Pseudo-nitzschia sp cf. pseudodelicatissima - a confirmed producer of domoic acid from the northern Gulf of Mexico

Domoic acid (DA), a potent neurotoxin, is synthesized by certain members of the ubiquitous marine diatom genus Pseudo-nitzschia. We recently detected elevated concentrations of DA in phytoplankton field samples from the north ern Gulf of Mexico. In searching for a possible source of the toxin, we use d a receptor-binding assay to detect DA activity in cultures of R sp, cf. p seudodelicatissima (Hasle) isolated from this region and confirmed its pres ence in 2 of 7 clones using liquid chromatography coupled with tandem mass- spectrometric detection (LC-MS/MS). Unlike other toxic Pseudo-nitzschia spe cies examined previously (e,g., P. multiseries, P. australis), cellular lev els and net production of DA in these clones were highest in the early expo nential phase, while the population growth rate was high and cell concentra tion was low. There was a negative correlation between cellular DA and cell concentration. The maximum cellular DA activity in cultures was 36 fg DA e quiv. cell(-1). No net toxin production was evident in the stationary phase ,. yet extracellular DA levels increased markedly during this period to as much as 88% of the total DA in the cultures. Interestingly, these 2 toxic c lones were able to enlarge their cell size after the apical axes declined t o 15 to 25 mum, and these larger cells had considerably higher levels of DA than the original small cells. This study unequivocally establishes P. sp. cf. pseudodelicatissima as a source of DA in the northern Gulf of Mexico. Moreover, our work suggests that rapidly growing, rather than nutrient-limi ted, populations of this diatom should yield maximum net DA production rate s and DA cell quotas. Thus, the presence of P. sp, cf. pseudodelicatissima cells, even at the low levels of early, rapidly growing bloom stages, can p otentially lead to toxic events.