[B-PT01] Biomineralization and Geochemistry of Proxies

[BPT01-03] Macromolecular analysis of modern aquatic palynomorphs using micro-Raman spectroscopy

*Takuto Ando¹, Tatsuyuki Yamamoto², Mika Ishigaki³, Kazumi Matsuoka⁴ (1.Estuary Research Center, Shimane University, 2.Faculty of Life and Environmental Sciences, Shimane University, 3.Center for Promotion of Project Research. Shimane University, 4.Institute for East China Sea Research, Nagasaki University, Japan)

Keywords:micro-Raman, palynomorph, biomacromolecule, polysaccharide

Palynomorphs are composed of resistant biomacromolecules and preserved partly in sedimentary rocks. Aquatic palynomorphs have great potential to reconstruct ecosystem or community composition of aquatic organisms in the past due to that they include several taxa with different ecological roles. For example, dinoflagellate cysts, ciliate cysts/loricas, foraminiferal linings, raphidophycean cysts, prasinophycean phycomata, flagments/eggs of zooplanktons, and acritarchs including Micrhystridium ariakense are deposited as aquatic palynomorphs in the sediments of Nakaumi Lagoon and Lake Shinji. However, production, preservation and distribution of these aquatic palynomorphs are not fully understood. Macromolecular analysis using micro-FTIR is common technique to know preservation/decomposition process of palynomorphs. In fact, we used micro-FTIR for chemotaxonomy and understanding preservation bias of several aquatic palynomorphs, and showed the results in this session at the last JpGU-AGU Meeting. Raman microscopy is another spectroscopic method to observe macromolecular structure and can analyze the area with micron- or submicron size. However, it is difficult to get good spectra due to emission of autofluorescence from targeted palynomorphs under the light with low wavelength (e.g., 532nm laser). In this presentation, we would like to introduce the results of micro-Raman analysis for aquatic palynomorphs succeeded by using the laser with higher wavelength (785 nm).
We analyzed authentic standards (e.g., cellulose and chitin) to compare with macromolecular structures of aquatic palynomorphs. IR spectra of polysaccharides are characterized by a dominant absorption band between 1200-1000 cm^-1 (fingerprint region) related to the stretching vibration of C-O bond. On the other hand, Raman spectra of polysaccharides are more sharp and smaller number of peaks in the same region. The data using authentic standards suggested that IR and Raman spectra are sensitive to differences in the position of glycosidic bond (e.g., α-/β- and 1,3-/1,4-/1,6-) and the type of functional groups in fingerprint region, respectively. In fact, a shift in the Raman spectra around fingerprint region of chitin with acetamide groups was observed as compared with those of cellulose with hydroxyl groups. Alexandrium catenella/pacificum is one of causative dinoflagellates for paralytic shellfish poisoning and forms a translucent ellipsoidal cyst. The highest peak of IR spectra around fingerprint region in the cyst of A. catenella/pacificum is at ~1033cm^-1 and lower than for cellulose (~1059cm^-1). On the other hand, there is no change in Raman spectra of cellulose and A. catenella/pacificum in fingerprint region. However, Raman spectra of the cyst wall in A. catenella/pacificum included the higher peaks around 1500-1200 cm^-1 region characterized by polysaccharides with α-glucoside bond and mainly related to the scissoring vibration of C-H bond. We suggested the macromolecules in cyst wall of A. catenella/pacificum is possibly composed of polysaccharides with β-glucoside bonds including smaller number of α-glucoside bonds. The cysts of A. catenella/pacificum are more labile than the dinoflagellate cysts for degradation in the sediments, which might be resulted from higher α-/β-glucoside bonds ratio in macromolecule of cyst wall.