Abstract: Background, aim, and scope Cyanobacteria are prokaryotic auxoautotrophs with small size and huge amount. Cyanobacteria exist widely in the ocean, lakes and soils, and some of them can fix nitrogen. Therefore, cyanobacteria play an important role in global carbon cycle and nitrogen cycle. It is important for understanding the evolution of earth environment to reconstruct the change in cyanobacteria. Owing to short lifetime of cyanobacteria, researchers used the biomarkers in sediments to reconstruct the interglacial and glacial evolution of cyanobacteria. In this paper, we summarize the study progress and the application of 2-methylhopanoids (2-methylbacteriohopanepolyols and 2-methylhopane) which are commonly used as biomarkers for cyanobacteria. Materials and methods Bacteriohopanepolyols (BHPs) are pentacyclic triterpenoids biosynthesised by many prokaryotes, the most well-known structural feature of BHPs associated with cyanobacteria is the presence of methylation at C-2. During the process of diagenesis，the branched chain of 2-methylbacteriohopanepolyols (2-MeBHPs) could be degraded, and they would be converted into the 2-methylhopane. Therefore, 2-MeBHPs are the precursor of 2-methyhopane. Results 2-Methylhopane and 2-MeBHPs are commonly used as biomarkers of cyanobacteria. 2-MeBHPs are usually used to estimate the biomass and community structure of cyanobacteria in recent and modern environments, while 2-methylhopane can trace the past change in cyanobacteria. Additionally, the relative abundance of 2-methylhopane (2-MeHI index) can indicate the cyanobacterial contribution to biological nitrogen fixation. Discussion Recently, the studies also indicated that there are some problems about 2-methyhopanoids as biomarkers of cyanobacteria to reconstruct the abundance of cyanobacteria and the past environmental change. 1) The sources of 2-methyhopanoids are still not unclear. 2-methyhopanoids are not only originated from cyanobacteria, and not all cyanobacteria can produce the 2-methyhopanoids. The studies have revealed that the hpnP gene is found not only in some cyanobacteria but also in α-proteobacteria and acidobacteria. 2) The content of the 2-methylhopane in marine sediments may be affected by terrestrial input. 2-MeBHPs can also be produced by terrestrial cyanobacteria and can be carried into the marginal sea by the river runoff. Consequently, there is a deviation using the 2-methylhopane index to reconstruct paleoenvironment in the marginal sea. Conclusions 2-Methylhopanoids have been widely used as cyanobacterial biomarkers to indicate the biomass and evolution of cyanobacteria. However, recent studied have revealed there are some problems about 2-methylhopanoids as cyanobacterial biomarker. Recommendations and perspectives In future, biological sources of 2-methylhopanoids need to be confirmed. The influence of terrestrial 2-methylhopanoids on environmental reconstruction needs to be evaluated in the marginal sea areas. Cyanobacteria culture and molecular biological methods can help to solve these problems.
Keywords: cyanobacteria；biomarker；2-methylbacteriohopanepolyol；2-methylhopanoid index