1:45 PM - 3:15 PM
[BCG05-P02] Diversity and early evolution of protistan biopolymers
Keywords:biopolymer, palynomorph, cellulose, chitin
Protistan organic ‘hard’ shells and membranes preserved as palynomorphs (organic microfossils) are more resistant for degradation compared with other organic matter. Cellulose and chitin are representative biopolymers. Protistan biopolymers can be divided into cellulose-like and chitinous. Cell wall of Zygnematophyceae and dinoflagellate theca are composed of pure cellulose. Some autotrophic dinocysts (e.g., Spiniferites) include more resistant cellulose-like biopolymers composed of network structure by alkyl-chain linkages among polysaccharides. Biopolymers of spore and pollen, so called as sporopollenin, have the similar network structures with alkyl chains. Moreover, algaenans formed by some species of Chlorophyta (e.g., Pediastrum) are also more resistant due to existence of alkyl chains. On the other hand, simple-shape, like spherical and ellipsoidal, cysts formed by some autotrophic dinoflagellates (e.g., Alexandrium) are composed of branched polysaccharide with cellulose skeleton and easily decomposed by cellulase. Heterotrophic dinocysts, ciliate loricae/cysts, shells of testate amoebae and foraminiferal linings are chitinous palynomorphs, mainly composed of peptide and sugar chains like glycoproteins. Simple-shape chitinous palynomorphs are easily decomposed while the reason is still uncertain. In summary, higher preservation of protistan biopolymers are resulted from more complex structure of polysaccharidic macromolecules with alkyl chains for autotrophs and peptide chains for heterotrophs.
How have the structures of these biopolymers changed through the evolutionary history of protists? It is easily understood that cellulose, its synthase originated from cyanobacteria, is the most accessible resistant biopolymer for photoautotroph. In contrast, the chitin synthesis system is assumed to originate from Opisthokonta and SAR supergroups, with the former inherited by fungi and Metazoa and the latter by dinoflagellate, ciliate and foraminifera (Hoshino and Ando, JpGU2022). Therefore, it suggests that cellulose and chitin synthases were acquired by each taxon during the early evolution of protist. On the other hand, only some species of Chlorophyta synthesize algaenans and difference in macromolecular structures between Spiniferites and Alexandrium cyst in Gonyaulacales dinoflagellate, which indicate that utilization of alkyl chains for biopolymer are occurred independently in genus or species level. For these reasons, macromolecular analyses of palynomorphs from the Proterozoic, especially focus on whether cellulose or chitin, are important for understanding predator-prey relationship and the divergence of supergroups during the early evolution of protist.
It is assumed that cellulose and chitin were easily decomposed under higher concentrations of cellulase and chitinase due to flourishing of vascular plants and fungi on land after Devonian. Interestingly, acritarchs originated from early protist are simple spherical which are similar to modern labile cellulose-like or chitinous palynomorphs. In addition, the species diversity of acritarchs suddenly decreased after the Hangenberg event, the latest Devonian, which seems to be synchronous with the expansion of terrestrial ecosystem. Cellulose and chitin are resistant to thermal maturation, and both pyrolysates after heating to 400℃ maintain their characteristic structures. Hence, macromolecular analyses of acritarchs deposited before the Permian can provide the insight to aquatic ecosystem development. In fact, the macromolecular in some acritarchs from the Mesoproterozoic samples have similar structures with the pyrolysates of cellulose and chitin. However, from the analyses using modern species, these palynomorphs were mostly experienced attachment and/or chemical reaction with amorphous organic matter during early diagenesis. We are recently developing the new methods to analyze macromolecular structures of palynomorphs in detail with higher spatial resolution than micro-FTIR.
How have the structures of these biopolymers changed through the evolutionary history of protists? It is easily understood that cellulose, its synthase originated from cyanobacteria, is the most accessible resistant biopolymer for photoautotroph. In contrast, the chitin synthesis system is assumed to originate from Opisthokonta and SAR supergroups, with the former inherited by fungi and Metazoa and the latter by dinoflagellate, ciliate and foraminifera (Hoshino and Ando, JpGU2022). Therefore, it suggests that cellulose and chitin synthases were acquired by each taxon during the early evolution of protist. On the other hand, only some species of Chlorophyta synthesize algaenans and difference in macromolecular structures between Spiniferites and Alexandrium cyst in Gonyaulacales dinoflagellate, which indicate that utilization of alkyl chains for biopolymer are occurred independently in genus or species level. For these reasons, macromolecular analyses of palynomorphs from the Proterozoic, especially focus on whether cellulose or chitin, are important for understanding predator-prey relationship and the divergence of supergroups during the early evolution of protist.
It is assumed that cellulose and chitin were easily decomposed under higher concentrations of cellulase and chitinase due to flourishing of vascular plants and fungi on land after Devonian. Interestingly, acritarchs originated from early protist are simple spherical which are similar to modern labile cellulose-like or chitinous palynomorphs. In addition, the species diversity of acritarchs suddenly decreased after the Hangenberg event, the latest Devonian, which seems to be synchronous with the expansion of terrestrial ecosystem. Cellulose and chitin are resistant to thermal maturation, and both pyrolysates after heating to 400℃ maintain their characteristic structures. Hence, macromolecular analyses of acritarchs deposited before the Permian can provide the insight to aquatic ecosystem development. In fact, the macromolecular in some acritarchs from the Mesoproterozoic samples have similar structures with the pyrolysates of cellulose and chitin. However, from the analyses using modern species, these palynomorphs were mostly experienced attachment and/or chemical reaction with amorphous organic matter during early diagenesis. We are recently developing the new methods to analyze macromolecular structures of palynomorphs in detail with higher spatial resolution than micro-FTIR.