The biogeochemical research of microbial hopanoids in Dajiuhu peatland, central China

Peatlands are a vast store of organic carbon and play a significant role in the global carbon cycle. The abundance of lipids, especially microbial hopanoids are, directly or indirectly, participating in the carbon cycle in peatlands. Although the hopanoids and their compound-specific carbon isotope composition have been applied to some extent in the study of paleoenvironment and paleoecology, it is still misty about the diagenetic transformation process and the main controlling factors of the early diagenetic transformation of hopanoids. The potential of paleo-ecological application of hopanoids is still lack of systematic understanding. Previously we investigated several typical peatlands in China and found pH plays an important role in the early diagenetic transformation of bacteriohopanepolyols (BHPs) into geohopanoids. We also found the pH value promotes the isomerization of geohopanoids. Here, we focus on the biogeochemical research of microbial hopanoids in Dajiuhu peatland. We  first carried out a series of modern process monitoring on a seasonal scale, such as monthly investigating the climate factors (air temperature, air humidity, rainfall), water, soil temperature, soil moisture, water chemistry (pH, ORP, conductivity, dissolved oxygen), the main nutrient (nitrate, phosphate, etc.), dissolved organic carbon (content, composition and isotopic composition). On this basis, we discussed the relationship between those compounds (composition, carbon and hydrogen isotopes) and the climate-environmental conditions as well as the carbon dynamics. Further on, we examined the response of the carbon cycle based on hopanoid index in a peat core (18ka BP) in Dajiuhu peatland. Our results showed that in acidic peat deposits in Dajiuhu peatland, the carbon isotopes of hopane are generally more than 5‰ positive compared with the carbon isotopes of n-alkanes from higher plants both the surface peat samples and core peat samples, which indicates that in acidic peat environment, the hopanoid-produced bacteria mainly take carbohydrate as carbon source. We also showed that the difference of the carbon isotopes between hopane and n-alkanes is not stable, suggesting that these hopanoids may use different carbon sources.