Assessing the potential of amino acid 13C patterns as a carbon source tracer in marine sediments: effects of algal growthconditions and sedimentary diagenesis

[摘要] Burial of organic carbon in marine sediments has a profound influence inmarine biogeochemical cycles and provides a sink for greenhouse gases suchas CO₂ and CH₄. However, tracing organic carbon from primaryproduction sources as well as its transformations in the sediment recordremains challenging. Here we examine a novel but growing tool for tracingthe biosynthetic origin of amino acid carbon skeletons, based on naturallyoccurring stable carbon isotope patterns in individual amino acids (δ¹³C_AA). We focus on two important aspects for δ¹³C_AA utility in sedimentary paleoarchives: first, the fidelityof source diagnostic of algal δ¹³C_AA patterns acrossdifferent oceanographic growth conditions, and second, the ability ofδ¹³C_AA patterns to record the degree of subsequentmicrobial amino acid synthesis after sedimentary burial. Using the marinediatom Thalassiosira weissflogii, we tested under controlled conditions how δ¹³C_AApatterns respond to changing environmental conditions, including light,salinity, temperature, and pH. Our findings show that while differingoceanic growth conditions can change macromolecular cellular composition,δ¹³C_AA isotopic patterns remain largely invariant. Theseresults emphasize that δ¹³C_AA patterns should accuratelyrecord biosynthetic sources across widely disparate oceanographicconditions. We also explored how δ¹³C_AA patterns change asa function of age, total nitrogen and organic carbon content after burial,in a marine sediment core from a coastal upwelling area off Peru. Based onthe four most informative amino acids for distinguishing between diatom andbacterial sources (i.e., isoleucine, lysine, leucine and tyrosine),bacterially derived amino acids ranged from 10 to 15 % in the sediment layers from thelast 5000 years, and up to 35 % during the last glacial period. Thegreater bacterial contributions in older sediments indicate that bacterialactivity and amino acid resynthesis progressed, approximately as a functionof sediment age, to a substantially larger degree than suggested by changesin total organic nitrogen and carbon content. It is uncertain whether archaea mayhave contributed to sedimentary δ¹³C_AA patterns weobserve, and controlled culturing studies will be needed to investigatewhether δ¹³C_AA patterns can differentiate bacterial from archealsources. Further research efforts are also needed to understand how closelyδ¹³C_AA patterns derived from hydrolyzable amino acidsrepresent total sedimentary proteineincous material, and more broadlysedimentary organic nitrogen. Overall, however, both our culturing andsediment studies suggest that δ¹³C_AA patterns in sedimentswill represent a novel proxy for understanding both primary productionsources, and the direct bacterial role in the ultimate preservationof sedimentary organic matter.

[学科分类] 地球化学与岩石