Shaul, N. J.; Dodder, N. G.; Aluwihare, L. I.; Mackintosh, S. A.; Maruya, K. A.; Chivers, S. J.; Danil, K.; Weller, D. W.; Hoh, E. Nontargeted biomonitoring of halogenated organic compounds in two ecotypes of bottlenose dolphins (Tursiops truncatus) from the Southern California Bight. Environ. Sci. Technol. 2015. DOI: 10.1021/es505156q
Organic Contaminants and the Global Experiment
Human beings have irrevocably altered the chemistry of Earth’s water and air. It’s a global experiment with unknown consequences. By synthesizing and releasing chemicals that don’t break down easily via natural processes, we are exposing people and wildlife to a complex mixture of synthetic chemicals that we haven’t evolved to handle. This issue has traversed in and out of the public consciousness for decades, but the chemicals in question aren’t going anywhere: As far back as the 1960s, the widely used pesticide DDT was found to be poisonous and banned, yet we are still finding its breakdown products in our foods today. Chronic exposure of humans and animals to DDT breakdown products and other pollutants like brominated flame retardants (some of which are still being manufactured today) is a global problem that could be costing us billions in health care.
While monitoring programs have been put in place to measure and track concentrations of some pollutants that are known to be harmful, these regularly monitored compounds make up only a small fraction of the total present in the environment. Many contaminants, as well as their degradation and transformation products, are not well studied – we know next to nothing about how long they persist, where they end up, and what levels wildlife and humans might be exposed to. Halogenated organic compounds are especially concerning: as large, hydrophobic (water-hating) chemicals, they are often persistent and bioaccumulative, meaning that they are fat-soluble and accumulate in the fatty tissues of living things.
Dolphins as Sentinels
Predators at the top of marine food chains, like dolphins and whales, are some of the creatures that stand to be hardest hit by these chemicals. In this study, researchers used stranded dolphins as sentinels for unknown halogenated contaminants. Because halogenated organic contaminants tend to accumulate in fatty tissues, dolphin blubber is an ideal place to look for them – they’re likely to be present at much higher concentrations in blubber than in water, which facilitates analysis and detection.
The Southern California Bight is a densely populated and historically polluted area that is home to very productive and complex ecosystems. Blubber samples were collected from 8 stranded dolphins that died of natural causes found on southern California beaches. These dolphins were identified as belonging to one of two “ecotype” groups which inhabit different regions: coastal (within one km from shore) and offshore (typically found a few km from shore). With these habitat differences come differences in diet, body size, and other characteristics, and the researchers were interested to see whether these differences were reflected by different contaminant levels in the dolphin’s bodies.
Samples were analyzed by a very advanced separation and detection technique called two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GCxGC-TOF/MS). Analyzing samples using “non-targeted analysis” (an analysis method that does not single out specific known compounds) is challenging: thousands of peaks can be found in a single sample, some of which are unidentifiable. In the first blubber sample the researchers analyzed, they found 17,038 peaks! They prioritized the peaks that were largest because they were most likely to represent compounds in high abundance. As they analyzed each new sample, they added new peaks to their database library. Using this recursive method, they were able to identify when these peaks were also found in other samples, and track similarities between samples. This process resulted in identification of 327 unique compounds present in some or all of the samples, with 182 – 253 compounds found in a single sample.
Chemicals were divided into groups based on structural similarities and known sources. 180 compounds were identified as synthetic manmade chemicals and their breakdown products, while 41 compounds were identified as having natural origins. 4 compounds were known to have both natural and human origins. Of the remainder, 8 compounds had identified structures, but no information could be found about their sources, while 94 compounds’ structures could not be elucidated with certainty.
The most abundant chemicals in the samples were identified as “DDT-related compounds” – breakdown products related to the pesticide DDT, which was manufactured in the Bight region. While DDT and its most common breakdown products are routinely monitored in many regions, additional suspected breakdown products were also identified in this study, as were compounds associated with the historically used insecticide chlordane. A number of compounds were identified that are not routinely monitored, but may be cause for concern, including man-made polychlorinated terphenyls (PCTs) and some natural halogenated compounds that could accumulate similarly to manmade pollutants and enter human diets via seafood.
The researchers found no significant difference between loads of manmade chemicals between the two dolphin ecotypes. However, they did find significantly greater concentrations of natural halogenated compounds in the offshore ecotype. While sources of these compounds aren’t well understood, the authors hypothesize that this is due to differences in the dolphins’ habitats and diets.
Like dolphins, humans are continuously exposed to low levels of various halogenated, persistent compounds through their diets. This study highlights a different strategy to prioritize contaminants for monitoring. Rather than a “top-down” approach where certain pre-defined pollutants are searched for in the samples, these researchers used a “bottom-up” approach, analyzing non-selectively and then identifying which compounds should be prioritized for further study.
86% of the compounds identified in the study are not routinely monitored. We know very little about where they’re found and at what levels, and we know even less about their possible toxic effects as single chemicals or in a mixture. These findings suggest many routine monitoring programs may underestimate the exposure of marine mammals and other organisms to toxins, as they only measure a small fraction of bioaccumulative halogenated contaminants.