The metabolism of nitrogen in Ulva prolifera blooming in a marginal sea of China - a case study based on a 15N-enriched culturing experiment

Ulva prolifera (U. prolifera) bloom are a severe annual ecological disaster in a marginal sea of China. Earlier studies have suggested that the growth of U. prolifera is very responsive to nitrogen nutrients in seawater. However, there is still a lack of knowledge about cell-level nitrogen metabolism in U. prolifera and its quantification. With the assistance of ¹⁵N-labeled NO₃ tracers and transcriptome analysis, we focused on nitrogen uptake, in-cell metabolism and inter-form conversion throughout the U. prolifera growth-decay phase by continuous culturing experiments. U. prolifera took up NO₃-N (17.37 μmol/g/d) in the growth phase, which led to the dominance of NO₃-N (73.75-92.15%) in the dissolved inorganic nitrogen (DIN) pool of U. prolifera cells. In the decay phase, NH₄-N accounted for 60.87-92.13% of the cell DIN pool, thus a NH₄-N release rate of 0.84 μmol/g/d was observed in the extracellular culturing media. Over the whole period of the growth-decay phase, a high release rate of DON, 59.57 μmol/g/d, from the cell to the culture media suggested intensive DON syntheses inside U. prolifera cells. The mass-balance model showed that 60% of the added NO₃-N was converted to DON by the end of cultivation and was secreted extracellularly, of which the < 1 kDa component accounted for 63.82 - 98.17% while the > 1 kDa component accounting for 1.83 - 36.18%. A comparable result was obtained from the ¹⁵N-labeled NO₃-N tracer: approximately 73.61% of NO₃-N was transformed to DON by the end of culturing, with the < 1 kDa component accounting for 67.46 - 90.86% and the > 1 kDa component accounting for 9.14 - 32.54% of the DON. The high efficiency of NO₃-N utilization and DON synthesis is attributed to the up-regulated of nitrate/nitrite transporter protein and glutamine synthetase inside the U.prolifera cell, respectively. Our study sheds light on the efficiency of U. prolifera in metabolizing nitrogen and quantifies the nitrogen interspecies transformation. Our study further suggests that an outbreak of U. prolifera could have a significant impact on the coastal nitrogen nutrient hierarchy within just one month.