3.3.1 Source organism(s)
DSP toxins are produced by dinoflagellates that belong to the genera Dinophysis spp. and Prorocentrum spp. These algae, under favourable environmental conditions, may grow to large numbers and produce algal blooms.
The production of DSP toxins has been confirmed in seven Dinophysis species, D. fortii (in Japan), D. acuminata (in Europe), D. acuta, D. norvegica (in Scandinavia), D. mitra, D. rotundata and D. tripos, and in the benthic dinoflagellates Prorocentrum lima, Prorocentrum concavum (or P. maculosum) and Prorocentrum redfieldi (Viviani, 1992). Three other Dinophysis species, D. caudata, D. hastata and D. sacculus, are also suspected (Hallegraeff et al., 1995). Giacobbe et al. (2000) showed that D. sacculus contained OA and DTX1 concentrations of 110-400 and 8-65 fg/cell, respectively. Maximum DSP toxins (OA+DTX1 455 fg/cell) were found in early spring blooms. The authors suggested that the role of D. sacculus in harmful events in the Mediterranean area may be far from negligible despite their low toxicity.
Detection of DSP toxins in the heterotrophic dinoflagellates Protoperidinium oceanicum and P. pellucidum may reflect their feeding on Dinophysis. Toxin productivity varies considerably among species and among regional and seasonal morphotypes in one species. For example, D. fortii in northern Japan during March and June contains high concentrations of toxins and is associated with significant accumulation of toxins in shellfish. But the same species in southern Japan during May and July does show slight toxicity and shellfish is free from toxins (Hallegraeff et al., 1995).
Pan et al. (1999) reported the production of OA, OA diol ester, DTX1 and DTX4 by Prorocentrum lima. Caroppo et al. (1999) demonstrated the potential of the non-photosynthetic species Phalacroma rotundatum in the southern Adriatic Sea to produce OA, DTX1 and DTX2. The benthic dinoflagellate Prorocentrum arenarium isolated from the reef ecosystem of Europa Island (Mozambic Channel, France) (Ten Hage et al., 2000) and also Prorocentrum belizeanum from the Belizean coral reef ecosystem were found to produce OA (Morton et al., 1998). Gonyaulax polyhedra was implicated as responsible for YTX contamination in the Adriatic mussels by Tubaro et al. (1998). Satake et al. (1997) isolated YTX from cultured cells of the marine dinoflagellate Protoceratium reticulatum.
3.3.2 Predisposing conditions
The appearance of Dinophysis, even at low densities such as 200 cells per litre, can cause already a toxification of shellfish that is enough to affect humans (Botana et al., 1996). On the other hand, only blooms greater than 20 000 cells per litre were associated with DSP in the Dutch Wadden Sea. A study of Dinophysis in the Portuguese waters revealed that the time needed for shellfish to become toxic depends not only on the presence of toxic algae but also on the relative abundance of the non-toxic accompanying species (Aune and Yndestad, 1993). Not all DSP outbreaks are accompanied by macroscopic blooms of Dinophysis spp. or Prorocentrum spp. (Viviani, 1992). Toxicity of specific Dinophysis species varies spatially and temporally, and the number of cells per litre needed to contaminate shellfish is highly variable. Significant accumulation of cells in blue mussels (20 000 to 30 000 cells per digestive gland) resulting in a high toxicity in mussels in Norway, was seen at already 1 000 to 2 000 cells of Dinophysis spp. per litre of seawater (Aune and Yndestad, 1993).
In an isolated fjord in Sweden where levels of dissolved inorganic phosphate (DIP) and nitrogen (DIN) were relatively high, low OA levels in mussels (M. edulis) were detected. Deep water in this area was rich in dissolved silicate (DSi). Areas with low DIN/DSi and DIP/DSi ratios during the end of the summer coincided with low OA levels in mussels. High OA levels in mussels occurred in areas where DSi was almost totally depleted in July and remained low during the rest of the production season. Apparently the absence of silicate favours dinoflagellates including the DSP toxin producing Dinophysis spp. (Haamer, 1995).
In the north of the Gulf of California, Mexico, promoting factors for dinoflagellate dominance, such as disappearance of diatoms, low grazing pressure, probably nitrate-limited environment, a 20 to 23 °C temperature range and thermal stratification, were present in March 1993 and April to May 1994. During these periods maximal D. caudata densities of 75 to 90x103 cells per litre were observed (Lechuga-Devéze and Morquecho-Escamilla, 1998).
