Growth, chain formation, and toxin production by southern Brazilian Pseudo-nitzschia isolates under laboratory conditions.

Pseudo-nitzschia is a diatom genus capable of producing the neurotoxin domoic acid (DA), which has been related to mortalities of marine vertebrates, and the amnesic shellfish poisoning (ASP) in human consumers of contaminated bivalves. This study reports DA production among Pseudo-nitzschia strains isolated from shellfish farming areas in southern Brazil. Twenty-seven cultures of potentially toxigenic Pseudo-nitzschia species were established. Growth, stepped-chain formation, and DA production were evaluated in static, intermittently illuminated (12:12 photoperiod) batch cultures for 12 selected strains, and under continuous light and/or turbulence for a single Pseudo-nitzschia calliantha strain. Growth rate ranged from 0.16 to 0.39 day-1 among the 12 strains. Only P. calliantha and P. cf. multiseries yielded detectable levels of intracellular DA, reaching up to 0.054 fg cell-1 in late exponential and 0.15 pg cell-1 in early stationary phase, respectively. Continuous light impaired cell growth, and turbulence enhanced step-chain formation by threefold during exponential growth phase, but no DA production was detected under both conditions. The effect of turbulence on chain formation should be further evaluated in the field, once particle size is expected to affect the ingestion of toxic cells and thus toxin accumulation by certain DA vectors. The low cell toxicity revealed herein under laboratory conditions is in accordance with the low frequency of DA contamination episodes in south Brazilian waters. Nevertheless, monitoring of Pseudo-nitzschia abundance and DA presence in farming areas is continuously required to assure the quality of local shellfish products.