藿类生物标志物及其对海洋碳氮循环过程的指示

由微生物介导的海洋碳氮生物地球化学循环对全球气候变化具有重要影响.五环三萜的藿类化合物（hopanoids），包括以细菌藿多醇（BHPs）为主的生物藿类和藿烷等地质藿类，是指示近现代环境和重建古环境中碳氮循环过程的重要类脂生物标志物.本文总结了藿类化合物的生物合成途径和生理功能，及在海洋碳氮循环关键过程（固氮、硝化、厌氧氨氧化、甲烷氧化和陆源有机质输入）中的指示作用.微生物膜脂中的藿类化合物主要由hpn基因编码的藿类合成和修饰酶调控，与微生物理化特征及环境条件具有密切关系.2-甲基藿类、2-甲基环醇醚和不饱和环醇醚等藿类化合物可调节细胞膜的氧渗透性以发挥固氮酶保护作用，可指示环境中固氮蓝细菌及其固氮过程.细菌藿四醇异构体之一的BHT-x是海洋厌氧氨氧化菌Candidatus Scalindua的专属产物，可指示海洋厌氧氨氧化和低氧环境.土壤标志物BHPs及陆源输入指标R_soil可追踪陆源有机质向海洋环境的输入和迁移.35-氨基BHPs和3-甲基（氨基） BHPs可指示好氧甲烷氧化活动.细菌藿六醇和3-甲基细菌藿六醇可指示亚硝酸盐型甲烷氧化活动.未来，随着分子生物学、基因组学和仪器分析技术的不断发展，藿类化合物在指示海洋碳氮循环过程方面势必会发挥更加重要的作用.

Hopanoids biomarkers and their indications in the marine carbon and nitrogen cycles

The microbe-mediated marine biogeochemical cycles of carbon (C) and nitrogen (N) have an important impact on the global climate changes. The hopanoids of pentacyclic triterpenoids, including biological hopanoids (e.g., bacteriohopanepolyols (BHPs)) and geological hopanoids (e.g., hopanes), are important lipid biomarkers for indicating and reconstructing C and N cycles in modern and ancient environments. In this paper, we reviewed the biosynthetic pathways and physiological functions of hopanoids, and their roles as indicators in the key processes of the marine C and N cycles (i.e., nitrogen fixation, nitrification, anammox, methane oxidation and terrestrial organic matter input). The hopanoids in microbial membrane lipids are mainly regulated by enzymes of hopanoids synthesis and modification encoded with hpn gene, which are closely related to microbial physicochemical characteristics and environmental conditions. The oxygen permeability of cell membrane can be regulated by 2-methyl hopanoids, 2-methyl cyclitol ether and unsaturated cyclitol ether to protect nitrogenase, being able to indicate the diazotrophic cyanobacteria and N-fixation. Marine anammox bacteria Candidatus Scalindua is the only producer known as BHT-x (one of bacteriohopanetetrol isomers) so far, which can indicate marine anammox and hypoxic environment. Soil markers BHPs and terrestrial inputs indicator of R_soil can track the input and migration of terrestrial organic matter to the ocean. Biomarkers 35-amino BHPs and 3-methyl (amino) BHPs can indicate aerobic methane oxidation. Bacteriohopanehexol and 3-methyl bacteriohopanehexol can indicate nitrite-reducing methanotrophy. With the development of molecular biology, genomics and instrumental analysis technology, hopanoids will play a more important role in indicating marine C and N cycles.