Paper: Noël, M.; Loseto, L. L.; Helbing, C. C.; Veldhoen, N.; Dangerfield, N. J.; Ross, P. J. PCBs are associated with altered gene transcript profiles in Arctic beluga whales (Delphinapterus leucas). Environ. Sci. Technol. 2014. DOI: 10.1021/es403217r
Just as water vapor condenses on a cold surface, air pollutants emitted from temperate mid-latitude industrial centers condense and accumulate in cold polar regions. Because of this process, known to scientists as “global distillation”, the Arctic is the final sink (gathering place) for many pollutants. While the concentration of pollutants in the Arctic may be small compared to urban concentrations, even trace levels could have drastic impacts on the fragile ecosystems typical of these environments.
Many researchers have focused their efforts on investigating how pollutants enter Arctic food chains and magnify in top predators. Aside from being adorable, beluga whales are high trophic level organisms (top predators). As such, they are exposed to a variety of contaminants through biomagnification. In this study, Canadian researchers set out to determine whether the belugas of the Beaufort Sea, a relatively pristine region, were accumulating toxic pollutants at levels that could disrupt biological processes and potentially affect the future health of the beluga population.
While many studies measure concentrations of accumulated toxins in an organism, it can be difficult to determine whether these toxins are present at sufficient levels to have an effect, especially in marine organisms like belugas that might be difficult to observe or track over time. Visible symptoms associated with accumulation of toxic contaminants, such as increased incidence of tumors, have been observed in beluga whales that make their homes further south (in the Gulf of St. Lawrence) but these cleaner, northerly belugas may be suffering from less obvious effects; disruption of the immune system, impaired growth and reproduction, and behavioral changes have all been associated with accumulation of persistent bioaccumulative pollutants. In this study, researchers looked for changes in gene expression to better understand how pollutants might be affecting the growth, reproductive success, and resiliency of beluga populations.
Researchers collected samples from male whales harvested by native hunters in the Beaufort Sea. It is often important for scientists to control for gender in bioaccumulation studies because females can have seasonally varying fat content due to body changes associated with pregnancy. The researchers measured concentrations of mercury, polychlorinated biphenyls (PCBs), and flame retardants in the blubber of these whales. They also measured stable isotopes to get an idea of what the whales have been eating – a significant change in the ratio of carbon or nitrogen stable isotopes within an animal suggests that they’ve changed their diet.
The researchers then tackled the problem of whether higher concentrations of pollutants in blubber corresponded to changes in gene expression. They identified 11 specific genes that regulate hormonal pathways, growth, development, metabolism, as well as 2 genes that are usually expressed when the body is under stress from xenobiotics (foreign compounds that aren’t expected in the body). They also identified 3 genes that weren’t expected to be affected by pollutant concentrations to use as controls.
The group used a method called quantitative real-time polymerase chain reaction (qPCR) to amplify the expression of each gene. They compared the expression of the genes of interest in each whale to the concentration of pollutants measured in its blubber. They also compared gene expression to year of capture, age and size of the whale, blubber thickness, and stable isotopes to determine what other factors might impact gene expression.
PCBs were the dominant pollutant measured in the belugas, measured in the μg/g range (0.000001 g/g of blubber). Concentrations of pollutants were 8 – 12 times lower than in beluga populations found further south in the St. Lawrence estuary. Researchers found that the concentration of PCBs was higher in belugas that exhibited higher expression of the 2 biomarkers associated with exposure to xenobiotics, as expected. This confirms that toxic contaminants are affecting belugas’ biological processes even in clean areas where no visible symptoms have arisen. On the bright side, the authors did not find any significant change in the other gene expression profiles, the ones associated with various hormonal, developmental, and other processes.
Correlations were only seen between PCBs and gene expression (Figure 1): no relationship was seen with pollutants present at lower concentrations, such as brominated flame retardants and mercury. This suggests that PCBs, which are persistent organic chlorinated compounds that have been banned since the ‘70s, seem to pose the biggest threat to belugas out of the contaminants measured. The researchers noted, however, that they can’t rule out mixture effects on biological response: other pollutants that weren’t measured in the study could be interacting in complex ways with the more dominant compounds to produce the observed gene expression profiles.
While no relationship was seen between contaminant concentrations and expression of the 11 genes related to beluga health, the authors found that changes in the expression of these genes did relate to stable isotope levels, and could be related to changes in diet during years of low sea ice when the belugas must travel farther offshore to find food. Stable isotope data suggested the belugas fed on different prey during 2008 and 2010 (low-ice years in the Beaufort Sea), and this change in diet altered the expression of genes associated with hormonal processes, reproduction, growth, and development. Subtle changes in the health of beluga populations may be altered by humans via a more circuitous route than direct contamination with pollutants, as human-induced climate change is the main culprit associated with decreased ice coverage. The authors note that they don’t yet know how the observed impacts on gene expression will impact the health of individual belugas, or of the population as a whole, over time.
Food webs tend to be simpler in the Arctic, with less biodiversity and less choice of prey for large predators. This is intuitively true if you list animals in a tropical rainforest and animals on the tundra. Slight destabilization of a certain population of organisms in the Arctic could have much more serious consequences that it would at lower latitudes. Biomagnification also tends to be especially pronounced in these food webs.
This study highlights the stress put on populations due to a combination of anthropogenic (human-caused) factors, including food web alterations arising from climate change, and the introduction of toxic pollutants into marine food webs. Together, these forces are eliciting measurable changes in gene expression, even in pristine areas. However subtle these effects may seem, the described changes could leave populations more vulnerable to disease, habitat destruction, and other stressors.