Data from: Growth and nitrogen uptake characteristics reveal outbreak mechanism of the opportunistic macroalga Gracilaria tenuistipitata

Macroalgae has bloomed in the brackish lake of Shenzhen Bay, China continuously from 2010 to 2014. Gracilaria tenuistipitata was identified as the causative macroalgal species. The aim of this study was to explore the outbreak mechanism of G. tenuistipitata, by studying the effects of salinity and nitrogen sources on growth, and the different nitrogen sources uptake characteristic. Our experimental design was based on environmental conditions observed in the bloom areas, and these main factors were simulated in the laboratory. Results showed that salinity 12 to 20 ‰ was suitable for G. tenuistipitata growth. When the nitrogen sources' (NH4+, NO3−) concentrations reached 40 µM or above, the growth rate of G. tenuistipitata was significantly higher. Algal biomass was higher (approximately 1.4 times) when cultured with NH4+ than that with NO3− addition. Coincidentally, macroalgal bloom formed during times of moderate salinity (~12 ‰) and high nitrogen conditions. The NH4+ and NO3− uptake characteristic was studied to understand the potential mechanism of G. tenuistipitata bloom. NH4+ uptake was best described by a linear, rate-unsaturated response, with the slope decreasing with time intervals. In contrast, NO3− uptake followed a rate-saturating mechanism best described by the Michaelis-Menten model, with kinetic parameters Vmax = 37.2 µM g−1 DM h−1 and Ks = 61.5 µM. Further, based on the isotope 15N tracer method, we found that 15N from NH4+ accumulated faster and reached an atom% twice than that of 15N from NO3−, suggesting when both NH4+ and NO3− were available, NH4+ was assimilated more rapidly. The results of the present study indicate that in the estuarine environment, the combination of moderate salinity with high ammonium may stimulate bloom formation.

中国深圳湾咸淡水湖自2010年至2014年持续暴发大型藻类（macroalgae）水华。经鉴定，细基江蓠（Gracilaria tenuistipitata）为引发该水华的大型藻类物种。本研究旨在探究细基江蓠的暴发机制，通过分析盐度与氮源对其生长的影响，以及不同氮源的摄取特性。实验设计以水华发生区域的现场环境条件为依据，并在实验室中模拟了核心影响因子。研究结果显示，盐度12‰~20‰适宜细基江蓠的生长。当氮源（NH4+、NO3−）浓度达到40 μM及以上时，细基江蓠的生长速率显著升高。以NH4+为氮源培养时，藻体生物量较NO3−组高出约1.4倍。巧合的是，大型藻类水华恰好在中等盐度（约12‰）与高氮条件下形成。为解析细基江蓠水华的潜在机制，本研究进一步探究了其对NH4+与NO3−的摄取特性。NH4+的摄取过程呈线性、速率未饱和响应，且其斜率随培养时长的增加而降低。与之相反，NO3−的摄取遵循米氏-门坦（Michaelis-Menten）动力学饱和模型，动力学参数为Vmax=37.2 μM·g⁻¹（干重）·h⁻¹，Ks=61.5 μM。此外，基于15N同位素示踪法（isotope 15N tracer method），本研究发现NH4+中的15N积累速率更快，其原子百分比（atom%）达到NO3−组的2倍，表明当体系中同时存在NH4+与NO3−时，细基江蓠对NH4+的同化效率更高。本研究结果表明，在河口环境中，中等盐度与高铵态氮浓度的协同作用可能会促进大型藻类水华的形成。