Understanding the nitrogen uptake and assimilation of the Chinese strain of Aureococcus anophagefferens (Pelagophyceae)

Brown tides caused by Aureococcus anophagefferens have occurred annually and severely impacted on the local ecology and economy in the coastal waters of Qinhuangdao, China since 2009. The uptake preference, pigment composition and growth responses of the Chinese strain of A. anophagefferens to different nitrogen substrates were studied in the laboratory to better understand its nitrogen (N) strategies and compare the findings with those from the other strains in the U.S.A. Our results indicated that A. anophagefferens had a high absolute uptake rate (ρmax) for NH4+ and low half-saturation constants (Ks) for NO3− and urea. It grew faster on the organic substrates (urea and glutamic acid) than that on the inorganic substrates (NO3− and NH4+). A. anophagefferens had higher contents of cellular pigments for both photosynthesis (chlorophyll a, chlorophyll c2, 19′-butanoyloxyfucoxanthin and fucoxanthin) and photoprotection (β-Carotene and diadinoxanthin) on urea than did on NO3− and NH4+, suggesting a stronger capability for photosynthesis on urea. A. anophagefferens had higher contents of cellular carbon, N and proteins on urea than did on NO3− and NH4+, suggesting a stronger capability for the acclimation and storage of C and N on urea. A. anophagefferens produced more β-carotene rather than chlorophyll a on glutamic acid and thus inhibited photosynthesis and carbon fixation. We found in this study that urea was the best N substrate for the growth of A. anophagefferens. A. anophagefferens might have a competitive advantage over other phytoplankton in waters enriched with DON, particularly with urea.