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Incubation experiments with axenic cultures of four common phytoplankton species of the genera Chaetoeeros, Skeletonema, Prorocentrum, and Micromonas were performed to test for the production of fluorescent dissolved organic matter (FDOM) by marine phytoplankton. Our results prove that the four species exuded both fluorescent protein-like and marine humic--like materials in variable amounts, with more production by the diatoms Chaetoceros sp. and Skeletonema costatum and less by Prorocentrum minimum. Whereas the exudation of protein-like substances by healthy phytoplankton cells has been recognized, the in situ production of marine humic-like substances is still a matter of debate. Using axenic cultures, we demonstrate unequivocally that phytoplankton can directly contribute to the autochthonous production of colored humic--like substances in the ocean. Extrapolation of these findings to the field indicates that about 20% of the marine humic--like substances produced in the highly productive coastal upwelling system of the Ria de Vigo could originate from growing phytoplankton. Therefore, the exudation of FDOM by marine phytoplankton should be considered in future studies of the dynamics of colored DOM in marine systems.

Whereas it is accepted that the population dynamics of higher organisms is strongly grounded in life history processes, such as the time taken to mature and reproduce, regulation in the abundance of aquatic protists is generally attributed to proximate properties of the environment in which they grow. We used 10 yr of data from the Gulf of Naples to determine the life history of a planktonic diatom, Pseudo-nitzschia multistriata, through the analysis of cell abundances and cell size patterns. Asexual and sexual phases recurred with remarkable regularity. Maximum abundance occurred annually, between the end of summer and the beginning of autumn, but cohorts of large cells, derived from sexual events, were found only every second year. We developed a model of population growth, based on parameters determined from laboratory cultures, with further tuning using information from natural populations. The model simulated the birth, maturation, and disappearance of age classes like those in natural populations, but only when we imposed seasonal variation in the rate of cell division and timed sexual reproduction during the stationary phase of a bloom. The model predicts that P. multistriata will become locally extinct if sexual reproduction does not occur within 4 yr. Our data and model show that coherent life cycle properties can emerge in natural populations of unicellular organisms, analogous to those in multicellular organisms, as a result of finely tuned regulation of cell division and sexual competence.

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 copepods Neocalanus flemingeri and N. plumchrus are major components of the mesozooplankton on the shelf of the Gulf of Alaska, where they feed, grow and develop during April-June. The daily ingestion of phytoplankton by N. flemingeri and N. plumchrus varied widely because of mesoscale variability in phytoplankton, and these daily differences did not result in differences in final body size or lipid storage of the copepods.

A cross-ecosystem comparison of data obtained from 92 coastal zone ecosystems worldwide revealed a strong positive response of marine phytoplankton biomass to nutrient enrichment that is highly consistent with the general patterns reported previously in the limnological literature for freshwater lakes and reservoirs. Average concentrations of chlorophyll a in estuarine and coastal marine systems were strongly dependent on the mean concentrations of total nitrogen and total phosphorus in the water column. Moreover, as is true of freshwater ecosystems, the identity of the primary growth-limiting nutrient for marine phytoplankton appeared to be generally predictable from water-column total nitrogen : total phosphorus (TN : TP) ratios. This similarity in physiological response to nutrients likely derives from the shared evolutionary histories of marine and freshwater phytoplankton.

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.

A batch culture experimental method reveals that development of phytoplankton such as marine diatom and Skeletonema costatum is activated by solar irradiation which causes a rise in the bioavailability of iron. Irradiation of sea water having Fe(III) colloids leads to the formation of Fe(II). In sea water, photoproduction of Fe(II) is possible at pH 8.

The vertical and mesoscale distribution of prochlorophytes in the West Pacific Ocean is obtained by flow cytometry. Results show that picophytoplankton are omnipresent in the West Pacific Ocean, with a maximum concentration greater than 10(5) cells per ml. Moreover, their distribution is greatly affected by nitracline and light intensity. Lastly, their chlorophyll is a major contributor to the subsurface chlorophyll maximum in the West Pacific Ocean.

The photosynthetic carbon metabolism of six species of marine phytoplankton of different sizes and taxonomic categories were studied through their exponentialand lag phases of growth. The aim was to determine the extent to which the variability in the relationship among photosynthesis, growth and carbon metabolism is determined by interspecific dissimilarities. The results showed significant effects of interspecific differences.

Research has been conducted on viruses infecting coccolithophorid Emiliania huxleyi. The study of the diversity and population dynamics of these viruses has been carried out during E. huxeleyi bloom via the use of denaturing gradiant gel electrophoresis, and the authors report that only a few virus genotypes kill the bloom.