RATES AND MECHANISMS OF FATTY-ACID DEGRADATION IN OXIC AND ANOXIC COASTAL MARINE-SEDIMENTS OF LONG-ISLAND SOUND, NEW-YORK, USA

The rates and pathways of labile organic matter degradation significan tly affect the cycling of organic carbon and nutrients in coastal sedi ments. In this study, we measured degradation rate constants of satura ted and unsaturated fatty acids by incubating radiolabeled C-14-palmit ic (16:0) and 1-C-14-oleic (18:1) acids and an unlabeled plankton mixt ure in oxic and anoxic sediments from Long Island Sound (LIS) under la boratory-controlled conditions. Rate constants for degradation of 16:0 and 18:1 fatty acids were higher in oxic sediments than in anoxic sed iments. Degradation of the unsaturated 18:1 acid in anoxic sediments w as two times faster than for 16:0, while there was little difference b etween the two fatty acids in oxic sediments. The incubation results c learly showed that fatty acids degrade through multiple pathways in bo th oxic and anoxic sediments. About 80-90% of the label was lost from the incubated sediments (presumably as (CO2)-C-14 or other volatile pr oducts), and 5-10% was incorporated into the sediment matrix. Both deg radation and incorporation into the sediment matrix were slightly grea ter under oxic conditions. A small part (5-10%) of the label was incor porated into what are presumed to be metabolic products. A higher perc entage of this incorporation occurred under anoxic conditions, implyin g that anaerobic bacteria are less efficient at degrading the labeled fatty acid to volatile products such as CO2. In the oxic sediments, mo re oleic than palmitic acid was converted into intermediate metabolite s, indicating that the unsaturated fatty acid was degraded less effici ently. There was little difference in formation of metabolites between oleic and palmitic acids under anoxic conditions. The seasonal distri butions of palmitic and oleic acids at two coastal LIS sites with dist inctive oxygen content were modeled to yield degradation rate constant s for these two fatty acids. The comparison between fatty acid degrada tion rate constants derived from incubation experiments and field prof iles is consistent with anaerobic degradation being predominant in LIS sediments. Copyright (C) 1997 Elsevier Science Ltd.