Gulf of Mexico hydrocarbon seep communities—IX. Sterol biosynthesis of seep mussels and its implications for host-symbiont association

The sterol analysis of six hydrocarbon seep mussels (mytilid II and mytilid Ib) from the Alaminos Canyon in the Gulf of Mexico is reported. The sterol composition of the mussel-symbiotic bacteria complex reflects a preponderant synthesis of 4α-methyl sterols (seep mytilid II), and a predominant biosynthesis of 4-desmethyl sterols with some amounts of 4α-methyl sterols (seep mytilid Ib). This suggests a methane-based symbiotic relationship between the mussels and methanotrophic bacteria. It also suggests that the biosynthesis of sterols in the mussel-bacteria complex is completed to the level of cholest-5-en-3ß-ol (mytilid II) or 5α-cholestan-3ß-ol (mytilid Ib).     

Use of microbial enrichments for the study of the anaerobic degradation of cholesterol

To demonstrate the potential of model microbial assemblages in studies of the biogeochemistry of complex organic molecules, anaerobic microbial populations capable of degrading cholesterol (cholest-5-en-3β-ol) have been enriched from marine sediment sources. The bacterial enrichments actively mineralized the C-4 nucleus of the cholesterol ring system to carbon dioxide when nitrate was present as the terminal electron acceptor. Nitrite was found as an intermediate in the reduction of nitrate, indicating the presence of denitrifying bacteria in the enrichments. When sulfate was supplied as the sole electron acceptor, active dissimilation of cholesterol was not observed. In enrichments containing 5 mM nitrate, 95–98% of the added cholesterol was recovered as carbon dioxide (2–5%), transformation products (20–30%), or as the unmodified sterol (70–80%). Cholesterol transformation products thus far identified include 5α- and 5β-cholestan-3β-ol, cholest-4-en-3-one, 5α-androstan-3, 17-dione and androst-4-en-3, 17 dione.     

青海湖沉积物中的甾醇及其演化

Ｃ_２７，Ｃ_２８和Ｃ_２９甾醇（△５.２２，△２２，△５和△５.２４（２８）和５α－甾烷醇）在青海湖三孔近代沉积物（ＱＨ，ＱＥ和ＱＯ）中被检出。其中以Ｃ_２９甾醇占优势。在ＱＥ和ＱＧ孔沉积物中还检出了低浓度的三个Ｃ_３０甾醇（４α，２３，２４－三甲基－５α－胆甾－２２Ｅ－烯醇（甲藻甾醇），４α－甲基－２４－乙基－５α－胆甾－２２Ｅ－烯醇和４α－甲基－２４－乙基－５α－胆甾烷醇］。在ＱＨ和ＱＧ孔上部层段沉积物中，Ｃ_２７，Ｃ_２９.饱和的和不饱和的甾醇的碳数分布的平行性，甾烷醇/△５－甾醇及△２２－/△５.２２－甾醇的比值随沉积物埋深而增加提供了甾酸氢化作用的间接证据。青海湖沉积物中的甾醇主要来自湖区丰富的高等植物和湖中的藻类等。在ＱＧ孔８０～１００ｃｍ和１２０～１４０ｃｍ层段沉积物中，５ａ－胆甾烷醇浓度迅速增加可能反映特殊的生物（介形类）对该二层段沉积物中高浓度的胆甾烷醇的贡献。     

δC and δD identification of sources of lipid biomarkers in sediments of Lake Haruna (Japan)

