This dataset includes primary data generated during our investigation of the effects of competing nitrogen sources on the assimilation of nitric oxide (NO) into algal biomass by the harmful algal species, Heterosigma akashiwo. H. akashiwo was grown in continuous culture on artificial seawater and provided with either nitrate, ammonium or nitrate plus ammonium as a sole source of nitrogen. Cultures were spiked with NO (100 µM), or NO3-, or NH4+ at concentrations equal to 10% ambient levels. Controls were spiked with an equal volume of water. Samples were collected for RT-qPCR analysis of gene expression at 15 and 60 minutes after spiking. Additional samples were collected for analysis of NR enzyme activity at 2 hours after spiking, and again at 4 hours and 24 hours to evaluate assimilation of NO into biomass.