海洋生物固氮速率与影响因素的研究进展

Research progresses in marine biological nitrogen fixation rate and affecting factors

海洋生物固氮是指固氮生物利用固氮酶将氮气转化为生物可利用铵盐的海洋氮元素输入过程,和反硝化及厌氧氨氧化等氮流失途径一起制约着大洋氮收支平衡。而固氮速率的测定是研究海洋生物固氮的最直接方式。自发现海洋生物固氮作用以来,固氮速率的测定方法在不断更新改进,但总体来说仍存在较大不确定性。最近用15N2同位素示踪法及其他相关数据综合得到全球海洋固氮量为196.1 Tg N·a−1,最高固氮速率发生在南太平洋热带地区。但分布受到多种因素的影响。其中,物理因素中的光照和温度是全球范围固氮速率分布的最佳预测因子,光照为固氮过程提供能量,温度通过影响固氮酶活性而发挥作用。在化学因素中,铁元素的缺乏成为固氮的重要限制因子。除此之外,还有生物因素,如浮游植物和异养固氮生物等,对固氮量的贡献影响较大。最近有研究对以往固氮作用区域和反硝化作用空间相互耦合的观点表示质疑,提出二者分布空间分离的新格局。研究多控制因素对固氮生物的耦合效应、明确不同物种对固氮总量的相对贡献以及进一步建立固氮速率的原位测定方法是未来海洋固氮作用研究的主要工作。

Marine biological nitrogen fixation is a nitrogen input process in which diazotrophs use nitrogenase to convert N2 to bioavailable ammonium.Nitrogen fixation maintains the balance between the marine nitrogen budget and denitrification,as well as anammox and other nitrogen output processes.The most direct way to study marine nitrogen fixation is to measure the nitrogen fixation rate.Since the discovery of marine nitrogen fixation,various methodologies have been used to measure the nitrogen fixation rate,but some limitations still exist.The global ma-rine nitrogen fixation rate was estimated to be 196.1 Tg N·y−1 using a 15N2 tracer assay combined with related data,and the highest nitrogen fixation rate is found in the southern tropics of the Pacific.The distribution of nitrogen fixation is affected by many factors.Among them,sunlight and temperature are the best predictors of the global marine nitrogen fixation distribution.Sunlight provides energy for the nitrogen fixation process,and temperature plays an important role in affecting nitrogenase activity.Iron deficiency,one of the controlling chemical factors,is an important factor limiting nitrogen fixation.In addition,biological factors such as phytoplankton and heterotro-phic diazotrophs affect nitrogen fixation estimates.Recent studies have questioned whether the nitrogen fixation zones are near the denitrification zones and proposed a new view of spatial decoupling of nitrogen fixation and de-nitrification.Studying the coupling effects of multiple factors affecting diazotrophs,disentangling the relative con-tribution of different species to the total nitrogen estimate,and establishing an in-situ rate measurement method are required in future research on nitrogen fixation.