| 摘要: |
| 以海链藻(Thalassiosira pseudonana)为研究对象，分别考察了以硝酸盐、铵盐和尿素为氮源的氮同位素分馏作用。在建立相应理论模型基础上，分别计算出各个实验体系的ε值。结果表明，在藻类生长初期，δ15N均较低，其δ15N的积累主要发生在指数增长期，在稳定期达到最高，与氮源的初始δ15N相同；不同氮源的氮同位素分馏作用也不相同，其中以铵盐最强，硝酸盐次之，尿素最弱。考虑到实际情况下氮化合物并非单一存在，作者还进一步考察了上述3种氮源混合后对其同位素分馏作用的影响，发现混合氮源体系的表观ε值介于单一氮源时的最大与最小ε值之间，该结果较好地解释了Montoya等(1991)在Chesapeak湾的现场实验结果。 |
| 关键词: 氮同位素分馏作用,混合氮源,海洋微藻 |
| DOI： |
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| 基金项目:国家自然科学基金资助项目，49976029号；国家杰出青年基金资助项目，40025614号及国家自然科学基金重点项目，50339040号 |
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| EFFECT OF DIFFERENT NITROGEN POOLS ON NITROGEN ISOTOPIC FRACTIONATION DURING THE UPTAKE BY MARINE MICROALGEA |
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YU Zhi-Ming1, Waser N A D2, Harrison P J2
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1.Key laboratory of Marine Ecology and Environmental Sciences，Institute of Oceanology，Chinese Academy of Sciences;2.Department of Earth and Ocean Sciences，University of British Columbia
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| Abstract: |
| Nitrogen isotopic fractionation during the uptake by marine microalgae is of great importance in biogeochemical cycle of nitrogen in marine system. Nitrogen isotopic fractionation controls particulate δ15N in ocean surface layer where nitrate and ammonium are abundant, and the scientific interest in this issue is ever growing. However, understanding of the fractionation and its controlling mechanism are still very limited. Most studies in this field focused on single nitrogen pool, especially nitrate pool, and few of them on other major nitrogen pools, such as ammonium, nitrite and organic nitrogen. Research on mixed nitrogen pool which is close to the real case, is nearly nil. In this paper, effect of various nitrogen pools including nitrate, ammonium and urea on isotopic fractionation during the uptake by marine diatom, Thalassiosira pseudonarm, was studied. Furthermore, effect of mixed pool of these three nitrogen compounds was determined. Finally, suggestion is made for further research on the mechanism of isotopic fractionation when nitrogen compounds were assimilated by algae. In this study, the algae were cultured in artificial seawater in order to avoid disturbance from other nitrogen compounds in natural seawater. Experimental system was designed to be nitrogen-limited, in which N/P was 4:1 and Si/N was 2:1, to the purpose that the nitrogen compounds were assimilated completely by algae. Particulate δ15N and concentration of nitrogen compounds were measured. Based on corresponding theoretical model, the per mil enrichment factor(ε)of each experimental system was calculated. Results showed that the change pattern of δ15N in particulate in three nitrogen pools was very similar. The δ15N was low in the initial stage of algal growth. With consumption of dissolved nitrogen, particulate δ15N increased gradually. The accumulation of the δ15N occurred mainly in the period of algae exponential growth and reached the highest level during the algae steady growth period. When nitrate, ammonium and urea pools were consumed completely, their particulate δ15N were 4‰, -0.3‰, 0.1‰, respectively, which were the same or very similar to the initial δ15N values respectively. Isotopic mass balance was kept during the whole assimilation process. In addition, it was found that there was a great difference in magnitude of isotopic fractionation among nitrogen pools during algal uptake. εof ammonium, nitrate and urea pools was 20‰, 4.1‰, 0.45‰ respectively, indicating that isotopic fractionation in ammonium pool was the strongest and that in urea pool was the weakest. Considering that single nitrogen pool does not exist in real circumstance, effect of isotopic fractionation in mixed pool of three compounds including nitrate, ammonium and urea was determined. The data showed that the pattern of the δ15N variation in mixed pool was similar to that of single nitrogen pool. However, major change occurred when ammonium was assimilated by algae. Therefore, the process of nitrogen isotopic fractionation in mixed pool could be divided into two stages: in the first stage ammonium was basically assimilated by algae, and in the second stage nitrate and urea were assimilated. εof ammonium an d nitrate/urea pools were 10‰ and 0.7‰, indicating that compared with εof their single nitrogen pool, apparentεvalue of ammonium in mixed pool was relatively low, and apparentεvalue of nitrate/urea in the mixed pool was between that of their single pools. In brief, the apparentεof mixed nitrogen pool was between the maximum and minimum ofεof their single pools. The results suggested that apparentεof mixed pool depended on the contribution of all nitrogen compounds, and the pool whoseεwas higher could have a greater influence on the apparentεof mixed pool. The results in this paper could explain the data of field experiment in Chesapeak Bay by Montoya et al. |
| Key words: Nitrogen isotopic fractionation, Nitrogen pools, Thalassiosira pseudonana |
