Production of domoic acid (DA) by Pseudo-nitzschia multiseries (Hasle) was studied using continuous cultures with growth rdtcs ranging from 0 06 to 0.67 d-' At steady states, DA concentrations were 1.65 to 553.20 pg 1-' and production rates were 0 007 to 1.354 pg DA cell-' d-' Both were negatively correlated with rates of growth and silicate uptake. DA production was studled further by stopping the addition 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 pM. In those where silicate declined, DA production increased by a factor of 3. The maximum production rate attained was 3.17 pg DA cell-' d-' and the highest DA concentration in the culture was 768.5 1. 19 DA I-'. of which 664 pg 1. ' was in the cells (1 1.9 pg DA cell-'). In the experiment where silicate was enriched. DA 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 of slhcate. There appear to be 2 types of conditions associated with DA production. When dissolved silicate is moderately low and there is a decline in overall physiological activity, intnnsic factors probably trigger the formation of a moderate amount of DA When dissolved silicate IS severely limiting, the extrinsic stress leads to considerably enhanced production of DA. 1989). During these blooms, silicate concentration in the sea water was low (<2 PM; Subba Rao et al. 1988), but the bloom population survived and produced the neurotoxin domoic acid.In batch culture, the production of domoic acid (DA) by Pseudo-nitzschia nlultiseries generally occurs in the stationary phase when cell division has stopped (Subba Rao et al. 1990, Bates et al. 1991 or when growth of the culture populations decline due to silicon limitation (Pan 1994). Pan et al. (1996 -this issue) proposed that DA product~on can be divided into 2 stages: the first coincides with the late exponential phase when growth proceeds slowly, and the second occurs when silicate in the medium is depleted and the culture is in the stationary phase. The rates of DA production during the second stage (13.67 to 30.20 fg DA cell-' d-l) were an order of magnitude higher than those during the first stage (0.97 to 4.98 fg DA cell-' d-l). This implies that (1) toxin production is not necessarily associated with complete cessation of cell division, and (2) this diatom produces more DA when cells are under severe silicate stress. 0

Abundance and activity of picoplankton (here defined as cells passing a 1 pm Nucleporem screen) were studied in northern Foxe Basin, eastern Canadian Arctic. Substantial proportions (10 to 70 %) of the chlorophyll a content, ribulose-1,5-bisphosphate carboxylase activity (RuBPC, E.C. 4.1.1.39) and autofluorescent bodies present in whole seawater samples passed a 1 p screen in intact, photoautotrophic particles. A smaller fraction (10 to 25 %) of the light-dependent 14C fixation was found in this picoplankton fraction, the difference possibly explained by a selective effect of screening on photosynthetic activity rather than by heterotrophic uptake of algal exudates. About 10 % of the whole sample RuBPC was found to pass a 0.2 pm diameter screen, indicating the presence of autotrophy in marine ultramicrobacteria (ferntoplankton). A potential for growth in the ferntoplankton fraction was also indicated by substantial fixation of tritiated nucleic acid precursors into macromolecules.

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Comparison of 2 water samples, one collected from 10 m, the other from the aphotic zone (1000 m) on the Costa Rica Dome in the eastern tropical Pacific Ocean, revealed the presence in both samples of pigmented cells of several diatoms, dinoflagellates and coccoid organisms. Measurements of carbon assimilation rates in temperature-controlled incubators across a light gradlent demonstrated that the assimilation number (mg C [mg Chl a]-' h-') of the 1000 m sample was about 0.8, slmilar to that of the 10 m sample. The ratios of RuBP carboxylase to other carboxylating enzymes were also similar between 10 m and the aphotic zone. However, the initial slope a and the inhlbltion parameter P were considerably higher for the deep sample than for the 10 m sample. Possible mechanisms by which these viable algae reached the aphotic zone are discussed.

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Photosynthesis in Cylindrotheca closterium was greatest in 1-day-old cells (2-4 � O2 per 106 cells per hour), declining to about 50% of this at 3 days and to about 10 % at 14 days. Due mainly to a decrease in the intensity of photosynthesis the ratio of photosynthesis to respiration fell from 6-8 at 1 day to 34 at 3 days, and to 0.6-1.6 at 14 days. The decline in photosynthesis started before phosphate or nitrate in the culture medium was exhausted, but was accompanied by a pH change from 7.4 before inoculation to 8.4 at 1 day and 9.2 at 3 days and thereafter. White light of 1000-1500f.c. (6.4-9.6 mW/cm2) saturated photosynthesis, the compensation point varying from 50 to 100 f.c. (0.3-0.6 mW/cm2). In blue light similar in spectral composition to that found at 10m below the surface of the ocean, the compensation point was 0.5-0.6 mW/cm2.

A shipboard conductivity technique to measure the total carbon dioxide in dilute tropical water is described. The standard error of a single determination in the range 40-100 mg CO2/I is 6.0 mg/l. There were 41-102 mg CO2/l in waters of the Gulf of Thailand. In these waters, primary production determined with the usually assumed value of 90 mg CO2 gave errors ranging from 11 to 120% for the individual depth samples and from 6 to 92 % for column values as compared to the values obtained using the measured CO2 concentration.

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