Organic materials in lacustrine sediments are from multiple terrestrial and aquatic sources. In this study, carbon (δ¹³C) and hydrogen isotopic compositions (δD) of phytol, various sterols, and major n-fatty acids in sediments at Lake Haruna, Japan, were determined in their solvent-extractable (free) and saponification-released forms (bound). The δ¹³C-δD distributions of these lipid molecules in sediments are compared with those of terrestrial C3 and C4 plants, aquatic C3 plants, and plankton to evaluate their relative contributions. δ¹³C-δD of free phytol in sediments is very close to that of phytol in plankton samples, whereas δ¹³C-δD of bound phytol in sediments is on a mixing line between terrestrial C3 plant and plankton material. Unlike phytol, no significant δ¹³C-δD difference between free and bound forms was found in sterols and n-fatty acids. δ¹³C-δD values of algal sterols such as 24-methylcholesta-5,22-dien-3β-ol in sediments are close to those of plankton, whereas δ¹³C-δD of multiple-source sterols such as 24-ethylcholest-5-en-3β-ol and of major n-fatty acids such as n-hexadecanoic acid in sediments are between those of terrestrial C3 plants and plankton samples. Thus, δ¹³C-δD distributions clearly indicate the specific source contributions of biomarkers preserved in a lacustrine environment. Free phytol and algal sterols can be attributed to phytoplankton, and bound phytol, multiple source sterols, and major n-fatty acids are contributed by both terrestrial C3 plants and phytoplankton.     

Aspects of the geochemistry of sedimentary sterols in the Chang Jiang estuary

Sediment samples have been collected during the Donghai 1 cruise in January 1986 in the Chang Jiang estuarine region for which solvent extractable sterols have been analysed by GC and GC/MS and the data examined using Correspondence Factorial Analysis (CFA). The main autochthonous sterols in these sediments were cholesterol, 24-methylcholesta-5,22-dien-3β-ol and 24-methylcholesta-5,24(28)-dien-3β-ol, whereas 24-ethylcholesterol, 24-ethylcholesta-5,22-dien-3β-ol and 24-methylcholesterol were principally allochthonous. Autochthonous sterols prevailed in regions of high primary production observed in summer, and allochthonous ones dominated the sterol distribution in the accumulation region of fine sediments just outside the river mouth and in the reaches of coastal currents. The cholesterol/cholesterol ratio values exhibited higher values close to the river mouth and in the region where the turbidity maximum extends. A comparison between sediment and suspended matter indicated a great difference in the sterol content in regions of high autochthonous production which suggested that sterols have been transformed in the course of sedimentation and at the sediment/water interface. In contrast, sedimentary and suspended sterol concentrations were comparable close to the mouth of the Chang Jiang River and in the region where the turbidity maximum extends, a feature that may be attributable to sediment resuspension episodes and lower production of autochthonous sterols. These results demonstrate the distinct impact of biogeochemical processes on the sediment sterol features in different zones of the estuary.     

Vertical transport of steroid alcohols and ketones measured in a sediment trap experiment in the equatorial Atlantic Ocean

The vertical flux and free steroid alcohol (sterol) and ketone composition of particulate material was determined using sediment traps deployed at 389, 988, 3755 and 5068 m at a station in the equatorial North Atlantic, PARFLUX E. Cholest-5-en-3β-ol (cholesterol) was found to be the dominant sterol in all the traps. This compound had a maximum flux at 988 m, accounting for more than 90% of the sterols at this depth. Inputs from mesopelagic Zooplankton populations living in or migrating to depths between the 389 and 988 m traps appear to be responsible for this distribution. The deeper two traps exhibited an increased flux of phytosterols relative to cholesterol, probably due to (a) the incorporation of labile phytoplankton remains in fecal pellets and rapid transport into the deep sea and (b) differential dissolution of heterogeneous large particles. A maximum of 5–22% of the sterols produced in the euphotic zone were present in the 389 m trap. This value drops to less than 1% for the 5068 m trap, 200 m above the sediment surface.In general steroid ketone fluxes gradually decreased with depth. Δ4-Stenones were found in greater abundance than their saturated counterparts. Cholest-4-en-3-one was the major steroid ketone detected in all the traps. A five-fold increase with depth in the cholest-4-en-3-one to cholesterol ratio is most likely due to microbial oxidation of sterols to steroid ketones, or higher Δ4-stenone inputs relative to sterols from organisms.     