P. lima is known from the benthos and plankton and is common in both warm and cool-temperate waters. Growth of cultures of P. lima (from Nova Scotia, Canada) was preceded by a prolonged lag phase. During the initial lag phase toxin levels per cell remained relatively high if nitrogen had been added to the medium. When cells began to grow total toxin level per cell generally decreased and remained between 5 and 10 pg. Cells of P. lima survived 0 °C for five weeks and recovered when brought to a higher temperature. During the cold period, some cell damage probably occurred with concomitant loss of toxins to the medium. Nitrogen concentration (NO3-) in the medium was used to limit growth or stress the cells physiologically. When growth was limited, increases in toxin associated with cells were recorded. Maximum accumulation of toxins in the cells occurred during the stationary phase. Ratios of OA/DTX1 were around five. Low OA/DTX1 ratios were associated with growing cells and higher ratios with cells in the stationary phase (McLachlan et al., 1994).
Pan et al. (1999) reported that DSP toxin synthesis by P. lima is restricted to the light period and is coupled to cell division cycle events. DTX4 synthesis is initiated in the G1 phase of the cell cycle and persists into S-phase (morning of the photoperiod), whereas OA and DTX1 production occur later during S and G2 phases (afternoon). No toxin production was measured during cytokinesis, which happened early in the dark.
The DSP incidences, or at least the presence of DSP, appear to be increasing. This may be partly due to increasing knowledge about the disease and better surveillance programmes. However, it must be noted that toxin-producing algae and toxic molluscs are frequently reported from new areas (Aune and Yndestad, 1993). DSP was first documented in 1976 from Japan where it caused major problems for the scallop fishery. Between 1976 and 1982, some 1 300 DSP cases were reported in Japan, in 1981 more than 5 000 cases were reported in Spain, and in 1983 some 3 300 cases were reported in France. In 1984, DSP caused a shutdown of the mussel industry for almost a year in Sweden. The known global distribution of DSP includes Japan, Europe, Chile, Thailand, Canada (Nova Scotia) and possibly Tasmania (Australia) and New Zealand (Hallegraeff et al., 1995).
In Japan, Dinophysis fortii has been incriminated as the organism producing DSP toxins (Van Egmond et al., 1993 and Viviani, 1992). However, the OA-producing Prorocentrum lima occurred on the Sanriku coast of northern Japan. The dinoflagellate was distributed on the surface of the algae, Sargassum confusum and Carpopeltis flabellata. This P. lima strain grew well in T1 medium at 15 °C at which tropical strains do not grow, indicating that it is a local strain which adapted to cooler environments (Koike et al., 1998).
On European Atlantic coasts, other dinoflagellate species are also involved: D. acuminata and D. acuta in Spain, D. acuminata, D. sacculus, P. lima in France; D. acuminata, P. redfieldii and P. micans in the Netherlands (Van Egmond et al., 1993 and Viviani, 1992); D. acuta, D. sacculus, D. acuminata, D. caudata and P. lima in Portugal (Van Egmond et al., 1993), D. acuta, D. acuminata, P. lima and P. concavum in Ireland; D. acuta, D. acuminata, D. norvegica, P. micans, P. minimum, P. lima in Scandinavia; and D. sacculus, D. acuminata, D. tripos, D. caudata and D. fortii in the Adriatic sea (Van Apeldoorn et al., 1998; Ciminiello et al., 1997; Marasoviæ et al., 1998; Giacobbe et al., 2000). Draisci et al. (1996a) reported the detection of PTX2 in Dinophysis fortii collected in the northern Adriatic Sea. This was the first report of such a toxin in Europe.
In the Gulf of Mexico, D. caudata was involved; in the Australian region D. fortii, D. acuminata and P. lima and at eastern Canada D. norvegica and P. lima (Van Apeldoorn et al., 1998). In Johor Strait, Singapore, D. caudata was the most frequent and abundant species from March 1997 to February 1998. Other dinoflagellates observed were Prorocentrum micans and Protoperidinium spp. (Holmes et al., 1999).
Phalacroma rotundatum which has the potential to produce toxins of the okadaic acid group, was observed in Japanese waters, in North West Spain (Ria Pontevedra) and along the southern Adriatic coast of Puglia (Italy) (Caroppo et al., 1999).
Along the Chinese coasts in the South and East China Sea, DTX1 and OA were detected in shellfish species implying that DSP toxins producers also exist in this area. Frequency and shellfish toxin levels in southern parts of the coast were greater than those in northern areas (Zhou et al., 1999).
D. acuminata and Prorocentrum minimum occurred in large numbers in the Peter the Great Bay (Sea of Japan, the Russian Federation) during summer 1995 and 1996 (Orlova et al., 1998).
The benthic dinoflagellate Prorocentrum arenarium isolated from the reef ecosystem of Europa Island (Mozambic Channel, France) (Ten Hage et al., 2000) and also Prorocentrum belizeanum from the Belizean coral reef ecosystem (USA) were found to produce OA (Morton et al., 1998).