大洋球石藻（Gephyrocapsa oceanica）是一种具有代表性的钙化浮游植物（颗石藻类），进行光合作用的同时，通过钙化作用合成碳酸质的外壳，在海洋碳循环中发挥着重要作用。在大洋海域，浮游植物的生长受限于营养盐；而人们对不同氮源（硝氮和氨氮）限制条件下颗石藻对光环境变化的响应认识甚微。
高坤山教授课题组采用半连续培养的方式，研究了在硝氮和氨氮限制条件下，大洋球石藻钙化和其它生理过程的变化。结果显示，在氮限制情况下，该藻的钙化作用随着光强的升高而降低；当用氨氮代替硝氮作为氮源的时候，较高光强下钙化也会降低。理论上，以氨氮（相对于硝氮）为氮源，在能量学有优势，但在较高光强或光抑制条件下，则无差异。从生态意义的角度上看，硝氮与氨氮比例变化较大的海域，颗石藻类钙化作用及其与光合作用的固碳量比例，受光强变化影响的程度会较大。
该研究成果在7月29日以“不同光强和氮源对氮限制生长下的颗石藻（大洋球石藻）的钙化和生理的影响”为题，发表在Limnology and Oceanography期刊上。论文的第一作者是2014级博士生佟善英，通讯作者为高坤山教授。
Figure. Cellular PIC content (a), PIC/POC ratio (b) and PIC production rate (c) in NO3- and NH4+ grown cultures under nitrogen limited conditions at LL (50 μmol m-1s-1), ML (190 μmol m-1s-1) and HL (400 μmol m-1s-1). The different letters above the bars indicate significant differences among the treatments (p < 0.05). The values are the means and error bars are standard deviations for triplicate cultures at each treatment.
Abstract
Gephyrocapsa oceanica is a widespread species of coccolithophore that has a significant impact on the global carbon cycle through photosynthesis and calcium carbonate precipitation. We investigated combined effects of light (50, 190 and 400 μmol m-2s-1) and the nitrogen sources and NH4+ on its physiological performance under nitrogen-limited conditions. The specific growth rate was highest at the mid-range light level of 190 μmol m-2s-1, where it was further accelerated by NH4+ relative to NO3-. There were no significant growth rate differences between NO3- and NH4+ grown cells at the two light levels either above or below this optimum irradiance. Cellular particulate organic carbon (POC) and nitrogen (PON) content were not significantly affected by different light intensities and nitrogen sources. However, both the cellular particulate inorganic carbon (PIC) content and the PIC to POC ratio were greatly decreased by increased light levels, and were further decreased by NH4+ only at the highest light level. Non-photochemical quenching (NPQ) increased with increasing light intensity, and was higher in NO3- rather than in NH4+ grown cells at medium and high light intensities. Our results demonstrate that under low, relatively realistic oceanic nitrogen concentrations, increasing light intensity and the replacement of NO3- and NH4+ would have a significant negative effect on the calcification of the coccolithophore G. oceanica. If these findings are also applicable to other coccolithophore species, the future ocean carbon cycle may be greatly affected.
Reference: Tong S, Hutchins D, Fu F, Gao K (2016). Effects of varying growth irradiance and nitrogen sources on calcification and physiological performance of the coccolithophore Gephyrocapsa oceanica grown under nitrogen limitation. Limnology and Oceanography, doi: 10.1002/lno.10371
Link to full text: http://onlinelibrary.wiley.com/doi/10.1002/lno.10371/full
