Metal contaminants in estuarine sediments are a major concern at Department of Defense sites and in the environment globally. Metals comprise eight of the top 50 substances of concern on the Agency for Toxic Substances and Disease Registry (ATSDR) priority list and are contaminants of concern at many Superfund sites. Estuarine sediments are repositories for metal contaminants transported from watersheds, and they also provide important habitat for many invertebrate and fish species. Benthic and epibenthic organisms are exposed to metal concentrations producing molecular-, population-, and community-level effects. These biota also are vectors to human exposure through the consumption of seafood.
The objectives of this research project were to 1) characterize metal trophic transfer from sediments to intertidal food webs and determine food web structural variables that are sensitive to biological exposure and bioaccumulation in studies of reference vs. contaminated sites; 2) understand processes regulating metal bioaccumulation in animals from contaminated sediments, measure rates and routes of metal bioaccumulation in benthic animals, and develop a predictive biokinetic model of metal bioaccumulation in benthic fauna representing different functional groups (deposit feeder, filter feeder, and omnivore/predator); and 3) produce a set of robust genomic biomarkers that are predictive of the site-specific hazards posed by contaminated sediments.
Novel comparative approaches were used to link metal bioaccumulation in natural food webs to bioaccumulation and assimilation of metals by sentinel species in laboratory experiments and to genomic and physiological measures of metal toxicity in the species, Fundulus heteroclitus. These tools were applied in field sites ranging from relatively clean reference sites to industrialized contaminated sites. The ecosystem and community level fate of contamination were examined by relating metal burdens to food web structure and functional feeding groups. The transfer of metal contaminants in the food chain was investigated directly in bioaccumulation studies and development of a biokinetic model that is linked to metal trophic transfer measured in field studies.
Differences in metal concentrations in the food web were not found to vary strongly with sediment concentration. In fact, benthic-sediment concentration factors for sediments varied significantly with total organic carbon (TOC) and not with simultaneously extracted metal-acid volatile sulfide (SEM-AVS). When comparing the various invertebrate and vertebrate taxa, concentrations in the filter feeders, ribbed and blue mussels, are highest for all metals. Moreover, the biota that are more depleted in13C appear to have higher concentrations of methyl mercury, suggesting that pelagic feeding organisms bioaccumulate more methyl mercury than benthic feeding organisms. Finally, relationships of metal concentrations in biota with trophic level (measured as15N) are only apparent with percent methyl mercury which indicates biomagnification and with lead which decreases in concentration with increasing trophic level, indicating biodiminution, as has been seen in freshwater systems.
The bioaccumulation experiments indicated that there are differences in the retention of metals and their distribution in tissues dependent on the metal. In general, as metal concentrations in water increase, metal concentrations in fish also increase but the metal uptake rate constant (ku) does not. Salinity and dissolved organic carbon (DOC) also affected the amount of uptake and rate of uptake for cadmium (Cd) and chromium (Cr). Ingestion of worms by killifish results in bioaccumulation of mercury (Hg), but less so for Cd and Cr. In comparisons of bioaccumulation of methyl mercury when exposed in different waters (varying in salinity and DOC), experiments showed that there was a decrease in uptake with increasing salinity suggesting that the Cl- reduces the bioavailability of methyl mercury, but that efflux was not influenced by water chemistry. However, standardized salinity and DOC experiments suggest that salinity may enhance the uptake of metals. In all experiments, retention of methyl mercury was extremely high (~90%).
A transcriptome database was sequenced and assembled for killifish. This information was used to upgrade the microarray platform from individual 50 basepair oligonucleotides representing only 500 genes, to over 35,000 genes and gene fragments represented by multiple long oligonucleotides (i.e., ~95000 elements). This array platform was employed to successfully obtain functionally significant biomarkers of exposure that were able to differentiate individual (i.e., genetic) variation and physical modifiers of effects (e.g., salinity). The tools constructed under this project complement other resources (including a complete genome sequence for this species) that make the killifish a model system for environmental genomics and enhance the ability to detect and determine the effects of exposures to environmental toxicants.
This project provided tools for assessing the site-specific availability of metals and evaluating changes in food webs resulting from metal contamination. Important processes of metal transfer from sediments to receptors and the relative importance of diet versus water exposure to metal uptake were identified. Results also led to a method to link biotic responses across multiple levels of biological organization, genomic, physiological, individual, and community levels.