Interaction of nitrogen source and light intensity on the growth and photosynthesis of the brown tide alga Aureococcus anophagefferens

High ratios of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) have been suggested to favor the growth of the brown tide alga Aureococcus anophagefferens. DON could provide a particular advantage in low light levels, as occur when blooms induce self-shading. We examined the effects of varying DON:DIN ratios on the photosynthetic abilities of cultured Aureococcus at two light intensities, 93 and 17 μmol photons m−2 s−1. Glutamic acid and urea were used as DON sources, and the remainder of the nitrogen was added as nitrate. In experiments examining Aureococcus growth with varying ratios of DONGlu:DINNitrate at two light intensities in batch culture, higher growth rates and biomass were observed in treatments containing DIN than in those with DON only, which contrasts with the results of previous studies. In semi-continuous growth experiments with varying DONUrea:DINNitrate ratios, low light cultures with urea had higher growth rates than those without urea. Also, the effective target area for light absorption per cell and photosystem II efficiency were greater for the low light cultures of each nutrient treatment, particularly when DON:DIN mixtures (33 and 67% NUrea) were used. The same pattern was seen in the maximum photosynthetic rates per cell in the light-saturated (Pmcell) and in the initial slope (αcell) of the PE (photosynthesis versus irradiance) curve, and in PON, POC and chlorophyll a cell−1. This indicates that the ability of Aureococcus to acclimate to low light conditions may be enhanced by the presence of both organic and inorganic nitrogen sources. These results suggest that Aureococcus physiology and photosynthesis are different during growth on a mixture of urea-N and nitrate than when either nitrogen source is present alone. Results of this study suggest that Aureococcus may not respond to all DON substrates in the same way, and that mixtures of DON and DIN may provide for higher photosynthetic rates, especially at low light. Our results did not, however, support earlier suggestions that growth on DON alone provides the brown tide alga with a large advantage at low light levels.