186-12 Abundant Chromans and a Diaryl Isoprenoid in Italian OAE2 Sequences: A Response to Environmental Stressors?

Abstract:

The occurrence of tocopherols in the sedimentary record, notably in OAE1a, reflects their biosynthesis by photoautotrophs to serve as anti-oxidants to counteract production of free radicals during photosynthesis. Enhanced production of tocopherols by photosynthetic algae represents a direct response to environmental stressors. Chromans are structural analogs of tocopherols that lack a phenolic hydroxyl group but possess similar anti-oxidant properties. Examination of 'aromatic hydrocarbon' fractions of samples from two Italian OAE2 sequences collected at Vispi and Furlo revealed that they contained familiar suites of aromatic steroidal hydrocarbons, methylphenanthrenes, and benzohopanes, but their two most abundant components were a-chroman (trimethylMTTC or trimethyl-2-methyl-2-(4,8,12-trimethyltridecyl)chroman) and a diaryl isoprenoid not previously reported. a-Chroman is the major component of the Vispi samples, whereas the Furlo samples are dominated by the unknown diaryl isoprenoid. The relative abundances of the two compounds are remarkedly consistent among each of the sequences, yielding strong correlations (r² = 0.94 for Vispi, r² = 0.81 for Furlo), which suggests a common biological source or similar environmental controls on their production. The mass spectrum of the unknown resembles those of tocopherols and chromans exhibiting a molecular ion (M^+. at m/z 374; C₂₈H₃₈) and two prominent fragment ions (m/z 159, 119/118) that, coupled with its elution position are consistent with expectations for a symmetric, diaromatic hydrocarbon, tentatively assigned as 1,4-bis[2,5-dimethyl-1,2,3,4-tetrahydronapthal-2-yl]butane. Sequential cyclization and aromatization of squalene represents a viable biosynthetic pathway for formation of this compound, which seems more probable than generation by diagenetic alteration. The d¹³C composition (-29 ‰) of the unknown is consistent with production by photoautotrophs and excludes a possible origin from anoxygenic sulfur bacteria that are known to produce C₄₀ aryl isoprenoids (e.g., isorenieratane). Significantly, its structural affinities to tocopherols, related chromans, and carotenoids suggest that this unknown undoubtedly serves a similar function and its dual aromatic rings likely make it more effective anti-oxidant than chromans. The prominence of these anti-oxidant compounds in the aromatic hydrocarbon fractions from Furlo and Vispi likely represent a direct response to environmental stressors, including high light intensity, elevated amounts of heavy metals, high temperatures, and low dissolved CO₂, while the higher abundance of the unknown in the Furlo sequence perhaps reflecting a greater influence of these stressors relative to Vispi.

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