Two stralns of the paralytic shellfish tox~n (PST) producing dinoflagellate Alexandrium lninutum Halim (highly toxic ALlV and weakly toxic AL2V) were grown in batch culture with either nitrate or phosphate as the limiting nutrient. In comparison with cells of the strain ALlV, cells of AL2V grew at a similar C-specific rate, had a higher C/N ratio, and lower ratios of chl a/chl c2 and chl a/peridinin. Neither chlorophylls nor carotenoids could be used to estimate C-biomass, N-biomass or toxin content for this organism. The toxin profile for both strains was dominated (up to 95 %) by the gonyautoxin GTX4, with smaller proportions of GTX1. GTX2 and GTX3. The rate of toxin synthesis for both strains was greatest 1 to 2 d after the N-refeeding of N-deprived cells, with the net rate of toxin synthesis exceeding that of C-biomass and cell division by a factor of up to 4. Toxin synthesis was not enhanced by short-term P-stress. N-stress alone led to a decrease in toxin cell-', but P-stress followed by N-stress did not result in such a decline, implicating phosphorus in the regulation of toxin metabolism. Although arginine is a major precursor for PST synthesis, taurine, glycine, glutamine, and cell N showed similar relations to that observed for arginine with respect to toxin content. Furthermore, the mole ratio of arginine/toxin could vary by a factor of up to 5 between ALlV and AL2V at peak values of toxin cell-', and by more than 5 within a strain when growing under different conditions. These observations suggest that the relationship between free arginine content and toxin content is complex. No explanation for the higher toxin content of A L l V IS apparent, except that ALlV has a higher N-content per cell and this may be conducive to a higher rate of synthesis of the N-rich toxins.

Dissolved free amino acids (DFAA) form a significant proportion of dissolved fixed nitrogen in marine waters and could provide an additional source of nitrogen for the growth of marine microalgae. With the advent of DFAA analysis by HPLC, recent studies of algal physiology, and an increased awareness of potential experimental errors, doubts are cast over the conclusion that marine microalgae are not net users of DFAA. Results obtained by testing the use of a few amino acids cannot be extrapolated; the full range of amino acids should be used in field experiments. A high rate of uptake of any one amino acid is not to be expected. It is more probable that a simultaneous uptake of several amino acids will occur at a lower rate. It is essential that environmental conditions be taken into account in interpretation of results from field experiments. Results from laboratory and field studies suggest that maximum rates of DFAA uptake would occur in dark conditions in waters depleted of dissolved inorganic nitrogen. Turbid estuaries and coastal waters would also be environments likely to induce a significant uptake of amino acids, because levels of nFA.4 i.n. sl~ch WB!P~S S~C :~!st;l.~.~!y high and the periods of enforced darkness due to turbidity are likely to induce the development of microalgal amino acid uptake systems.

Present methods used for the determination of N-status in microalgae typically involve testing for the existence of gross metabolic changes which develop in response to N-stress. Such approaches have 2 problems. First, the experimental tech~uques may be inappropriate for the species present and may perturb the organisms, possibly creating artifacts. Second, these gross changes, such as changes in rates of CO2-fixation and N-source uptake, may be affected by factors other than N-stress. There is a need to develop methods to detect metabolic changes which themselves trigger the genetic response to N-stress rather than to detect the products of that response. Such changes are likely to be in relative proportions of key metabolites of C and N metabolism, as In bacteria. It is suggested that only in the presence of excess NH: are the processes of cellular response to N-stress fully suppressed. As a consequence, microalgae throughout the oceans may show some symptoms of N-stress. The level of derepression of the N-stress responses which corresponds to growth-limitation, and hence is of ecological significance, needs to b e determined.

An unidentified o-phthaldialdehyde positive substance is often present at significant concentrations in extracts of intracellular amino acids from diatoms, dinoflagellates and prymnesiophytes. This compound elutes in reverse phase HPLC protocols similar to that of Lindroth & Mopper (1979; Analyt. Chem. 51: 1667-1674), near or with glutarnine or histidine and may thus result in errors of quantification of those amino acids. As a consequence, errors in the estimation of the Gln/Glu ratio, an index of C-N status in algae, are possible. In addition there are other compounds, some present at significant levels (notably a compound in prymnesiophytes), which elute near or with other protein amino acids. A modification to the HPLC protocol is described which not only results in the elution of the major compounds away from any of the other common 20 amino acids, but which also gives a better separation of glycine and threonine which formally were often resolved poorly.

The physiological status of 2 dinoflagellate populations in Bantry Bay, Ireland, was examined by use of amino acid analys~s. Observations were made durlng two 24 h periods. In one, during an upwelling event, the phytoplankton population was of low density (ca 1 yg chl a I-') and dominated mainly by Ceratium, Prorocentrum and Scrippsiella species. In the second (1 wk later), following relaxation of the upwelling and an incursion of surface water from the adjacent open coastal region, the phytoplankton community was dominated by Ceratium spp. and Gyrodiniurn aureolum with chl a concentrations up to 10 times higher. During the first sampling period, there was evidence of slight N-stress at the surface; the ratio of intracellular glutamine/glutamate (Gln/Glu) was 0.35, increasing with ammonium spiking There was evidence of C-stress at the chl maximum, where Gln/Glu was high (1.2) and decreased on exposure to increased irradiance. During the second samphng period, Gln/Glu was low (0.1) suggesting a poor N-status. However, there was little or even a negative response of Gln/Glu to ammonium splking; glutamate responded to the spiklng rather than glutamine. The general composition of the intracellular amino acid pool was similar to that at the first period, with high proportions of N-rich amino acids such as arginine. This second population appeared to be physiologically damaged in some way; this would not appear to have been simply a result of N-stress, but perhaps reflected the consequences of the associated advective process and exposure of the algae to high photon flux densities. The concentration of dissolved free a m n o acids (DFAA) during the second period was up to 4 times higher than at the first, but still dominated by serine and glycine. DFAA appeared to relate more to the presence of zooplankton than to the phytoplankton. KEY WORDS: Dinoflagellate . DFAA . Intracellular amino acids. Gln/Glu . Nitrogen physiology. Gyrodinium aureolurn . Ceratiurn spp. -Prorocentrurn spp. . ScrippsieUa spp.