Concentrations and sources of polycyclic aromatic hydrocarbons in surface coastal sediments of the northern Gulf of Mexico.

BACKGROUND: Coastal sediments in the northern Gulf of Mexico have a high potential of being contaminated by petroleum hydrocarbons, such as polycyclic aromatic hydrocarbons (PAHs), due to extensive petroleum exploration and transportation activities. In this study we evaluated the spatial distribution and contamination sources of PAHs, as well as the bioavailable fraction in the bulk PAH pool, in surface marsh and shelf sediments (top 5 cm) of the northern Gulf of Mexico. RESULTS: PAH concentrations in this region ranged from 100 to 856 ng g-1, with the highest concentrations in Mississippi River mouth sediments followed by marsh sediments and then the lowest concentrations in shelf sediments. The PAH concentrations correlated positively with atomic C/N ratios of sedimentary organic matter (OM), suggesting that terrestrial OM preferentially sorbs PAHs relative to marine OM. PAHs with 2 rings were more abundant than those with 5-6 rings in continental shelf sediments, while the opposite was found in marsh sediments. This distribution pattern suggests different contamination sources between shelf and marsh sediments. Based on diagnostic ratios of PAH isomers and principal component analysis, shelf sediment PAHs were petrogenic and those from marsh sediments were pyrogenic. The proportions of bioavailable PAHs in total PAHs were low, ranging from 0.02% to 0.06%, with higher fractions found in marsh than shelf sediments. CONCLUSION: PAH distribution and composition differences between marsh and shelf sediments were influenced by grain size, contamination sources, and the types of organic matter associated with PAHs. Concentrations of PAHs in the study area were below effects low-range, suggesting a low risk to organisms and limited transfer of PAHs into food web. From the source analysis, PAHs in shelf sediments mainly originated from direct petroleum contamination, while those in marsh sediments were from combustion of fossil fuels.

The impact of drying on structure of sedimentary organic matter in wetlands: Probing with native and amended polycyclic aromatic hydrocarbons.

Wetland sediments undergo dry-wet cycles that may change their structural properties and affect geochemical behavior of associated organic compounds. In this study, we examined the effect of drying on particle size distributions and the rapid (24h) sorption reactions of polycyclic aromatic hydrocarbons (PAHs) with salt marsh sediments in Nueces Delta, South Texas. Drying reduced the fraction of fine particles in organically richer sediments, indicating structural rearrangement of organic matter and mineral aggregates. Among the 16 EPA priority PAHs examined, dried sediment preferentially released 1.0-7.5% of phenanthrene, fluoranthene and pyrene to added seawater (solid: water mass ratio of 1/100) - significantly greater than release from sediments maintained in the wet state. On the other hand, drying also increased the affinity of sedimentary organic matter (SOM) for experimentally amended (deuterated) phenanthrene relative to continually wet sediments. Further, deuterated phenanthrene was even more effectively retained when it was added to wet sediment that was subsequently dried and rewetted. These apparently contradictory results can be reconciled and explained by SOM having a heterogeneous distribution of hydrophobic and hydrophilic zones - e.g., a zonal model. We propose that drying changed the orientation of amphiphilic SOM, exposing hydrophobic zones and promoting the release of some of their native PAHs to water. Freshly amended PAHs were only able to penetrate into the surface hydrophobic zone and/or deeper but rapidly accessible ("kinetic") zone in wet sediments due to the brief adsorption contact time. Subsequent drying presumably then induced structural changes in SOM that isolated these amended PAHs in sites inaccessible to water exchange in the next rewetting. These results provide insights into structural changes of SOM upon drying, and help predict the fate of compounds such as organic contaminants during drought/flood oscillations.

1-Hydroxypyrene glucuronide as the major aqueous pyrene metabolite in tissue and gut fluid from the marine deposit-feeding polychaete Nereis diversicolor.

Both 1-hydroxypyrene and 1-hydroxypyrene glucuronide are identified as the primary phase I and phase II metabolites of the four-ringed polycyclic aromatic hydrocarbon (PAH) pyrene in the marine deposit-feeding polychaete Nereis diversicolor. Identification of pyrene and primary metabolites was performed using high-pressure liquid chromatography (HPLC) with diode-array detection and fluorescence detection (HPLC/DAD/F) and an ion-trap mass spectrometer for positive identification of 1-hydroxypyrene glucuronide. Besides 1-hydroxypyrene and 1-hydroxypyrene glucuronide, the HPLC/F trace of tissue samples from pyrene-exposed worms showed three additional low-intensity peaks that may be related to pyrene metabolism based on similar excitation/emission wavelengths. The peaks were all too low in intensity to be positively identified. Of the total PAH in tissue, 1-hydroxypyrene glucuronide, 1-hydroxypyrene, and pyrene constituted 73%, 2%, and 25% respectively. Gut elimination of metabolic products is supported by the identification of 1-hydroxypyrene and 1-hydroxypyrene glucuronide in both gut fluid and defecation water. Being the only phase I metabolite of pyrene, 1-hydroxypyrene becomes a useful marker for PAH exposure, and it may serve as a valuable model compound for assessing species-specific PAH metabolic capabilities.

