Transformations of Carotenoids in the Oceanic Water Column

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-