EFFECTS OF AMMONIUM AND NITRATE ON GROWTH AND DOMOIC ACID PRODUCTION BY NITZSCHIA-PUNGENS IN BATCH CULTURE

Four clones of Nitzschia pungens f. multiseries (toxin-producing form) and two of N. pungens f. pungens (non-toxic) were grown in batch culture at initial nitrogen concentrations of 55, 110, 220, 440, and 880 muM in the form of either nitrate or ammonium. As expected, only N. pungens f. multiseries clones produced the neurotoxin, domoic acid (DA). Ammonium at 880 muM prevented the growth of all N. pungens clones but not of Skeletonema costatum, although division rate was reduced. At 440 muM, ammonium lowered the cell yield obtained during the stationary phase, inhibited photosynthesis, and caused a lag phase during which DA was produced (by f. multiseries). At 220 and 440 muM ammonium, in contrast with the same concentrations of nitrate, stationary phase cellular DA production was enhanced by two- to fourfold. At 110 and 55 muM nitrogen, cell yield and DA production were equivalent for nitrate and ammonium but less DA was produced relative to the higher nitrogen concentrations, possibly due to nitrogen depletion. Enhanced production of DA at elevated ammonium concentrations may be a response to ammonia toxicity. This could be a useful tool for studying mechanisms of DA production and for maximizing the yield of this valuable toxin in large-scale cultures.