ABSTRACT. Dornoic acid (DA) production by Pseudo-nitzschia multiseries (Hasle) was studied at various silicate concentrations and under silicate perturbation. Both slowly dividing and non-dividing population~ produced DA, and the production rates were inversely correlated with the ambient silicate concentrations. Production of DA was significantly enhanced when overall cell metabolism (i.e growth rate) declined as a result of silicate stress. Following silicate starvation, cultures supplemented with silicate registered uptake, but suspended DA production. Results suggest that luxury uptake of Si by P. multisenes may happen only in phys~ologically active populations, i.e, the exponential phase, but not in the stationary phase. There were 2 stages of DA production The first stage corresponded to a decline in growth caused by moderately low lcvels of remaining s~licate in the medium, wh.ile the second stage was caused by severe silicate limitation. The production rate during the second stage (13.67 to 30.20 fg DA cell-' d-') was about an order of magnitude higher than during the first stage (0.97 to 4.98 fg DA cell-' d-l). Increases and decreases in cellular DA content corresponded to decreases and increases in growth rates.KEY WORDS: Domoic acid . Pseudo-nitzschia multiseries . Silicate limitation . Batch culture INTRODUCTIONPseudo-nitzschia multiseries, formerly known as Pseudonitzschia pungens f. multiseries (Hasle, 1995), produces domoic acid (DA), which has caused amnesic shellfish poisoning (ASP) in Atlantic Canada (Addison & Stewart 1989). The ASP problem has now been observed on both the Atlantic and Pacific coasts of North America (Fritz et al. 1992, Garrison et al. 1992. Studies on DA production have shown it to occur only during the stationary phase, coinciding with low levels of silicate in the medium (Subba Rao et al. 1990, Bates et al. 1991, Pan et al. 1991). These observations suggest a possible relationship between silicate limitation and DA production. However, direct linkage between DA production and silicon depletion is unlikely, as silicate is neither a component of DA nor apparently involved in its synthesis.The magnitude of a diatom bloom is often directly related to the availability of silicon in sea water. Silicon regulates the growth and frustule formation of diatoms. Decreases in silicate concentrations to low or undetectable levels in marine and freshwater habitats during diatom blooms have been well documented (Paasche 1973a, Sommer & Stabel 1983, Egge & Aksnes 1992, Harrison et al. 1993.In cultures, silicate concentrations in the medium may regulate the yield of diatom cells (Taguchi et al. 1987). Cessation of cell division, which may be due to cessation of DNA synthesis (Darley & Volcani 1969, Sullivan & Volcani 1973, was found to accompany depletion of silicon in the culture medium (Lewin 1955, Lewin & Chen 1968, Vaulot et al. 1987, Brzezinski et al. 1990. Bates et al. (1991) first showed a connection between silicate limitation and DA production. Two of their 5 treatments with low s...

Abstract-A simple, manually diluted, semicontinuous, 96-h algal growth assay technique was developed to measure changing rates of population growth following sublethal chemical exposure. Rates were estimated directly from changing cell counts in a fixed volume of cell suspension. Short-term rate fluctuations in exponential rate parameters were observed in Selenastrum capricornutum populations using this method and similar fluctuations were also documented by reanalyzing conventional static culture assay data. Replicate cultures tended to fluctuate in unison, and patterns of population increase were similar in static assays initiated on different dates. The latter suggested that nonuniform rates of S. capricornutum population increase were not due simply to environmental variation. All populations were preacclimated to test conditions, but growth lags were consistently observed for 12-24 h following inoculation. Subsequent rate fluctuations probably resulted from a high degree of cell-cycle synchronization. Treating systematic rate fluctuations as random error lowered measurement precision, especially with respect to estimates of rate changes over time. Systematic variance may be difficult to eliminate in practice, but repeated-measures regression methods can account for this effect and substantially reduce rate parameter confidence intervals. Findings are expected to apply to endpoints such as dry weight, total cell volume, chlorophyll, or DNA.