Effect of extraction conditions on deposit feeder in trace element solubilization in only a digestive fluid.

In vitro extraction of sediments using digestive fluid of deposit-feeding invertebrates has been advocated as a measure of in vivo bioavailability, but the standard procedure used for the extractions alters fluid properties relative to in vivo conditions. In vivo fluid properties (redox potential [Eh], pH, dissolved oxygen) were measured in the gut of the polychaete Arenicola brasiliensis, and then the effects of gut evacuation, elevated temperature, and atmospheric exposure on these properties and/or metal and metalloid extraction capability of the fluid was assessed.The midgut of A. brasiliensis has an electropositive Eh of about +170 to +230 mV, a neutral pH, and little or no dissolved oxygen. Exposure to the atmosphere during in vitro extraction increased gut fluid oxygen content and affected the ability of the fluid to extract some trace elements from sediment particles, particularly Pb, Ni, Cu, Mn, and As. Even for these substances, however, the effects of oxygen exposure on extractability by gut fluid typically were small (<2-fold difference in amount extracted). For Cr, Hg, Se, Cd, Fe, and Zn, the effects of oxygen exposure usually were minor and inconsistent in direction. Maintaining anaerobic conditions during in vitro contaminant extractions is necessary when the experimental objective is to mimic accurately in vivo conditions and predict trace element extraction in the gut. If using metal extractability as a relative measure for sediment risk assessment purposes, a high degree of accuracy may not be required, and aerobic in vitro extractions could be suitable and produce only minor errors in extractability relative to in vivo conditions.

Inhibition of digestive enzyme activities by copper in the guts of various marine benthic invertebrates.

Digestive systems of deposit and suspension feeders can be exposed to high concentrations of copper (Cu) by ingestion of contaminated sediments. We assessed a potential impact of this Cu exposure on digestive enzyme activities in a wide range of benthic organisms by monitoring enzyme activities in their gut fluids during in vitro titrations with dissolved Cu, which mimics Cu solubilization from sediments. Increasing Cu inhibited digestive protease activities at threshold values, which varied widely among organisms, from 8 microM for an echinoderm to 0.4 M for an echiuran. More Cu was required to inhibit proteases in guts containing higher amino acid concentrations because strong Cu-binding sites on amino acids prevent Cu interaction with the enzymatically active sites. Threshold Cu concentrations were similar for proteases, esterases, lipases, and alpha- and beta-glucosidases, suggesting the same inhibition mechanism. Copper was less effective at inhibiting enzymes at lower pH, suggesting that protons can compete with Cu ion for binding to enzymatically active sites or that enzyme conformation is less vulnerable to Cu inhibition at lower pH. These results lead to the counterintuitive conclusion that deposit feeders with low enzyme activity, low amino acid concentration, and high pH values are most vulnerable to harm from sedimentary Cu by this mechanism, although they solubilize less sedimentary Cu than their counterparts with high enzyme activity, high amino acid concentrations, and low gut pH. In general, digestive systems of echinoderms may therefore be more susceptible to Cu contamination than those of polychaetes, with various other phyla showing intermediate susceptibilities. If threshold Cu values are converted to solid-phase sedimentary Cu concentrations, the thresholds are at least consistent with Cu loadings that have been observed to lead to biological impacts in the field.

Digestive bioavailability to a deposit feeder (Arenicola marina) of polycyclic aromatic hydrocarbons associated with anthropogenic particles.

Marine sediments around urban areas serve as catch basins for anthropogenic particles containing polycyclic aromatic hydrocarbons (PAHs). Using incubations with gut fluids extracted from a deposit-feeding polychaete (Arenicola marina), we determined the digestive bioavailability of PAHs from fly ashes, coal dusts, diesel soots, tire tread materials, and urban particulates. We found that gut fluids solubilize significant concentrations of PAHs from two tire treads, two diesel soots, and the urban particulates. However, PAHs in fly ashes and coal dusts were not available to the digestive agents in gut fluid. Potential digestive exposure to PAHs is much greater than that predicted to be available from these materials using equilibrium partitioning theory (EqP). Amending an already-contaminated sediment with fly ash decreased phenanthrene solubilization by gut fluid. In contrast, addition of tire tread to the sediment resulted in increased solubilization of four PAHs by gut fluid. Therefore, addition of certain types of anthropogenic particles to sediments may result in an increase in bioavailable PAHs rather than a net decrease, as predicted by EqP. Difficulty in predicting the amount of change due to amendment may be due to interactions occurring among the mixture of compounds solubilized by gut fluid.