Occurrence of gorgosterol in diatoms of the genus Delphineis

The presence of the C₃₀ sterol gorgosterol (22,23-methylene-23,24-dimethylcholest-5-en-3ß-ol) and its analogues in some marine and freshwater environments is generally associated with invertebrate animals or dinoflagellates since there have been no reports of them in other microalgal classes. Here we show that two unialgal cultures of different species of the marine diatom Delphineis contain gorgosterol in addition to sterols more commonly found in diatoms. Our findings suggest that some reports of gorgosterol in seawater and marine sediments may well have an origin, at least in part, from diatoms.     

The occurrence of stanols in various living organisms and the behavior of sterols in contemporary sediments

Unsaturated sterols (stenols) and saturated sterols (stanols) in phytoplankton and Zooplankton from Lake Suwa and from higher plants around the lake were analyzed by combined GLC and MS. In all the organisms investigated, 5α-cholestanol, 24-methylcholestan-3β-ol and 24-ethyl-5α-cholestanol were found, although in low concentrations, together with large quantities of stenols. This strongly suggests the contribution of stanols from living organisms to recent sediments.Findings from incubation experiments of cholesterol and 5α-cholestanol in the surface sediment from Lake Suwa extending through 450 days suggest the following: (1) stanols are slowly degraded and tend to survive unaltered in sediments in comparison with stenols, (2) in a relatively oxidative depositional environment such as Lake Suwa, the greater part of the stanols in the surface sediment originates from living organisms, (3) the reduction zone in which the degradation of sterols is suppressed and the rapid hydrogenation of stenols takes place may be in the microbiologically active sediment from 1 cm to about 10 cm in Lake Suwa, and (4) the increase in the ratio of stanols to stenols with depth below the zone may be caused by the simultaneous progress of preferential degradation and slow hydrogenation of stenols during long-term preservation in sediments.     

Rate of mercury loss from contaminated estuarine sediments

The concentration of mercury in contaminated estuarine sediments of Bellingham Bay, Washington was found to decrease with a half-time of about 1.3 yr after the primary anthropogenic source of mercury was removed. In situ measurements of the mercury flux from sediments, in both dissolved and volatile forms, could not account for this decrease. This result suggests that the removal of mercury is associated with sediment particles transported out of the study area. This decrease was modeled using a steady-state mixing model.Mercury concentrations in anoxic interstitial waters reached 3.5 μg/l, 126 times higher than observed in the overlying seawater. Mercury fluxes from these sediments ranged from 1.2 to 2.8 × 10^?5 ng/cm²/sec, all in a soluble form. In general, higher Hg fluxes were associated with low oxygen or reducing conditions in the overlying seawater. In contrast, no flux was measurable from oxidizing interstitial water having mercury concentrations of 0.01-0.06 μ/l.     

Transport of particulate organic carbon by the Mississippi River and its fate in the Gulf of Mexico

This study was designed to determine the amount of particulate organic carbon (POC) introduced to the Gulf of Mexico by the Mississippi River and assess the influence of POC inputs on the development of hypoxia and burial of organic carbon on the Louisiana continental shelf. Samples of suspended sediment and supporting hydrographic data were collected from the river and >50 sites on the adjacent shelf. Suspended particles collected in the river averaged 1.8±0.3% organic carbon. Because of this uniformity, POC values (in μmol l^?1) correlated well with concentrations of total suspended matter. Net transport of total organic carbon by the Mississippi-Atchafalaya River system averaged 0.48×10¹² moles y^?1 with 66% of the total organic carbon carried as POC. Concentrations of POC decreased from as high as 600 μmol l^?1 in the river to <0.8 μmol l^?1 in offshore waters. In contrast, the organic carbon fraction of the suspended matter increased from <2% of the total mass in the river to >35% along the shelf at ≥10 km from the river mouth. River flow was a dominant factor in controlling particle and POC distributions; however, time-series data showed that tides and weather fronts can influence particle movement and POC concentrations. Values for apparent oxygen utilization (AOU) increased from ～60 μmol l^?1 to >200 μmol l^?1 along the shelf on approach to the region of chronic hypoxia. Short-term increases in AOU were related to transport of more particle-rich waters. Sediments buried on the shelf contained less organic carbon than incoming river particles. Orgamic carbon and δ¹³C values for shelf sediments indicated 3 that large amounts of both terrigenous and marine organic carbon are being decomposed in shelf waters and sediments to fuel observed hypoxia.     

