42 research outputs found

    Phosphorus limitation might promote more toxin content in the marine invader dinoflagellate Alexandrium minutum

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    Alexandrium minutum (strain AM89BM) has been grown in semi-continuous culture (0.2 vol. d super(-1)) in N-limiting (NO sub(3) super(-): PO sub(4) super(3-)=1.6 and 3.16), in P-limiting (N: P=160 and 80), and in N and P balanced (N: P=16) media. The toxin content in cells changed greatly according to the N: P ratio. Cells grown in N: P balanced condition showed an average total paralytic shellfish poisoning (PSP) content of 1.24 fmol cell super(-1). In N-limiting conditions, cells contained ca. 3 times less toxin with mean values of 0.41-0.45 fmol cell super(-1). In contrast, cells grown in P-limiting conditions contained on average 3.5 and 7 times more toxins than in the balanced N: P condition: 4.31 fmol cell super(-1) in the N: P=160 medium and 8.01 fmol cell super(-1) in the N: P=80 medium. The toxin content per carbon unit shows the same trend; the lowest content in N: P16 conditions at 59.0 fmol PSP nmol C super(-1). Since present trends in nutrient loading are assumed to have skewed some historically stable situations towards higher N: P ratios, the stimulation of toxin production by P-limiting media has important ecological consequences; in niches where DIN: DIP is >16, populations of A. minutum sufficiently concentrated to represent a significant fraction of shellfish food could pose a serious proble

    Interactions between a marine dinoflagellate (Alexandrium catenella) and a bacterial community utilizing reverine humic substances

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    Dissolved organic matter in the form of riverine humic substances stimulated the growth of both axenic nitrogen-limited Alexandrium catenella cultures and nitrogen-limited cultures with a marine bacterial community present. The biomass increase of A. catenella could not be accounted for by utilization of inorganic nitrogen compounds. However, there was a considerable release of dissolved free and combined amino acids from the humic substances that was utilized by A. catenella. About 40% of the nitrogen used by A. catenella in the axenic treatment with humic substances added was taken up as organic nitrogen. Bacterial aminopeptidase and beta -glucosidase activity was stimulated by the addition of humic substances and bacterial growth increased several-fold. Bacteria also utilized the released amino acids from the humic substances, but did not remineralize nitrogen, since no increase in ammonium concentrations could be detected in the bacteria treatments with humic substances added. In the axenic A. catenella treatment there was no significant aminopeptidase activity, suggesting that A. catenella was able to utilize the dissolved combined amino acids directly. Moreover, large fluorescently labeled dextran molecules (2000 kDa) were taken up by A. catenella in the humic treatments, showing up in vacuoles inside the cells. These results suggests that A. catenella can grow well utilizing macromolecular organic compounds containing nitrogen, probably by a direct uptake

    Nitrogenous organic substances as potential nitrogen sources, for summer phytoplankton in the Gulf of Riga, eastern Baltic Sea

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    To investigate whether uptake of dissolved organic compounds might favour the growth potential of toxic cyanobacteria and dinoflagellates, surface water was collected in late July 1997 in the Gulf of Riga, and enriched with different nitrogen sources. (1) Organic substances were added (25 mu M N) under P-, Si- and Fe-replete conditions. Urea sustained a biomass increase (protein content) 20% greater than that with NH sub(4) super(+). Glycine, hypoxanthine and putrescine gave only 50% of the biomass obtained with NH sub(4) super(+). Glutamic acid and spermine supported growth in only one case each, and guanine in none. (2) The >1000 dalton fraction of dissolved organic matter (DOM) taken from the Daugava River was provided at concentrations of 5, 15 and 49 mu M dissolved organic nitrogen (DON); P, Si and Fe were not added, except in the control. DON sustained algal growth as much as ammonium and NO sub(3) super(-), but data were somewhat inconsistent. (3) The cyanobacterium Microcystis aeruginosa was cultured in filtered (0.45 mu m) water supplemented with organic substances (25 mu M N) and DOM (15 mu M DON), under P-, Si-, and Fe-replete conditions. Growth with urea, glycine, hypoxanthine, putrescine or spermine ranged between 145 and 175% of that obtained with NO sub(3) super(-). That 15 mu M DON sustained a biomass increase representing only 70% that gained with NO sub(3) super(-) might be explained by the different relative concentrations of DON and NO sub(3) super(-). Altogether, it is inferred that urea is an important potential nitrogen source for summer phytoplankton as a whole, while dissolved free amino acids (DFAA) and DON of terrestrial origin may partly sustain growth in some individual species

    Exoproteolytic activity determined by flow injection analysis: its potential importance for bacterial growth in coastal marine ponds

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    The measurement of the fluorescent 4-methyl-7-coumarinylamine released from the hydrolysis of a non-fluorescent peptide model substrate by exoproteolytic enzymes has been adapted to flow injection analysis (FIA). FIA allows samples to be processed very quickly (less than 2 min. for triplicate determinations) with good sensitivity (< 0.1 muM) and reproducibility (relative standard deviation < 3 % at the 0.2 muM level). In a coastal marine pond, exoproteolytic activity was closely related to bacterioplankton biomass. The high activity measured in pond water (maximum velocity: V(M) almost-equal-to 1.46 to 2.54 muM.h-1) emphasizes the importance of dissolved protein hydrolysis for bacterial growth and for dissolved organic nitrogen cycling. The mean turnover time of dissolved peptides was 7.6 days, and amino acids liberated by exoproteolytic activity could potentially support, on average, 40 % of the bacterial nitrogen demand.La mesure du fluorochrome 4-méthyl-7-coumarinylamide, libéré par l'hydrolyse exoprotéolytique d'un substrat peptidique non fluorescent, a été automatisée au moyen d'un système d'analyse en flux continu. Cette technique permet l'analyse très rapide des échantillons (moins de 2 minutes pour des triplicats) avec une bonne sensibilité (< 0,1 μM) et une bonne reproductibilité (coefficient de variation < 3 % pour des concentrations supérieures à 0,2 μM). Dans un marais maritime, l'activité exoprotéolytique est étroitement liée à la biomasse bactérioplanctonique. Le haut niveau de ces activités dans les eaux (VM: 1,46 à 2,54 μM.h−1) souligne l'importance de ce processus d'hydrolyse des protéines dissoutes tant pour la croissance bactérienne que pour le cycle de l'azote organique dissous. Dans ces eaux, le "turn over " moyen des protéines dissoutes est de 7,6 jours, et les acides aminés ainsi libérés par l'activité exoprotéolytique peuvent potentiellement supporter, en moyenne, 40 % de la demande bactérienne en azote
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