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Chl a and C-normalized pigment ratios were studied in two dinophytes (Prorocentrum minimum and Karlodinium micrum), three haptophytes (Chrysochromulina leadbeateri, Prymnesium parvum cf. patelliferum, Phaeocystis globosa), two prasinophytes (Pseudoscourfieldia marina, Bathycoccus prasinos) and the raphidophyte Heterosigma akashiwo, in low (LL, 35 [micro]mol photons [m.sup.-2] [s.sup.-1]) and high light (HL, 500 [micro]mol photons [m.sup.-2] [s.sup.-1]). Pigment ratios in LL and HL were compared against a general rule of photo-acclimation: LL versus HL ratios [greater than or equal to]1 are typical for light-harvesting pigments (LHP) and < 1 for photo-protective carotenoids. Peridinin, prasinoxanthin, gyroxanthin-diester and 19'-butanoyloxy-fucoxanthin were stable chemotaxonomic markers with less than 25% variation between LL versus HL Chl a-normalized ratios. As expected, Chls exhibited LL/HL to Chl a ratios > 1 with some exceptions such as Chl [c.sub.3] in P. globosa and MV Chl [c.sub.3] in C. leadbeateri. LL/HL to Chl a ratios of photosynthetic carotenoids were close to 1, except Hex-fuco in P. globosa (four-fold higher Chl a ratio in HL vs LL). Although pigment ratios in P. globosa clearly responded to the light conditions the diadinoxanthin-diatoxanthin cycle remained almost unaltered at HL. Total averaged pigment and LHP to C ratios were significantly higher in LL versus HL, reflecting the photoacclimation status of the studied species. By contrast, the same Chl a-normalized ratios were weakly affected by the light intensity due to co-variation with Chl a. Based on our data, we suggest that the interpretation of PPC and LHP are highly dependent on biomass normalization (Chl a vs. C)., Introduction The assessment of phytoplankton populations constitutes a major task in many oceanographic studies, due to their important role in the pelagic food webs )Fenchel 1988) and their implications in [...]

The availability of different forms of nitrogen in coastal and estuarine waters may be important in determining the abundance and productivity of different phytoplankton species. Although urea has been shown to contribute as much as 50% of the nitrogen for phytoplankton nutrition, relatively little is known of the activity and expression of urease in phytoplankton. Using an in vitro enzyme assay, urease activities were examined in laboratory cultures of three species: Aureococcus anophagefferens Hargraves et Sieburth, Prorocentrum minimum (Pavillard) Schiller, and Thalassiosira weissflogii (Grunow) Fryxell et Hasle. Cultures of P. minimum and T. weissflogii were grown on three nitrogen sources (N[O.sub.3.sup.-], N[H.sub.4.sup.+], and urea), while A. anophagefferens was grown only on N[O.sub.3.sup.-] and urea. Urease was found to be constitutive in all cultures, but activity varied with growth rate and assay temperature for the different cultures. For A. anophagefferens, urease activity varied positively with growth rate regardless of the N source, while for P. minimum, urease activity varied positively with growth rate only for cultures grown on urea and N[H.sub.4.sup.+]. In contrast, for T. weissflogii, activity did not vary with growth rate for any of the N sources. For all species, urease activity increased with assay temperature, but with different apparent temperature optima. For A. anophagefferens, in vitro activity increased from near 0-30[degrees]C, and remained stable to 50[degrees]C, while for P. minimum, increased in vitro activity was noted from near 0-20[degrees]C, but constant activity was observed between 20[degrees]C and 50[degrees]C. For T. weissfloggii, while activity also increased from 0[degrees]C to 20[degrees]C, subsequent decreases were noted when temperature was elevated above 20[degrees]C. Urease activity had a half-saturation constant of 120-165 [micro]g atom N [l.sup.-1] in all three species. On both an hourly and daily basis, urease activity in A. anophagefferens exceeded nitrogen demand for growth. In P. minimum, urease activity on an hourly basis matched the nitrogen demand, but was less than the demand on a daily basis. For T. weissflogii, urease activity was always less than the nitrogen demand. These patterns in urease activity in three different species demonstrate that while apparently constitutive, the regulation of activity was substantially different in the diatom. These differences in the physiological regulation of urease activity, as well as other enzymes, may play a role in their ecological success in different environments.