Physiological conditions favorable to domoic acid production by three Pseudo-nitzschia species

Twenty-six species of the cosmopolitan genus Pseudo-nitzschia can produce domoic acid (DA), a neurotoxin responsible for amnesic shellfish poisoning (ASP). To improve knowledge on this issue, we studied the physio-logical conditions favorable to DA production and accumulation by three Pseudo-nitzschia species from French coastal waters: P. australis, P. pungens, and P. fraudulenta. They were grown in batch cultures under silicate limitation to characterize their physiological traits and calculate their DA production rates. Three strains were studied per species to consider intraspecific diversity and better characterize interspecific differences. DA pro-duction was not influenced by growth or silicate limitation in any of the three species. In contrast, silicate limitation in the stationary phase led to DA accumulation by inhibiting cell division, while DA production was still active. The maximum cellular DA (cDA) production rate was 2.95 pg cell-1 d-1 for P. australis, 0.07 pg cell-1 d-1 for P. pungens, and 0.03 pg cell-1 d-1 for P. fraudulenta. The physiological conditions favorable to cDA production and accumulation by P. australis and P. pungens differed. The three species presented similar growth rates, but P. australis had higher photosynthetic capacities that could partly explain its higher DA production potential. The cDA production and the net dissolved DA (dDA) production in the culture medium were favored by different growth conditions. The cDA production to net dDA production ratio was species specific, with P. pungens excreting proportionally more of its produced DA. These laboratory results on cultures imply that cDA production and net dDA production can occur during all phases of P. australis, P. pungens, and P. fraudulenta blooms. The interactions between the species composition of the bloom, the species-specific capacity for DA production, and the effect of silicate limitation - among other factors - on DA cell quotas drive the toxigenicity of Pseudo-nitzschia blooms.