INTRACELLULAR AND EXTRACELLULAR AMINO ACID POOLS OF THE MARINE DIATOM PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE) GROWN ON UNENRICHED SEAWATER IN HIGH‐CELL‐DENSITY DIALYSIS CULTURE¹

Abstract

The consumption of inorganic macronutrients (NO3−+ NO2−, NH4+, and PO4−3) and the composition of intra‐ and extracellular dissolved free amino acid pools (IDFAA and EDFAA, respectively) were determined in continuous‐reservoir batch dialysis cultures of the marine diatom Phaeodactylum tricornutum Bohlin maintained on unenriched natural seawater as a growth medium. Nutrient diffusion (Nd), which equals the nutrient uptake of the culture, increased with the cell density and the age of the culture. A concentration of 6.77 × 107 cells · mL−1 was obtained in stationary phase, which coincided with the NO3−+ NO2− diffusion limit (Ndmax) of the dialysis apparatus. The Ndmax for NH4+ occurred much earlier, at the end of exponential growth, whereas Ndmax for PO4−3 was not attained during the growth cycle of the culture, even in early stationary phase. A significant depletion (77%) of the IDFAA pool during exponential phase was followed by a reestablishment–to approximately 60% of the initial level–of internal pools during linear and stationary growth phases. This recovery occurred during the illuminated portion of the photoperiod (12:12 h LD) and involved principally the amino acids GLN, GLU, β‐GLU, and ASN. The recovery of GLN and ASN levels was particularly significant, because the intracellular concentrations of these amino acids were higher at the end of the growth cycle than before. The EDFAA pool was generally dominated by the amino acids SER and GLY+THR; however, during active growth, ORN and LYS often constituted an important fraction. The EDFAA concentration increased until linear growth phase was reached, during which a higher concentration of total free amino acids was attained in darkness than under illumination. The EDFAA component diminished afterward, and in stationary phase this fraction returned to concentrations equivalent to those observed at the beginning of the growth cycle. The variations in EDFAA concentrations were expressed by a pronounced decrease in the cellular excretion of amino acids with increasing cell density. These cellular responses of Phaeodactylum tricornutum in dense culture, specifically the regulation of amino acid excretion and intracellular pool size, may affect the N‐conversion coefficient (YN). Consequently, by prolonging the linear phase of growth and reducing the concentration of autoinhibitory metabolites by diffusion, a markedly enhanced final cell density can be achieved in cultures grown on natural unenriched seawater.

Dynamics of Dissolved Organic Nitrogen

“…The types of amino acids released from Chaetoceros affinis (diatom) changes during exponential growth relative to stationary growth (Myklestad et al, 1989). In cultures of Phaeodactylum tricornutum, DFAA accumulate extracellularly, particularly glycine, threonine, and serine, which are all important in cellular respiration or are components of cell walls that are likely resistant to decomposition (Marsot et al, 1991).…”

Responses of marine planktonic protists to amino acids: feeding inhibition and swimming behavior in the ciliate Favella sp.

“…Direct release (exudation) by phytoplankton can be a significant source to the DFAA pool, as phytoplankton can contain substantial quantities of free amino acids (Marsot et al 1991, Granum et al 2002. Proline is an osmoregulator in some phytoplankton species and, as such, is present at high intracellular concentrations (Kirst 1989).…”

Activités métaboliques et toxicité du dinoflagellé Alexandrium tamarense en culture à dialyse exposée à un milieu estuarien

“…Les faibles valeurs de GlnIGlu enregistrkes durant la croissance de la culture en dialyse ( fig. 5A; 0,35 et 0,45) suggbrent une intervention importante du Glu via la voie du glutamate dChydrogtnase tandis que les rapports ClevCs (0,54-0,77) not& durant la phase stationnaire tCmoignent plut6t d'une implication de la Gln via les rCactions de la glutamine synthCtase et glutamate synthase (Marsot et al 1991a;Flynn et al 1994). Les concentrations cellulaires importantes en Orn et Lys, en culture B dialyse ( fig.…”

“…Le PmAm, qui est une constante relative aux opCrations de la culture B dialyse, est mesurC ? i partir des concentrations en NO3-+ NO,-du milieu de culture et de l'eau de mer B l'entrte (C,,) et i la sortie du dialyseur (C,,) et en appliquant 1'Cquation dCrivCe de la formule sur la diffusion molCculaire (Marsot et al 1991a Dawson et Liebezeit (1983). Cette mCthode a Ct C appliquCe avec succks pour la diatomke Phaeodactylum tricornutum (Marsot et al 1991a L'extraction des neurotoxines cellulaires est rtalisCe suivant le protocole dCcrit par Cembella et al (1987) et Turriff et al (1995.…”