Distribution And Significance Of Microbial Lipids In Lakes Along A Salinity Gradient

Lake is one of the most widespread natural resource all over the world. Even different lakes expericed different evolvement and development stage, but the sedimentation of lake is always constrained in the semi cutoff and cutoff lake basin, which leading to the formation of independent lake sediments. Lake sediments are high-resolution record of past environmental change and have been widely applied to paleoenvironmental and paleoclimate studies, especially in Quaternary. However, there are always some mismatch beween the proxies based on lake environments, which makes the interpretions much more confused and complicated. The reason of the mismatch is the proxies applied in this lake may not work well in another different type lake. So modern process study is essential to the application of proxies, especially in some extreme lakes such as hypersaline lakes. Hitherto there are few proxies for reconstruct the climate and environment change in hypesaline lakes and also there is little known about the biogeochemistry of hypersaline lakes.Glycerol dialkyl glycerol tetraethers (GDGTs) are microbial membrane lipids that ubiquitously occur in a wide range of environments, such as oceans, lakes, soils, peats as well as hot springs. The distribution pattern of GDGTs has led to the information that they might be indicators for certain environmental parameters, and proxies based on GDGTs are increasingly being utilized in palaeoclimatology to reconstruct palaeoenvironmental parameters. GDGTs have made a lot progress over the last decade in the biogeochemistry. The distribution pattern of GDGTs in sediments provides significant implication of organisms preserved in sediments according to the biological sources of GDGTs. Previous studies have proved the biological sources of GDGTs such as ammonia-oxidizing archaea (AOA) and methanotrophs, have played very important roles in N and C elemental biogeochemical cycles. Diphytanyl Glycerol Diethers (DGDs) also present in varies sediments and have a wide range of biological sources, which complicated the real biological source(s) of these compounds. In this research, the compound specific carbon stable analysis of DGDs is applied to determine the source of DGDs in lake sediments.Previous research has proved salinity is the principal driving forces of microbial flora change. But there is little known about relationship between salinity and microbal membrane lipids such as GDGTs, in other words does salinity change the distribution pattern of GDGTs and indices based on GDGTs in saline lakes? Furthermore, the significance of GDGTs in saline lakes should also be investigated for the biogeochemistry study. Base on these, the author focuses on two parts in this thesis, one is to investigate the distribution pattern of GDGTs in differen saline lakes to figure out the relationship beween salinity and GDGTs and GDGT-based indices, the other is to examine the distribution pattern of GDGTs in hypersaline lake, try to sort out if high salinity prohibit the production of GDGTs or not.In this study the author did two field works in 2011 and 2013 to collect the samples, respectively. The first is 11 lakes with a salinity range of 0.76～278.66%o located in New Barag Left Banner in Inner Mongolia NE China. The author sampled the surface lake sediments and surrounding soils. In this research, the distribution pattern and biological sources of GDGTs were examined and the relationship between concentration of anoions and cations (salinity) of lake water and GDGTs and GDGT-based indices in lakes sediments was discussed. The second is a hypersaline lake in Qinghai Province NW China. Chaka Salt Lake is an athalassohaline hypersaline lake with the salinity of 325‰. The author sampled the river surface sediments, lakes surface sediments and surrounding soils of Chaka Salt Lake and investigated the distribution pattern and biological sources of GDGTs. Moreover, the author applied the GDGT-based proxies established from freshwater lakes, soils environments to verify the applicability in this hypersaline lake.Based on the analysis of GDGTs in 11 lakes from New Barag Left Banner in Inner Mongolia, the main conclusions are as followed:1. GDGT-0 and crenarchaeol are the two most abundant GDGT compounds both in lake sediments and sourrounding soils. The ratio of GDGT-0 in lake sediments is higher than soils, while the ratio of crenarchaeol in soils is higher than lake sediments. The crenarchaeol’(region isomer of crenarchaeol) is more abundant in soils than lake sediments. brGDGTs II and I are dominated in lake sediments and soils, but soils produce more II and I than lake sediments. The ratio of Ib with one cyclohexane is higher than other types of brGDGTs with cyclohexane (s). brGDGTs II dominated in all lake sediments are the same as previous studies.2. The isomers of brGDGTs were examined both in lake sediments and surrounding soils. The Ⅲa’is observed as the most abundant isomers in lake sediments and surrounding