Transformations of carotenoids in the oceanic water column

Abstract

In an effort to understand the more general mechanisms and rates of pre-depositional reactions that transform organic matter, the types and relevant time scales of reactions that transform carotenoid pigments in the oceanic water column were studied.In the present study, a model was constructed for organic matter cycling that consisted of three parts: 1) the synthesis of carotenoid pigments by phytoplankton in the euphotic zone, 2) consumption and metabolism of some fraction of these pigments by heterotrophic organisms, and 3) removal of metabolic by-products to the sediments by large particle (e.g. fecal pellet) transport. The model separates particlate matter into reservoirs according to the degradation processes that have occurred since synthesis. The goal is to sample these particulate reservoirs, determine the compositional differences between them, and construct a mechanistic pathway for the transformations that occur as material is transferred between reservoirs.Suspended particulate matter collected in the surface waters of Buzzards Bay, Massachusetts and the Peru upwelling region has a carotenoid distribution reflecting the phytoplanktonic source of the material. The carotenoid distribution of sediment trap samples collected in these areas was dominated by transformation products. Fucoxanthin, the primary carotenoid of marine diatoms, typically constituted 77-100% of the total fucopigments in suspended particulate matter. In sediment trap samples this pigment constituted only 4-85% of the total. The remaining 15-96% of the pigments consisted of the fucoxanthin transformation products: free alcohols (2-94%), dehydrates (0-6%), and opened epoxides (0-19%).Postulated transformation products were synthesized to determine the structure of isolated compounds. Simultaneously, iodine catalyzed photoisomerization of fucopigments was tested as a potential method for the unambiguous identification of caotenoids requiring only the nanogram amounts of material typically found in samples.Preliminary results suggest that carotenoid esters are hydrolyzed at a rate determined by the turnover of primary productivity. The dehydrated and epoxide opened degradation products of fucoxanthin represent products of transformation reactions that operate over much longer time scales (0.1-10 yr). Dehydration and epoxide opening are not significant water column transformations, but are important in surface sediments. A trans-3 formation pathway of ester hydrolysis + dehydration + epoxide opening + further dehydration is proposed for fucoxanthin. Preliminary evidence for a parallel transformation squence for structurally similar carotenoids is also presented.

Phytoplankton pigments tn macrozooplankton feces: variability in carotenoid alterations

“…Fucoxanthin-like absorption and a polar elution position (lower retention time) relative to fucoxanthin are consistent with the characteristics of fucoxanthinol. Conversion of the 3'-acetate of fucoxanthin to the 3'-hydroxyl of fucoxanthinol results in a more polar molecule and does not alter the visible absorption of the molecule (Bonnett et al 1969, Repeta 1982. The slight differences in the absorption of fucoxanthin in the eluting solvent (Fig.…”

“…Fucoxanthinol is not reported to b e among the reaction products of fucoxanthin in basic solutions (Bonnett et al 1969, Repeta 1982. The products of fucoxanthin obtained from diagnostic laboratory reactions are concisely summarized by Liaaen-Jensen (1971).…”

“…Absorption maxima for the 2 major products and 1 minor product are listed in Table 4D. From its absorption spectrum, and by analogy to the base hydrolysis products of fucoxanthin (Bonnett et al 1969, Repeta 1982, it is possible that the major product at 4.4 min was 19'-hydroxy~sofucoxanthinol. Little of the minor product at 5.1 min was recovered, but its absorption spectrum appears to be fucoxanthin-like (Table 4; Nelson 1986) suggesting that it could be 19'-hydroxyfucoxanthinol.…”

“…Data presented by Repeta (1982) and Repeta & Gagosian (1984) suggests that the conversion of fucoxanthin to fucoxanthinol may vary with the composition of the macrozooplankton community. This question is addressed in the present study.…”

Stable isotope geochemistry of nitrogen in marine particulates

“…The C/N ratio did decrease from 14 to 11, suggesting that the pellets were not chemically identical. In addition, carotenoid analyses of these particles suggest that rapid transformations of this specific class of organic compounds had al so occurred (Repeta, 1982).…”