By Cynthia M. Erickson
The Gulf of Mexico Offshore Operations Monitoring Experiment (GOOMEX) was designed to investigate the effects of petroleum products and discharges from offshore drilling rigs on the marine environment. As part of my thesis research, I attempted to determine the exposure of marine biota, specifically bottom-feeding fish such as flounder and red snapper, and invertebrates such as shrimp and crabs, to these effluents. These organisms were chosen as indicators of contaminant exposure due to their close association with marine sediments which serve as sinks for petroleum-based compounds. Most of the compounds, and those of most concern due to their abundance in marine ecosystems, are polycyclic aromatic hydrocarbons (PAHs) such as naphthalene, phenanthrene and benzopyrene.
When marine organisms are exposed to PAHs, metabolic processes are induced to break down these foreign compounds in the body. I was specifically interested in the induction of a gene family known to be responsible for producing proteins and enzymes necessary to eliminate PAHs from an organism's tissues. This induction was measured using three bioassays, or biomolecular procedures used to measure the nature, amount or composition of a compound in tissues or chemical mixtures. These techniques are now highly valuable tools in risk assessment and toxicological analysis as their use often serves as an early warning system for environmental contamination.
The first assay, the mRNA Northern blot, allows determination of the increased amount of mRNA produced by the exposed fish versus an unexposed (control) fish. Enhanced mRNA production by this gene family leads to higher protein levels and consequently enzyme activity. These levels can be tested in fish by measuring ethoxyresorufin-O-deethylase (EROD) activity. Invertebrates are not as sensitive to PAHs as fish and thus their tissues were homogenized and extracted for foreign chemicals present into a small volume. These extracts were then combined with mammalian-cell cultures which are sensitive to PAHs. The EROD assay was then used to measure the cells' resulting gene enzyme activity.
All three assays can be compared with one another and with sediment loads of PAHs or other contaminants to assess exposure. Such early biomolecular testing can save millions of dollars in remediation and cleanup because it can potentially catch contamination before the ecosystem becomes highly polluted and many marine species die.
The results of my experiments on organisms from around three offshore platforms in the northwest Gulf of Mexico indicated that the waters and sediments around the platforms were not highly contaminated. Although sediment PAH concentrations generally decreased away from each site, the bioassays did not detect significant changes in fish or invertebrates with respect to distance from the platforms. No seasonal differences were noted. The mRNA levels and EROD (enzyme) activities remained low overall, but varied with marine species. Wenchman and flounder species showed highest levels and most activity, probably due to their constant proximity to sediment. As both bioassays measured the same gene activity I believed the two assays would yield similar trends. I observed no overall correlation, however. The reason for this is not yet known.
Editor's Note: Cynthia graduated in August with her master's degree and will be employed with the Geochemical and Environmental Research Group until she begins her Ph.D. in environmental toxicology next fall. She intends to continue with marine and aquatic research, and eventually work in environmental consulting or with state or federal government.
Oceanography, Texas A&M University
Updated July 24, 1995