two transects from a river mouth to the open sea (one transect in the Atlantic near important commercial shellfish installations; one transect in the Balti Sea, in a region important for fisheries). In waters showing critically different values of DOM concentration, rates of carbon uptake and biomass of different size classes will be measured, and community structure will be determined;
2. To investigate whether dissolved organic nutrients from terrestrial sources can promote toxin production in toxin-forming microalgae. The effect of addition of DOM to the production of toxin by potential-toxic species will be quantified (i) with cultured strains in laboratory-controlled conditions, and (ii) in land-based mesocosm experiments, in relation to the specific structuration of natural communities. The effects of addition of inorganic nutrients will be also investigated for comparison;
3. To determine whether the predation of cyanobacteria may be a source of toxin in toxic dinoflagellate blooms, and to investigate whether the predation of cyanobacteria by ciliates and toxic dinoflagellates is a pathway for the transfer of toxin to commercial filter feeding shellfish and fish larvae. Two sets of laboratory experiments will be carried out: (i) Larvae of herring will be grown with toxic food; the toxin-carrying food will be provided bycopepods grown in the presence of ciliates feeding upon toxic picocyanobacteriaand/or bacteria thriving on decaying filamentous cyanobacteria. (ii) Potentialltoxic picocyanobacteria will first be offered to ciliates or dinoflagellates. Second, these ciliates or dinoflagellates will be then offered as potential prey to oysters and/or mussels. The toxin concentration will be estimated a teach step along the experimental food chains.


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