High tolerance to tributyltin in embryos and larvae of the horseshoe crab,Limulus polyphemus

The effects of acute and chronic exposure to tributyltin (TBT) were examined in bioassays using horseshoe crab (Limulus polyphemus) embryos and “trilobite” larvae. Larvae had>95% survival after 24-h exposure to nominal concentrations of 1–500 μg l^?1 TBT. Survival was also high following 48-h and 72-h exposure to ≤100 μg l^?1 TBT; >50% mortality was seen only after 48-h and 72-h exposure to 500 μg l^?1 TBT. Estimated median lethal concentrations (LC₅₀) were >1000 μg l^?1, 742 μg l^?1, and 594 μg l^?1 for 24-h, 48-h, and 72-h exposure, respectively. Much higher toxicity LC₅₀=42 μg l^?1) was seen following chronic exposure of larvae to TBT. Acute exposure to TBT significantly increased the time required by larvae to molt into the first-tailed stage. LC₅₀ for horseshoe crab embryos exposed to TBT were 44 μg l^?1, 20 μg l^?1, and 14 μg l^?1 for 24, 48, and 72 h acute exposure, indicating that this earlier developmental stage was about 30–40 fold more susceptible to TBT than larvae. Horseshoe crabs are highly tolerant of TBT in comparison to early developmental stages of other marine arthropods. The ability of horseshoe crab embryos and larvae to survive in the presence of organotin pollution suggests the possibility of bioaccumulation and movement into the estuarine food chain via shorebirds, gulls, and fish.     

Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns,controlling factors and ecological significance

Sediment-water oxygen and nutrient (NH₄ ⁺, NO₃ ^?+NO₂ ^?, DON, PO₄ ^3?, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l^?1, apparently limiting SOC rates. Fluxes of NH₄ ⁺ were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations (<5 mg l^?1), very large releases (>500 μmol N m^?2 h^?1) were observed. Nitrate + nitrite (NO₃ ^?+NO₂ ^?) exchanges were directed into sediments in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were high (>18 μM N); sediment efflux of NO₃ ^?+NO₂ ^? occurred only in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m^?2 h^?1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH₄ ^?, PO₄ ^3?, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.     

Free,esterified and residual bound sterols in Black Sea Unit I sediments

Detailed compositional data for the sterols isolated from a surface, Unit I, sediment from the Black Sea are reported. A procedure based on digitonin precipitation has been used to separate the more abundant free sterols from those occurring in esterified forms. Saponification of the solvent extracted sediment residue liberated only a small quantity of residual bound sterols in contrast to studies of other sediments. 4-Methylsterols are much more abundant than 4-desmethylsterols in both the free and esterified sterol fractions which we attribute to a major dinoflagellate input, as in deeper Unit II sediment. The desmethylsterol fraction appears to derive from a variety of sources including dinoflagellates, coccolithophores, diatoms, terrigenous detritus and perhaps invertebrates. 5α(H)-Stanols are particularly abundant in the free sterol fraction. An analysis of the stanol/stenol ratios suggests that the 4-desmethyl-5α(H)-stanols are the result of specific microbial reductions of Δ⁵-sterols and/or the reflection of a contribution of stanol containing source organisms.     

Sterol geochemistry of sediments from the western North Atlantic Ocean and adjacent coastal areas

Core sections from coastal bay, continental slope, and continental rise surface sediments of the western North Atlantic were analyzed for sterols. Changing rate or type of sediment input, bioturbation, and chemical conversion appear to be processes important in controlling the distribution of sterols in these sediments. Comparisons of individual sterol distributions and variations in the ratio of Soxhlet-extractable to non-extractable saponified sterols indicate that for the western North Atlantic, the extent to which each process is dominant varies with proximity to shore. Evidence is presented to show that sterols in the deep sea may be at least partially terrigenous in origin and not all biogenically derived in the surface waters. These sterols, and by analogy other labile organic compounds, may serve as a source of carbon for benthic organism metabolism.     