Synchronous fluorescence spectrometry of 1-hydroxypyrene: a rapid screening method for identification of PAH exposure in tissue from marine polychaetes.

The uptake of polycyclic aromatic hydrocarbons (PAHs) by marine deposit-feeding invertebrates can be determined by screening for PAH-derived metabolites. We identified 1-hydroxypyrene as the only intermediate metabolite in tissue of four species of deposit-feeding polychaetes, Nereis diversicolor, Nereis virens, Arenicola marina, and Capitella sp. I exposed to pyrene spiked sediment. Synchronous fluorescence spectroscopy (SFS) provides a fast and simple method for both qualitative and quantitative analysis of 1-hydroxypyrene in all four species. The SFS assay was validated using HPLC with ultraviolet detection. A good correlation between 1-hydroxypyrene concentrations determined by the two methods was observed. We used HPLC with fluorescence detection combined with enzymatic hydrolysis of conjugated metabolites to investigate species specific metabolite patterns. A tentative aqueous metabolite identification scheme indicates that Nereid polychaetes predominately make use of glucuronide conjugation whereas Capitella sp. I. and Arenicola marina appear to utilize predominantly sulfate and/or glucoside conjugation. The usefulness of 1-hydroxypyrene as a biomarker for PAH exposure in deposit-feeding invertebrates is discussed.

Late Precambrian oxygenation; inception of the clay mineral factory.

An enigmatic stepwise increase in oxygen in the late Precambrian is widely considered a prerequisite for the expansion of animal life. Accumulation of oxygen requires organic matter burial in sediments, which is largely controlled by the sheltering or preservational effects of detrital clay minerals in modern marine continental margin depocenters. Here, we show mineralogical and geochemical evidence for an increase in clay mineral deposition in the Neoproterozoic that immediately predated the first metazoans. Today most clay minerals originate in biologically active soils, so initial expansion of a primitive land biota would greatly enhance production of pedogenic clay minerals (the "clay mineral factory"), leading to increased marine burial of organic carbon via mineral surface preservation.

Commercially available chemicals that mimic a deposit feeder's (Arenicola marina) digestive solubilization of lipids.

To develop a simple and cost-effective bioavailability test for sediment-bound contaminants, the solubilization strengths of mixtures of four commercially available surfactants and four proteins were compared to that of digestive fluids from a deposit-feeding benthic polychaete Arenicola marina. Initial tests indicated that sodium taurocholate, a vertebrate bile salt, was the most accurate mimic of A. marina gut fluids' solubilization of individual polycyclic aromatic hydrocarbons (PAH). Further testing with nutritional lipids and other hydrophobic contaminants confirmed the similarities of these fluids. Bovine serum albumin (BSA) solubilization of PAH was the most efficient of all the proteins tested. A cocktail of sodium taurocholate and BSA was compared to A. marina's solubilization of 12 PAH from four different contaminated sediments (from Boston, Charleston, Jacksonville, and San Diego harbors). The two solutions released most PAH to similar extents; 40 of 48 PAH-sediment combinations were released at amounts within a factor of 2 in cocktail and gut fluid solutions. Therefore, the cocktail may serve as a surrogate for real gut fluids and allow easier adoption of the in vitro incubation approach to bioavailability testing.

Interactions among contaminants and nutritional lipids during mobilization by digestive fluids of marine invertebrates.

Coastal sediments contain complex mixtures of hydrophobic compounds including organic contaminants such as polycyclic aromatic hydrocarbons and biogenic compounds such as cholesterol and phospholipids. Within the guts of benthic invertebrates, these mixtures are subjected to digestive, chemical conditions that can be rich in surfactants and proteinaceous material. Using in vitro incubations as proxy for digestive exposure, we studied the solubilization of binary mixtures of nutritional and contaminant lipids into artificial seawater and six marine invertebrate gut fluids (Molpadia intermedia, Cucumaria frondosa, Arenicola marina, Arenicola brasiliensis, Parastichopus californicus, and Nereis virens). For animals with surfactant micelles or high protein concentrations, solubilization interactions were frequent. For example, in Arenicola marina gut fluid, benzo(a)pyrene enhanced the solubilization of hexadecane (491% of the compound alone) and palmitic acid (130%) but hindered cholesterol (83%). Benzo(a)pyrene concentrations increased in gut fluids in the presence of cholesterol (137% of BaP alone), phenanthrene (154%), lecithin (140%), and hexadecanol (232%). In A. marina gut fluid, dilution with seawater indicated that these enhancements occur only when micelles are present. Sediment-water partitioning models, used to predict the bioavailability of hydrophobic organic chemicals, do not account for such interactions between solubilizates (compounds solubilized in micelles). However, for animals exposed via a digestive tract containing micelles or high protein concentrations, digestive bioavailability and perhaps bioaccumulation are likely influenced by these interactions.