The effect of salinity on domoic acid production by the diatom Pseudo-nitzschia multiseries

Domoic acid (DA) is a potent algal neurotoxin produced primarily by members of the diatom genus Pseudo-nitzschia, most of which are considered cosmopolitan and can produce harmful blooms in estuarine and coastal waters. Many of these habitats are subject to extreme fluctuations in salinity and are extensively utilised as shellfish growing/harvesting areas. Knowledge of how salinity influences DA production is essential to evaluating and ultimately predicting the potential impact of Pseudo-nitzschia blooms on shellfish resources as well as various wildlife populations (e. g., marine mammals). Herein, we examine the effect of different salinities (10, 20, 30, 40 psu) on the growth and DA production rates of P multiseries. Specific growth rates remained maximal (ca. 0.9 d(-1)) and essentially unchanged at the three highest salinities tested, but decreased by about half at 10 psu (ca. 0.4 d(-1)). By comparison, total (particulate and dissolved) DA quotas (ca. 30 fmol DA cell(-1)) and toxin production rates (ca. 12 fmol DA cell(-1)day(-1)) were similar and maximal at 30 and 40 psu, yet both declined significantly (three-to seven-fold) once adapted to 10 and 20 psu. These results suggest that P multiseries is able to maintain a high growth rate at 20 psu, but at the expense of continuing to produce DA at elevated levels. Because DA production requires a supply of bioenergetic metabolites generated by photosynthesis, we propose that the additional energy needed to grow rapidly while maintaining an osmotic balance at a likely sub-optimal salinity of 20 psu reduces that available for toxin synthesis, leading to the observed decline in DA levels. Our findings suggest that DA levels should be greatest in higher salinity coastal waters versus estuaries, which is consistent with recent field observations along the Louisiana coast and may help to explain the lack of DA outbreaks in this and other low salinity estuarine-based shellfish harvesting areas.