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Keywords:

  • ammonium;
  • cell quota;
  • Heterosigma akashiwo;
  • Heterosigma carterae;
  • intracellular amino acids;
  • nitrate;
  • photon flux density;
  • pigments;
  • Raphido-phyceae

ABSTRACT

The marine alga Heterosigma carterae Hulburt (Raphidophyta) was grown in N-limiting batch cultures using either nitrate or ammonium as the N source, at photon flux densities (PFDs) of 50, 200, and 350 μmol·m-2·s-1 in a 12:12 h LD cycle. Carbon content could be estimated from biovolume (μg C = 0.278 × nL; R =0.98) but not reliably from pigment content. During exponential growth, ammonium-grown cells (in comparison with nitrate-grown cells at the same PFD) attained higher growth rates by at least 20%, contained more N, and had a lower C:N ratio, higher concentrations of intracellular free amino acids, and higher ratios of glutamine: glutamate (Gln: Glu) and asparagine: aspartate (Asn:Asp). Growth was nearly light-saturated on ammonium at 200 μmol·m-2·s-1 (cell-specific growth rate of 1.2 d-1) but probably not saturated in nitrate-grown cells at 350 μmol·m-2·s-1. PFD did not affect Gln: Glu or Asn: Asp for a given N source. These results indicate that the nitrate-growing cells were more N-stressed than those using ammonium (which in contrast were relatively C-stressed) and that this organism would show an enhanced competitive advantage against other species when supplied with a transient supply of ammonium rather than nitrate.