Identification of carotenals in sediments

Abstract

High performance liquid chromatography with photodiode array UV–Vis detection and atmospheric pressure chemical ionization mass spectrometry was used to analyze sediment samples from the meromictic Ace Lake (Antarctica), as well as a Pliocene sapropel from the Mediterranean Sea and a Miocene marl from the Vena del Gesso formation (Italy). The Ace Lake samples contained intact chlorobactene, isorenieratene, γ- and β-carotene and two series of carotenoid breakdown products. The first and more abundant series was identified as aryl (ϕ)-carotenals of increasing chain length (C 13, C 15, C 18, C 20, C 22, C 25, C 27, C 30, and C 32) on the basis of their UV–Vis and mass spectra. They appear to be formed from chlorobactene through oxidative cleavage of the double bonds of the polyene chain. The minor series (β-carotenals) was also identified, and may be formed from γ-carotene in a similar manner. Intact isorenieratene and the ϕ-carotenal series were also found in the Mediterranean Sea sapropel. Only a trace of the C 32 ϕ-carotenal could be detected, suggesting isorenieratene as the predominant source of these ϕ-carotenals. Additionally, intact β-carotene and C 13–C 30 β-carotenals were found in the sapropel. The absence of the C 32 β-carotenal strongly suggests these breakdown products are derived from β-carotene. The Vena del Gesso marl contained both intact isorenieratene and the C 13–C 25 ϕ-carotenals. The absence of the longer chain length ϕ-carotenals from this sample may indicate more advanced oxidation of isorenieratene. The carotenals are likely formed during storage of samples and/or extracts under non-ideal conditions, i.e., not frozen and exposed to oxygen although they may also be formed by diagenetic processes. They can be relatively abundant and sometimes be an order of magnitude higher in concentration than the parent carotenoid. Hence, not including the ϕ-carotenals could lead to an underestimation of the persistence of photic zone euxinia.