Nitrogen uptake and assimilation preferences of the main green tide alga Ulva prolifera in the Yellow Sea, China

The successive outbreak of large-scale Ulva prolifera green tides in the Yellow Sea, China, from 2007 to 2017, is a serious regional environmental issue that attracts worldwide attention. The competitive advantage in nitrogen uptake and utilization is an important factor, making U. prolifera the dominant green-tide-forming seaweed. However, the detailed preference characteristics of U. prolifera for nitrogen uptake and assimilation in common inorganic and organic nitrogen sources is poorly understood and is studied using stable nitrogen isotope (N-15) analysis. Our results reveal that various nitrogen sources can be simultaneously and directly taken up by U. prolifera. The uptake rates are in the sequence of NO3- (nitrate)>NH4+ (ammonium)>CO(NH2)(2) (urea)>C2H5NO2 (glycine) and C3H7NO2 (alanine). In other words, U. prolifera mostly prefers inorganic nitrogen, such as nitrate and ammonium, although it can also utilize different organic nitrogen sources simultaneously. Moreover, the assimilation of NH4+ is inhibited and its uptake cannot be fitted by the Michaelis-Menten equation when the alga is exposed to multiple nitrogen sources. We propose that at the early and middle stages of green tides the rich inorganic nitrogen sources (especially NO3-) in seawater are very important to support the fast growth of U. prolifera, while at the later stage of green tides, when inorganic nitrogen sources have been exhausted, organic nitrogen sources may contribute importantly to maintaining the growth of U. prolifera, thus lengthening the duration of green tides.