Growth of Heterosigma carterae (Raphidophyceae) on nitrate and ammonium at three photon flux densities: Evidence for N stress in nitrate-growing cells'

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 pur densities (PFDs) of 50, 200, and 350 mu mol . m(-2). s(-1) in a 12:12 h LD cycle. Carbon content could be estimated from biovolume (mu g C = 0.278 x 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 mu mol . m(-2). s(-1) (cell-specific growth rate of 1.2 d(-1)) but probably not saturated in nitrate-grown cells at 350 mu mol . 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.