soils. The author infer the distribution pattern of Ⅲa’is realated to the pH of soil based on the recently published paper. There are more Ⅲa’than Ⅲa in alkaline soils with high pH, while the acid soils are with less Ⅲa’. The sources of Ⅲa’and Ⅱa’are different in lake sediments, Ⅲa’may be produced by the soils while Ⅱa’may be in situ production in lakes. The relationship between IR1036 and IR1034 is not linear in this study, the ratio of Ⅱb’is higher than Ⅱa’.3. The conclusions of biological sources of iGDGTs are as follow:molecular biology analysis indicated the mcrh gene (molecular metabolic marker of inelhanogenesis) detected in lake sediments correlates well with GDGT-0, there is also a mismatch between mcrA gene and GDGT-0/cren index. Based on the relationship between different types of iGDGTs, the author infers GDGT-0 has admixture of methanogensis and Thaumarchaeota in lake sediments. The crenarchaeol and crenarchaeol’s has a greater relationship in soils than lake sediements, which may have one common biological source. The author infers there are Thaumarchaeota Group Ⅰ.1 b in lake environment which leading to a lower cren/cren’ index of lake sediment compared to previous freshwater lakes data.4. There is a linear relationship between Ri/b index and salinity in high saline lakes, indicating high salinity may affect the distribution of iGDGTs and brGDGTs. The author also observed the relationship between ACE index and salinity. But the ACE index prefers high saline lakes. The concentrations of anions and cations of lake water may affect the contents of iGDGTs and brGDGTs, while salinity of lake water may affect GDGT-based indices. Interestingly, the concentrations of HCO3- correlates well with GDGTs distribution, indicating the biological source of GDGTs may us HCO3- other than as carbon source.Based on the analysis of GDGTs in river surface sediments, lakes surface sediments and surrounding soils from Chaka Salt Lake, the main conclusions are as followed:1. GDGTs present in river surface sediments, lakes surface sediments and surrounding soils from Chaka Salt Lake. Both iGDGTs from archaea and brGDGTs from bacteria are observed in sediments. High ratio of iGDGTs over brGDGTs in lake sediments and soils indicates these envronments are alkaline. GDGT-0 and crenarchaeol are the most two abundant GDGT compounds in these three types of samples. brGDGTs Ⅲa and Ⅱa are dominated in these three types of samples.2. Comparision analysis of brGDGTs distribution pattern of Chaka Salt Lake and freshwater lakes and soils indicating the distribution pattern of CK lake sediments is similar with previous studies, with a high abundance of Ⅱ than Ⅲ and Ⅰ. Interestingly, The ratio of Ⅲ is higher than Ⅰ in CK lake sediments, these distribution pattern was only observed in low temperature lakes. Ⅲ is the most abundant brGDGTs in surrounding soils, and the Ⅰ is the less abundant brGDGTs, this distribution pattern is different from other published soils data. The author speculated low temperature is the reason why lower ratio of Ⅰ was observed in lake sediments and surrounding soils.3. The conclusions of biological sources of iGDGTs are as follow:Based on the relationship between different types of iGDGTs, the author infers GDGT-0 has admixture of methanogensis and Thaumarchaeota in lake sediments, while Thaumarchaeota may be the only source of GDGT-0 in soils. The author infers crenarchaeol and crenarchaeol’s has different source because they have poor relationship. Also in Chaka Salt Lake, Thaumarchaeota Group Ⅰ.1b in lake environment which leading to a lower cren/cren’ index of lake sediment compared to previous freshwater lakes data.4. BIT and Ri/b indices record the alkaline condition of Chaka Salt Lake. GDGTs 1～3 produced by other microorgamisms lead to the mismatch of TEX86 reconstructed temperature. The MB/CBT reconstructed temperatures may affect the local mean summer temperature as the GDGTs-produced bacteria may grow in warmer season. The ACE index in lake sediments is higher than river sediments and surrounding soils.The distribution pattern of DGDs in lake sediments and surrounding soils indicates the lake sediments are prone to have more and higher DGDs. Among the DGDs, C20/C20 DGD are the most abundant, followed by C20/C25 DGD and OH-DGD. Salinity affects the distribution of C20/C25 DGD, but there is no linear relationship observed between salinity and C20/C25 DGD. However, there is a linear relationship observed between salinity and C20/C20 DGD, indicating high salinity leads high production of C20/C20 DGD. The compound specific carbon stable analysis of C20/C20DGD shows the carbon values of C20/C20DGD in these high saline lakes are depleted than C20/C20DGD from hypersaline environment and enriched than C20/C20DGD from methane related environments, such as AOM in marine sediments and hydrogenotrophic methanogensis. The carbon stable composition of C20/C20 DGD in these high saline lakes may indicate there are more than one biological sources of C20/C20 DGD, maybe admixture of halophiles and methanogensis.