Impacts of biogeochemical processes on phytoplankton-produced lipid biomarkers and their stable carbon isotopic compositions
MetadataShow full item record
Phytoplankton-produced lipid biomarkers and their compound-specific stable carbon isotopic compositions have been widely used to study organic carbon cycling and paleoceanography. One big uncertainty in their applications is the impact of biogeochemical cycling of organic matter on their stabilities. Although it has been assumed that the changes of these signals during biogeochemical processes are insignificant, accumulating evidence has indicated that these signals underwent diversified variations from their generation to preservation. This dissertation aims to examine (1) how physiological states of phytoplankton affect generation of cellular lipids and their stable carbon isotopic compositions, and (2) how cell respiration and microbial degradation processes alter these signals. Three series of laboratory experiments were conducted to reach my goals: (1) to grow two phytoplankton species (Thalassiosira weissflogii and Emiliania huxleyi) through exponential growth and stationary phases in batch culture, followed by dark respiration, and then microbial degradation; (2) to conduct microbial degradation of E. huxleyi cells collected from different growth phases; and (3) to incubate 13C-labeled tripalmitin in natural oxic and anoxic sediments. Bulk parameters (TOC, TN, C/N, and isotopes), lipids (alkenones, fatty acids, sterols and phytol) and associated molecular isotopic compositions were monitored for samples from these experiments. The results showed: (1) cell growth phases had an important influence on generation of chemical and isotopic signals; (2) cell respiration had little impact on chemical and isotopic signals although significant fractions of lipids were utilized; (3) microbial degradation could cause diversified alterations for chemical and isotopic signals but these alterations were more or less dependent on physiological states of phytoplankton cells; and (4) chemical reactivity of lipid compounds played one major role in altering compound-specific isotopic compositions when the compounds existed in several different pools in natural systems. Therefore, the general conclusion is that heterogeneous production of chemical and isotopic signals over different cell growth phases and selective degradation of lipids bound in different cellular components are major causes for alterations of chemical and isotopic signals during biogeochemical cycling. More studies are needed to elucidate the relations among cell physiological states, intracellular structures, and distributions of chemical and isotopic signals within these structures.