Ming-Yi Sun

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- 14 C-palmitic (16:0) and 1- 14 C-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 14 CO 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.

The influence of deposit-feeding on chlorophyll-a degradation in coastal marine sediments

To determine how macrofaunal activity affects rates and mechanisms of Chlorophyll- a (Chl-a) decomposition, we measured Chl- a concentrations during laboratory incubations of surface sediment with varying abundances of a subsurface deposit-feeder, Yoldia limatula. Decomposition patterns of Chl-a in sediment cores with and without a layer of algal-enriched sediment added to the surface were compared. Decomposition rate constants, kd, were calculated from the loss of reactive Chl- a and further quantie ed using a nonsteady state, depth-dependent, reaction-diffusion model. Values of kd decreased approximately exponentially with depth and were directly proportional to the number of Yoldia present.Yoldiaincreased the kd of both natural sedimentary Chl- a and algal enriched Chl- a in the upper 2 cm by up to 5.7 3 . Surface sediment porosity, penetration depths of a conservative tracer of diffusion (Br 2 ), and oxidized metabolic substrates (e.g. Fe(III)) all increased signie cantly in the presence of Yoldia. Macrofaunal bioturbation increased the importance of suboxic degradation pathways. These experiments demonstrated that organic compounds from a single source can have a continuum of degradation rate constants as a function of biogenically determined environmental conditions (Chl- a kd , 0.0043‐ 0.20 d 2 1 ). In particular, Chl- a can have a continuum of kd values related to redox conditions, transport, and macrofauna abundance as a function of depth.

Diagenesis of planktonic fatty acids and sterols in Long Island Sound sediments : Influences of a phytoplankton bloom and bottom water oxygen content

Diagenesis of organic matter in coastal sediments from Long Island Sound (LIS) was investigated by measuring fatty acids and sterols in (1) a time-series of surface sediment samples over a spring phytoplankton bloom; and (2) sediment cores collected during and after a bloom at two sites with distinctively different bottom-water oxygen contents. Time-dependent distributions of sedimentary fatty acids and sterols in LIS were strongly affected by pulsed inputs from the overlying water column, variations in benthic community, and redox-related degradation processes. The phytoplankton bloom delivered an intense pulse of unsaturated fatty acids (e.g., 16:1(ω7) and 20:5) to the surface sediments. Continuous increases of cholesterol and diunsaturated sterols after the bloom were related to zooplankton grazing processes and increase in benthic faunal abundance. High inventories of planktonic fatty acids and sterols in the upper 5 cm sediments were observed at the low oxygen site during summer, probably caused by a combination of higher input, reduced degradation rates and lower macrofaunal activity under anoxic conditions compared to oxic conditions.

Impact of seasonal hypoxia on diagenesis of phytol and its derivatives in Long Island Sound

Isoprenoid compound distributions were measured in sediment at two sites in Long Island Sound, one with oxygenated overlying water throughout the year, the other seasonally variable with hypoxia in summer. A comparison of these two sites showed that the initial rate and pathway of phytol degradation were strongly influenced by benthic faunal activity and environmental redox conditions. Solvent-extractable phytol (in detritus of phytoplankton origin) was rapidly converted into highly-bound phytol (in sediment matrix) after deposition. Paradoxically, dihydrophytol, a product of phytol reduction, was produced under oxic environmental conditions, apparently by macrofaunal digestion. In contrast, the 6,10,14-trimethylpentadecan-2-one and 4,8,12-trimethyltridecanoic acid, products of phytol oxidation, were formed under hypoxic environmental conditions during suboxic or sulfidic microbial metabolism. Incubation of planktonic material further revealed the difference in degradation pathway between oxic and anoxic conditions. Degradation processes of phytol and its derivatives were quantified based on a conceptual model and incubation data. Relatively greater preservation of isoprenoid compounds at the periodically low oxygen site may result from the higher input of planktonic detritus and slower degradation rate than at the continuously oxic site.