5α(H)-cholestan-3α-ol in sediments: Characterization and geochemical significance

The top five centimeters of sediment collected at the deepest point of Lake Léman (Switzerland), 309 meters below water level, contain concentrations of 5α(H)-cholestan-3α-ol (epicholestanol) up to 10% of the total sterol content. Isobutane chemical ionization mass spectrometry and coinjection with an authentic standard on a SP-1000 glass capillary column were used in order to successfully characterize this epimer of cholestanol.The distribution of this stanol throughout the sediment core studied suggests in situ bacterial production. The change in concentration of epicholestanol with depth is different from those found for other C₂₇ stanols, such as cholestanol or coprostanol. It is probably the result of a change in the bacterial fauna in the most recently deposited sediment, related to the increasing eutrophication of the lake rather than the consequence of its lower stability due to the axial conformation of the OH substituent.     

Diatoms as a source for 4-desmethyl-23,24-dimethyl steroids in sediments and petroleum

Analysis of the sterol composition of more than 100 diatom cultures, representing all major marine diatom orders, indicates that this group of algae may be an important source for 4-desmethyl-23,24-dimethyl steroids in sediments and petroleum, as their precursors, i.e. 4-desmethyl-23,24-dimethyl sterols, were present in 22 of the cultures. The phylogenetic positions of diatom species that produce 4-desmethyl-23,24-dimethyl sterols show that, within the centric diatoms, only a specific group of diatoms is able to produce these sterols, while within the pennate diatoms, a phylogenetic relationship between 4-desmethyl-23,24-dimethyl sterol-producing diatoms is less apparent. Based on the phylogenetic relationship, it is suggested that diatoms inherited the ability of producing these sterols from a single common ancestor, which originated between 150 and 100 Ma ago. Co-injection of an authentic 23R,24R-dimethyl-5α-cholestane standard with extracts confirmed its presence in sediments. We also tentatively identified three other 4-desmethyl-23,24-dimethyl sterane isomers having different side-chain stereo-configurations and observed that some of the isomers co-elute with other steranes including 24-ethyl-5α-cholestane.     

Marine and terrigenous lipids in coastal sediments from the Peru upwelling region at 15°S: Sterols and triterpene alcohols

The distributions of free 4-desmethyl sterols in sediments from the Peru coastal zone at 15°S have been determined. Major free sterols in the surface sediments include cholesterol, which is mainly derived from zooplankton, and two C₂₈ sterols: 24-methylcholesta-5,24(28)-dien-3β-ol and 24-methylcholesta-5,22E-dien-3β-ol both of which are derived fro diatoms. Their concentrations decrease by almost an order of magnitude in the top 20 cm of sediment depth, indicating that free sterols are rapidly degraded in this sedimentary environment. Lipids from higher plants were also detected: long chain fatty acids and alcohols and various triterpenoid alcohols, including taraxerol, lupeol and α- and β-amyrin. The concentrations of most terrigenous lipids varied by less than a factor of 3 over the same depth, and these changes were not correlated with changes in the concentrations of total organic carbon. Below 3 cm, lipids from higher plants predominated in the extractable lipid distributions due to the more rapid degradation of marine lipids. We postulate that there are significant marine sources of the higher plant sterols 24-ethylcholesterol, 24-ethylcholesta-5,22E-dien-3β-ol and 24-methylcholesterol in these sediments. A high proportion of many of the terrigenous lipids in these sediments are probably transported into the coastal zone by rivers, rather than from the atmosphere, and then redistributed by bottom currents.     

