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Connecting the dots: linking contaminant exposure to biological effects in marine mammals

Dr. Rebecca Lewison and Marisa Trego, San Diego State University

June 7, 2016

Exposure to toxic and bioaccumulative contaminants is widespread in marine systems and contaminants have been implicated as a direct threat to marine mammal populations. Contaminants come from many sources. Most contaminants enter the ocean environment from industrial and commercial factories, oil and chemical spills, run-off from roads, parking lots, and storm drains and wastewater treatment plants and sewage systems. These contaminants are all man-made.

Studying the exposure and effects of anthropogenic contaminants on marine mammals in the wild is an important, but challenging, area of research. Marine mammals are exposed to many contaminants at once and even sublethal levels of contaminants can compromise the health of an animal (e.g., impaired immune function, reduced reproduction). A new collaborative project between SDSU (Marisa Trego and Dr. Rebecca Lewison, Biology; Dr. Eunha Hoh, Public Health), NOAA (Dr. Nick Kellar (SWFSC)) and UC Davis (Dr. Andrew Whitehead, Environmental Toxicology) is developing next-generation approaches to innovate contaminant screening as well as our understanding of the biological effects of contaminants in marine mammals.

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This COAST-sponsored research is changing the way we study contaminant exposure in wild marine mammals using a trio of molecular approaches: two-dimensional gas chromatography with time-of-flight mass spectrometry (GC x GC TOF-MS), hormonal assays, and gene expression. Marisa Trego, the PhD candidate who is leading the research, is using tissue samples from wild animals to screen for known and emerging contaminants and pairing contaminant levels with hormonal and genetic assays to understand how contaminant levels are linked to stress, reproductive hormones and cellular pathways. With this approach, Marisa’s research is also aiming to track the effects of acute pulse contamination events, like the Refugio oil spill that occurred on May 19, 2015, in Santa Barbara County.

Traditional ecotoxicology approaches target known contaminant compounds to assess abundance and their potential effects in wildlife. While this targeted approach has provided invaluable information and serves as the foundation for the field of ecotoxicology, there are some limitations. Targeted analyses can only monitor known compounds that have been identified or linked to an ecological problem. That means that targeted analyses miss detecting new compounds that may pose a threat to marine animals and ecosystems.

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An analytical technique pioneered by Dr. Eunha Hoh has fueled Marisa’s innovative research. Dr. Hoh’s approach is based on the fact that many known persistent organic pollutants (POPs) have similar characteristics: they are often halogenated organic contaminants that are chlorinated or brominated, like DDT, a legacy chlorinated compound or PBDE, a brominated compound. We can use the structure of these known compounds to identify both known and unknown contaminants. To make an already complex problem even harder, there are also naturally occurring halogenated compounds that may interact or act synergistically with the man-made halogenated contaminants.

Although a comprehensive assessment of toxic compounds is a critical innovation, it is only the first step towards understanding how a complex mixture of contaminants affects marine mammal health. Marisa’s research is linking contaminant exposure to integrated measurements of molecular function, such as hormone levels and gene expression. To do this, she is using samples from several populations off the California coastline including tissue samples from short-beaked common dolphins (Delphinus delphis) that were taken as fisheries bycatch, and skin biopsy samples collected from wild bottlenose dolphins (Tursiops truncatus) populations in coastal and offshore areas of the Southern California Bight (SCB). Some of these wild dolphins were in the area of the Refugio oil spill and may also yield information on how the spill affected resident individuals.

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Figure 1. Relative abundance and classification of prevalent compound classes detected in D.delphis in the SCB.

Preliminary results on D. delphis suggest that both legacy and currently unmonitored contaminant compounds are widespread in some marine mammal populations in the SCB. Twenty-four contaminant compound classes were detected, classified according to source type (anthropogenic, natural or unknown), and relative abundance was quantified (Figure 1). Given the history of DDT deposition in the SCB, it is not surprising that DDT-related compounds (e.g., TCPM and TCPMOH) were found in the highest abundance.

Interestingly, DMBP, a naturally halogenated product, was found to occur in similar abundances to anthropogenic compounds, highlighting the lack of information on the combined effect of naturally and anthropogenically halogenated products. These results suggest that, in terms of relative abundance of contaminants, over 44% of the compounds found in our analyses are typically unmonitored.

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Figure 2. Significant differences in the relative abundance of contaminants detected in D. delphis by maturity class.

Our efforts to link results from the contaminant analyses with hormonal assays and gene expression have just begun. The first step of the transcriptome analysis (gene extraction and gene library construction) is underway at the Whitehead Lab at UCDavis. Hormonal data and analysis from the Kellar lab at SWFSC is also ongoing. To date, we have found a significantly higher numbers and greater relative abundance of contaminants in the tissues of older individuals (Figure 2), suggesting the bioaccumulation of contaminants over time. We will continue to investigate the relationships among contaminants, gene expression and endocrine biomarkers, including stress and reproductive hormones, so that we can improve our understanding of the impacts of these pollutants on marine mammal physiology and health.

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Dr. Rebecca Lewison is a Professor of Biology at San Diego State University and Marisa Trego is a PhD candidate in Biology in Dr. Lewison's lab. COAST provided funding for this project: Rapid Response Funding Program Award# COAST-RR-2015-001, July 2015.