Characterization of a highly resistant biomacromolecular material in the cell wall of a marine dinoflagellate resting cyst

The remarkable physical and chemical resistance of the organic cell walls enclosing resting cysts formed by several species of dinoflagellates has long invited questions regarding their composition. Traditionally, this resistance was thought to derive from the presence of "sporopollenin", a term originally coined to describe the highly refractory substance found in the walls of pollen and spores of higher plants. The lack of detailed chemical analyses of dinoflagellate materials, however, has left this practice open to question. Here we report the results of the first rigorous chemical characterization of resting cyst walls produced by a dinoflagellate, the extant marine species Lingulodinium polyedrum (formerly Gonyaulax polyedra). Resistant cell walls were isolated by sequentially treating cyst-producing laboratory cultures by solvent extraction, saponification, and acid hydrolysis. At each stage of processing, residues were characterized by light microscopy, FTIR microspectroscopy, elemental analysis, and direct ("in source") temperature-resolved mass spectrometry (DT-MS). Initial materials and final residues were further analyzed by Curie-point pyrolysis-gas chromatography-mass spectrometry (Py-GC/ MS) and cupric oxide (CuO) oxidation. Overall, our results indicate an absence of extended n-hydrocarbon chains which typify aliphatic macromolecules ("algaenans") dominating the resistant fractions of other algae studied to date. In contrast the data suggest that the cell wall contains relatively condensed, predominantly aromatic structures, possibly cross-linked via carbon-carbon or ether bonds. The presence of prist-1-ene among the most prominent pyrolysis products also suggests that bound tocopherols function as additional structural elements in the wall material(s). The L. polyedrum resting cyst cell wall thus appears to contain a biomacromolecular substance that is distinct from both sporopollenin and aliphatic algaenans. These findings help to further establish a chemical basis for the preservation potential of organic biomacromolecules, and illuminate possible chemical/functional relationships among highly refractory substances from diverse biological sources.