Mathematical aspects of non-steady-state diagenesis

Berner (1971) has solved the differential equation governing the concentration in interstitial water of substances produced or consumed during steady-state diagenesis. We have shown that closed solutions exist for non-steady-state diagenesis as well, and that these solutions are best obtained by means of Green's functions. Non-uniform distribution of decomposable organic matter in sediments is a major cause of non-steady-state diagenesis. However, the effect of such non-uniform distributions on the composition of interstitial water in sediments is pronounced only when the rate of deposition exceeds ca. 200 cm/1000 yr. At slower deposition rates, diffusion is sufficiently rapid to damp out major fluctuations in the concentration of ions such as SO^2?₄, NH⁺₄, PO^3?₄, and HCO^?₃. Concentration profiles of these ions therefore tend to be similar to steady-state profiles even if the concentration of decomposable organic matter is quite heterogeneous.The concentration of SO^2?₄ frequently approaches 0 in marine sediments rich in decomposable organic matter. In such sediments the total quantity of SO^2?₄ reduced during diagenesis is proportional to the concentration of SO^2?₄ in sea water even if the bacterial rate of decomposition of organic matter is nearly independent of the SO^2?₄ concentration in interstitial water. This implies that the rate of SO^2?₄ removal from sea water by reduction to sulfide is roughly proportional to the sulfate concentration in seawater.Solutions for the diagenetic equation exist for reasonable variations of the rate of ionic diffusion in interstitial waters and for changes in the rate of deposition due to compaction.     

Sterols in the western North Atlantic Ocean

The sterol concentrations in fourteen surface and nine deep water samples collected from the continental shelf and slope waters of the western North Atlantic and Sargasso Sea ranged from 0.1 to 1.3μ/l seawater. Isolation and structural elucidation by gas chromatography and combined gas chromatography-mass spectrometry show that cholesterol and β-sitosterol (or clionasterol) are the major free sterols in both the surface and deep water. Fucosterol, brassicasterol, 22-dehydrocholesterol, campesterol (or 22,23-dihydrobrassicasterol), 22-methylenecholesterol, norcholestadienol, and stigmasterol (or poriferasterol) are found in lower concentrations at the surface and in the deep sea. Cholesterol is the major sterol ester in both the surface and deep water, while very low concentrations of other sterol esters were found. The ratio of total free sterols to total esterified sterols is approximately two in both the surface and deep water.Marine sources of sterols in seawater include phytoplankton, yeasts, and marine animals such as Crustacea and molluscs. Terrestrial plants also may contribute. Sterol transport to the deep sea may occur by convective overturn and vertical diffusion or from vertical fluxes of large particles from the surface.     

Variability of dissolved organic carbon in sediments of a seagrass bed and an unvegetated area within an estuary in Southern Texas

Pore-water dissolved organic carbon (PWDOC) concentrations were examined in vegetated and bare sediments of aHalodule wrightii seagrass bed, and in a mud bottom sediment of a southern Texas estuary. Temporal variability was examined at diel (dawn and noon) and bimonthly time scales. Distribution patterns of PWDOC were compared with physical, chemical, and biological factors thought to exert control on PWDOC. Concentration of PWDOC, bacterial production, and resultant PWDOC turnover times displayed statistically significant spatial and temporal variability. Concentration of PWDOC ranged from 14 mg C 1^?1 to 107 mg C 1^?1 of pore water, or 9–71 μg C cm^?3 wet sediment. PWDOC was more variable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 μg C cm^?3) and high bacterial production rates (mean = 1.92 μg C cm^?3 h^?1) were observed at the mud station, whereas PWDOC concentrations were high (mean = 24.6 μg C cm^?3) and bacterial production rates were low (mean = 0.43 μg C cm^?3 h^?1) at the bare station. PWDOC turnover times (T_t), assuming 50% bacterial growth efficiency (1–840 h) were shortest at the mud station (mean=13 h) and longest at the bare station (mean=180 h). In the overlying water column, T_t values were longer, ranging from 1,000–10,000 h. PWDOC concentrations were 25% higher in vegetated sediments than in neighboring bare sediments. This difference was probably due to inputs of labile photosynthetic excretia, since bacterial production rates in vegetated sediments displayed significant diel variability and were 4 times greater than that of bare sediments. Based upon the entire data set, PWDOC was significantly related to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxygen metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that organism activities and detrital quality are the major determinants controlling variability in PWDOC.