PRODUCTION OF DOMOIC ACID, A NEUROTOXIC AMINO-ACID, BY AN AXENIC CULTURE OF THE MARINE DIATOM NITZSCHIA-PUNGENS F-MULTISERIES HASLE

A microbially contaminated culture of Nitzschia pungens f. multiseries (clone TKA-2) was treated with gentamicin followed by a combination of penicillin and streptomycin. An axenic culture was successfully isolated as evidenced by sterility tests using four different nutrient enrichment media and examination by epifluorescence microscopy. The isolate had a specific division rate (0.65.d-1) comparable with that reported for nonaxenic cultures. The axenic culture produced domoic acid (DA) during the lag and stationary phases, when cell division rates were low or zero, but not during exponential growth. Production of DA resumed within 1 d after cell division ceased and reached 0.5 pg DA.cell-1.d-1 2-3 d after the onset of the stationary phase. A maximum concentration of 2.0 pg DA.cell-1 was reached after 6 d in stationary phase. Our results provide the first evidence that N. pungens f. multiseries produces DA in the absence of other microorganisms.