Rates and mechanisms of fatty acid degradation in oxic and anoxic coastal marine sediments of Long Island Sound, New York, USA

Abstract

The rates and pathways of labile organic matter degradation significantly affect the cycling of organic carbon and nutrients in coastal sediments. In this study, we measured degradation rate constants of saturated and unsaturated fatty acids by incubating radiolabeled 1- 14C-palmitic (16:0) and 1- 14C-oleic (18:1) acids and an unlabeled plankton mixture in oxic and anoxic sediments from Long Island Sound (LIS) under laboratory-controlled conditions. Rate constants for degradation of 16:0 and 18:1 fatty acids were higher in oxic sediments than in anoxic sediments. Degradation of the unsaturated 18:1 acid in anoxic sediments was two times faster than for 16:0, while there was little difference between the two fatty acids in oxic sediments. The incubation results clearly showed that fatty acids degrade through multiple pathways in both oxic and anoxic sediments. About 80–90% of the label was lost from the incubated sediments (presumably as 14CO 2 or other volatile products), and 5–10% was incorporated into the sediment matrix. Both degradation and incorporation into the sediment matrix were slightly greater under oxic conditions. A small part (5–10%) of the label was incorporated into what are presumed to be metabolic products. A higher percentage of this incorporation occurred under anoxic conditions, implying that anaerobic bacteria are less efficient at degrading the labeled fatty acid to volatile products such as CO 2. In the oxic sediments, more oleic than palmitic acid was converted into intermediate metabolites, indicating that the unsaturated fatty acid was degraded less efficiently. There was little difference in formation of metabolites between oleic and palmitic acids under anoxic conditions. The seasonal distributions of palmitic and oleic acids at two coastal LIS sites with distinctive oxygen content were modeled to yield degradation rate constants for these two fatty acids. The comparison between fatty acid degradation rate constants derived from incubation experiments and field profiles is consistent with anaerobic degradation being predominant in LIS sediments.