Changes in carbon and nitrogen physiology during ammonium and nitrate nutrition and nitrogen starvation inIsochrysis galbana

Abstract

Isochrysis galbana was grown in a 12/12 h light/dark cycle with either nitrate or ammonium as the N-source, and then resuspended in N-flee medium. During exponential growth ammonium-growing cells contained half the fatty acid content, higher concentrations of intracellular free glutamine and asparagine, and little nitrate reductase activity in comparison with nitrate-growing cells. Differences in the glutamine/glutamate and asparagine/aspartate ratios suggest that nitrate-growing cells were relatively more N-stressed than ammoniumgrowing cells. During N-starvation the fatty acid content of ammonium-grown cells increased to levels similar to those in nitrate-grown cells and the proportion of oleic acid increased in both cultures. Possible reasons for the increased synthesis of fatty acids, with its additional demand for reductant, in cells growing on nitrate N are discussed. Nitrate reductase activity remained low in the ammoniumgrown cells and decreased rapidly in the nitrate-grown cells during N-starvation. Net chlorophyll synthesis ceased within 36 h of the start of N-deprivation but synthesis of some carotenoids continued, with proportions of echinenone increasing and fucoxanthin decreasing. The ratio of chlorophyll a to c was the same in both cultures and remained constant during N-deprivation. Electron microscopy showed increasing numbers of lipid bodies in the cells during N-starvation and the cells reacted to fixation by swelling, possibly because of decreased protein content in membranes.

An experimentally validated nitrate–ammonium–phytoplankton model including effects of starvation length and ammonium inhibition on nitrate uptake

“…This result is consistent with earlier work on Chlorella vulgaris, where the production of nitrate reductase, a specific enzyme required for nitrate assimilation, increased when cells were reared in ammonium following N starvation (Morris and Syrett, 1965). Also the marine haptophyte Isocrysis galbana registered a low but constant production of nitrate reductase when reared with ammonium as the only N source (Flynn et al, 1993). In contrast, synthesis of nitrate reductase did not occur in cells of the freshwater rhodophyte Cyanidium caldarium grown in ammonium-based media (Rigano and Violante, 1973).…”

Isochrysis galbana and Diacronema vlkianum biomass incorporation in pasta products as PUFA’s source

“…This Haptophyta is of substantial interest in aquaculture, principally to feed mollusk larvae, as well as fish and crustaceans in the early stages of growth (Pernet, Tremblay, Demers, & Roussy, 2003;Poisson & Ergan, 2001;Renaud, Thinh, Lambrinidis, & Parry, 2002;Sánchez, Martínez, & Espinola, 2000;Saoudi-Helis, Dubacq, Marty, Samain, & Gudin, 1994). Other important biomolecules such as sterols, tocopherols, pigments and pharmaceuticals, among others, are also synthesized by this microalga (Fabregas & Herrero, 1990;Flynn, Zapata, Garrido, Opik, & Hipkin, 1993;Volkman, Smith, Eglinton, Forsberg, & Corner, 1981).…”

Utilization of dissolved inorganic carbon (DIC) and the response of the marine flagellate Isochrysis galbana to carbon or nitrogen stress

“…In this study, Chl a : C ratios were highest in C-stressed cultures, and decreased with increased N stress. The high degree of variability in the Chl a : C ratio (by a factor of 2) of I. galbana due to nutrient deprivation (as also noted by Flynn et al, 1993) indicated that Chl a measurements could not be used to reliably estimate phytoplankton biomass. In addition, dinoflagellates generally have lower Chl a : C ratios than diatoms (Tang, 1996), and the use of Chl a measurements to estimate the biomass of a mixed population of unknown nutrient history is debatable.…”