Physiological response of a red tide alga (Skeletonema costatum) to nitrate enrichment, with special reference to inorganic carbon acquisition

A classical red tide alga Skeletonema costatum was cultured under various nitrate levels to investigate its physiological response to nitrate enrichment combined with CO2 limitation. The higher nitrate levels increased content of photosynthetic pigments (Chl a and Chl c), electron transport rate in photosystem II, photosynthetic O2 evolution, and thus growth rate in S. costatum. On the other hand, the lower CO2 levels (3.5-4.4 μmol kg−1 seawater) and higher pH (8.56-8.63) values in seawater were observed under higher nitrate conditions. Redox activity of plasma membrane and carbonic anhydrase in S. costatum was enhanced to address the reduced CO2 level at higher nitrate levels. In addition, the pH compensation point was enhanced and direct HCO3− use was induced at higher nitrate levels. These findings indicate that nitrate enrichment would stimulate the breakout of S. costatum dominated red tides via enhancing its photosynthetic performances, and maintain a quick growth rate under CO2 limitation conditions through improving its inorganic carbon acquisition capability. Our study sheds light on the mechanisms of S. costatum defeating CO2 limitation during algal bloom.