EFFECTS OF SILICATE LIMITATION ON PRODUCTION OF DOMOIC ACID, A NEUROTOXIN, BY THE DIATOM PSEUDO-NITZSCHIA MULTISERIES .2. CONTINUOUS-CULTURE STUDIES

Production of domoic acid (DA) by Pseudo-nitzschia multiseries (Hasle) was studied using continuous cultures with growth rates ranging from 0.06 to 0.67 d(-1). At steady states, DA concentrations were 1.65 to 5 53.20 mu g l(-1) and production rates were 0.007 to 1.354 pg DA cell(- 1) d(-1). Both were negatively correlated with rates of growth and sil icate uptake. DA production was studied further by stop ping the addit ion of fresh medium, thus producing batch mode experiments, in some of which silicate was allowed to decline, while in another silicate was increased to 85 mu M. In those where silicate declined, DA production increased by a factor of 3. The maximum production rate attained was 3 .17 pg DA cell(-1) d(-1) and the highest DA concentration in the cultu re was 768.5 mu g DA l(-1), of which 664 mu g l(-1) was in the cells ( 11.9 pg DA cell(-1)). In the experiment where silicate was enriched, D A production was suspended soon after the enrichment, but resumed when silicate in the medium became low. The results suggested differences in kinetics of DA production and growth under different supply rates o f silicate. There appear to be 2 types of conditions associated with D A production. When dissolved silicate is moderately low and there is a decline in overall physiological activity, intrinsic factors probably trigger the formation of a moderate amount of DA. When dissolved sili cate is severely limiting, the extrinsic stress leads to considerably enhanced production of DA.