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The mechanisms and energetics of nitrate uptake by marine phytoplankton Falkowski, Paul Gordon

Abstract

The results of this study suggest that NO^ uptake in many (but not all) species of marine phytoplankton is mediated by a membrane-bound (NO⁻₃, Cl⁻)-activated adenosine triphosphatase. In the presence of NO⁻₃ and Cl⁻, semi-purified membrane preparations exhibit enhanced adenosine triphosphatase activity. The enzyme has characteristics common to other membrane-bound proteins: a break in the Arrhenius plot of 30.9 Kcal/mole at 2.9 C, parallel purification with the (Na⁺ + K⁺)-activated transport adenosine triphosphatase, and activation of catalytic activity by non-ionic and anionic detergents. It is inferred from parallel purification of the (NO⁻₃,Cl⁻)-activated adenosine triphosphatase and the physiological kinetics of NO⁻₃ uptake by intact cells, that the enzyme translocates NO⁻₃ across the cell membrane, into the cytoplasm, against the chemical concentration gradient of the ion. The half-saturation constants for activation of the adenosine triphosphatase by NO⁻₃ are less than 1 µM for most species tested and correlate with half-saturation constants for NO⁻₃ uptake by whole cells. The three dinoflagellates tested did not exhibit any relationship between NO⁻₃ concentrations and ATP hydrolysis, and it is inferred that the (NO⁻₃,Cl⁻)-activated adenosine triphosphatase is probably absent from this group. Results of metabolic inhibitor studies (including KCN, 2,4-dinitrophenol, dichlorodimethylurea, and carbonyl cyanide m-chlorophenylhydrozone) imply that the source of ATP for the NO⁻₃ transport is primarily cyclic photo-phosphorylation in vivo. These results are consistent with observations of selective inhibition of NO⁻₃ uptake in unialgal cultures as well as in natural populations. Field studies with natural phytoplankton communities from Knight Inlet, B. C, suggest a physiological adaptation to external nitrogen concentrations may occur. This adaptation is characterized by increased intracellular chlorophyll a synthesis in response to 10-15% nitrogen enrichment over a 6-8 hr period. During the adaptive period carbon fixation is temporarily suppressed, apparently due to competition between inorganic carbon and inorganic nitrogen for high-energy nucleotides from the light reactions. The results of this study are related to previous proposals for the metabolic pathway of nitrogen in marine phytoplankton. In conclusion, a modified pathway is proposed stressing (1) group differences, in that nitrogen assimilation in dinoflagellates appears different from other groups, and (2) the energetics and biochemical feed-back controls of nitrogen assimilation.

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