| Résumé | Small-vol. batch cultures of the toxic marine dinoflagellate Alexandrium ostenfeldii (Paulsen) Balech et Tangen were exposed to gradients of salinity, light, and inorg. nitrogen to det. their effects on growth and toxin prodn. Cell growth was detd. by in vivo fluorometry and microscopic cell counts, and spirolide prodn. through the culture cycle was measured by liq. chromatog.-mass spectrometry. Growth of A. ostenfeldii was a function of environmental conditions, and was strongly limited under sub-optimal regimes of light (≥ 150 μmol m-2 s-1), salinity (≤ 15 psu), and low nutrients (≤ 20 μM initial nitrate). Cell concn. and in vivo fluorescence were highly correlated (r2 = 0.83 to 0.97, n = 6) but the relationship varied slightly depending on exptl. conditions. Nevertheless, except under nitrogen limitation, fluorometry was an accurate method for tracking cell growth. The growth characteristics indicated that this estuarine isolate of A. ostenfeldii from Atlantic Canada was moderately "shade-adapted", had a broad salinity tolerance, and could effectively utilize much higher levels of inorg. nitrogen than commonly occur in the natural environment. Anal. of toxin prodn. and compn. throughout the culture cycle showed that total concn. of spirolides in the cellular fraction of the culture (μmol l-1) increased with cell concn., but cell spirolide quota (Qs) and toxin compn. (% molar) did not vary substantially in response to variations in environmental regime. Spirolides accumulated in the growth medium, but the concn. remained low (< 15% of total) in comparison to that in the cellular fraction. This study supports the conclusion that cell growth is directly responsive to changes in environmental variables, but spirolide prodn. is constitutive and susceptible to acclimation in culture. |
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