Abstract
Domoic acid (DA), a potent neurotoxin, is synthesized by certain members of the ubiquitous marine diatom genusPseudo-nitzschia. We recently detected elevated concentrations of DA in phytoplankton field samples from the northern Gulf of Mexico. In searching for a possible source of the toxin, we used a receptor-binding assay to detect DA activity in cultures ofP. sp. cf.pseudodelicatissima(Hasle) isolated from this region and confirmed its presence in 2 of 7 clones using liquid chromatography coupled with tandem mass-spectrometric detection (LC-MS/MS). Unlike other toxicPseudo-nitzschiaspecies examined previously (e.g.,P. multiseries, P. australis), cellular levels and net production of DA in these clones were highest in the early exponential phase, while the population growth rate was high and cell concentration was low. There was a negative correlation between cellular DA and cell concentration. The maximum cellular DA activity in cultures was 36 fg DA equiv. 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 clones were able to enlarge their cell size after the apical axes declined to 15 to 25 μm, and these larger cells had considerably higher levels of DA than the original small cells. This study unequivocally establishesP. sp. cf.pseudodelicatissimaas a source of DA in the northern Gulf of Mexico. Moreover, our work suggests that rapidly growing, rather than nutrient-limited, populations of this diatom should yield maximum net DA production rates and DA cell quotas. Thus, the presence ofP. sp. cf.pseudodelicatissimacells, even at the low levels of early, rapidly growing bloom stages, can potentially lead to toxic events.