. Scientific Frontline: Environmental toxin PCB found in deep sea trench

Monday, April 17, 2023

Environmental toxin PCB found in deep sea trench

A sediment core has just been retrieved from the Atacama trench during an expedition with the research vessel R/V Sonne.
Photo Credit: Anni Glud / University of Southern Denmark

Despite being banned in numerous countries since the 1970s, PCBs continue to persist in the environment. Recent findings from deep-sea researchers reveal that PCBs have been detected at the depths of the Atacama Trench in the Pacific Ocean, highlighting the enduring impact of these toxic pollutants.

Throughout their deep-sea expedition, the research team retrieved sediment cores from multiple locations within the Atacama Trench and conducted meticulous analyses to detect PCB occurrences. Astonishingly, every single sample of surface sediment analyzed from all five locations within the trench was found to contain PCBs, indicating the widespread presence of these hazardous pollutants even in the remote depths of the ocean.

The groundbreaking study, helmed by Professor Anna Sobek from Stockholm University's Department of Environmental Science and Professor Ronnie N. Glud, esteemed director of the Danish Center for Hadal Research at the University of Southern Denmark, has been published in the prestigious scientific journal Nature Communications. This significant contribution sheds light on the alarming presence of PCBs in the Atacama Trench and underscores the urgent need for continued research and action to mitigate their adverse effects on marine ecosystems.

A sediment core from the Atacama trench is being cut open for further analysis
Photo Credit: Anni Glud / University of Southern Denmark

PCB, an abbreviation for Poly-Chlorinated Biphenyls, encompasses a group of 209 distinct chemical compounds. Originally introduced in the 1930s, PCBs found widespread use in building materials and technical components. However, due to their harmful effects, they have been banned in most countries and are now classified as highly persistent environmental toxins. Research has shown that PCBs can have detrimental health impacts, including being carcinogenic and causing reproductive harm. As such, ongoing efforts to monitor and mitigate the presence of PCBs in the environment are crucial to safeguarding human health and protecting vulnerable ecosystems.

Although the world's production of PCBs dropped significantly in the 1970s, the substances still pose an environmental threat. In 2018 researchers reported, for example, that half of the world's killer whale populations were weakened by PCB. 

Another study has found that scavenging amphipods in the deep sea contained large amounts of PCBs. 

Professor Ronnie N. Glud, a seasoned researcher with over 10 deep-sea trench expeditions under his belt, aptly captures the significance of the findings, stating, "It is thought-provoking that we find traces of human activity at the bottom of a deep-sea trench; a place that most people probably perceive as distant and isolated from our society." The discovery of PCBs in such remote and seemingly untouched areas serves as a stark reminder of the far-reaching and lasting impact of human actions on our planet, even in the most remote and unexplored corners of the deep sea. It underscores the pressing need for continued vigilance and proactive measures to address and mitigate the pervasive presence of harmful substances like PCBs in our environment.

Through these expeditions, a previously held myth that deep-sea trenches remain untouched by surface events has been debunked. Instead, they have revealed a rich, active, and diverse ecosystem thriving at the greatest depths of the ocean. Moreover, the studies have shed light on the significant accumulation of organic material in deep-sea trenches, which plays a crucial role in the oceans' capacity to absorb carbon emissions resulting from fossil fuel burning. These findings highlight the intricate interconnections between surface activities, deep-sea ecosystems, and global carbon cycles. It underscores the need for a comprehensive understanding of the complex interactions within our oceans and the importance of protecting these delicate and vital ecosystems from human-induced harm.

Indeed, deep-sea trenches, also known as hadal trenches, not only accumulate organic material but also other pollutants. Recent research from the Danish Center for Hadal Research has highlighted the presence of mercury in the sediments of these trenches, as reported in 2021. Furthermore, in 2022, a similar announcement was made regarding the accumulation of black carbon, which consists of particles primarily generated from the combustion of fossil fuels. These findings reveal that deep-sea trenches are susceptible to the accumulation of various pollutants, including heavy metals and particulate matter, originating from human activities on the surface. This underscores the need for comprehensive monitoring and management strategies to mitigate the impacts of anthropogenic pollution on these remote and fragile ecosystems, and to safeguard the health and integrity of our oceans for future generations.

Professor Ronnie N. Glud notes that while PCB concentrations were detected in samples from the Atacama Trench, they are not alarmingly high. In fact, he points out that much higher concentrations of PCBs have been found in other regions such as the Baltic Sea, North Sea, and Tokyo Bay. In the Baltic Sea, for example, concentrations of PCBs have been measured to be 300 to 1500 times higher than those detected in the Atacama Trench. This comparison underscores the persistent and widespread nature of PCB pollution in certain regions and highlights the urgent need for continued efforts to address and mitigate the impacts of these harmful substances on our oceans and their ecosystems. Even in remote areas such as deep-sea trenches, the presence of PCBs serves as a reminder of the far-reaching consequences of human activities on the environment.

Professor Ronnie N. Glud highlights that while regions such as the Baltic Sea, North Sea, and Tokyo Bay are known for having high PCB concentrations due to significant human activity in those areas, the samples from the Atacama Trench, though not alarmingly high, still showed relatively elevated levels considering they were retrieved from the bottom of a deep-sea trench. He notes that it is unexpected to find pollutants in such remote and isolated areas, but the presence of PCBs in the Atacama Trench serves as a reminder of the pervasive nature of human-induced pollution and its potential impacts on even the most remote and seemingly pristine parts of our oceans. It emphasizes the need for continued vigilance in monitoring and mitigating pollution to protect the health and integrity of marine environments worldwide.

PCBs are hydrophobic, meaning they are not very soluble in water. Instead, they bind to organic material that sinks to the bottom.

“The Atacama trench is located in an area with relatively high production of plankton in surface waters. When the plankton dies, it sinks to the bottom of the ocean”, explains Anna Sobek.

In addition, large amounts of material are transported down the steep slopes and deposited in the deepest areas.

Some of the organic material that reaches the bottom of the Atacama trench is eventually decomposed by microorganisms, and as a result, PCBs accumulate in sediment.

PCBs are known to be persistent compounds that can persist in the environment for a long time, and they have the ability to be redeposited and transported over time. This is one of the reasons why increasing concentrations of PCBs can be found in inaccessible areas such as the hadal trenches, despite being banned in most countries since the 1970s. The slow degradation and movement of these pollutants in the environment can result in their accumulation in remote and isolated areas, including the deep-sea trenches. This underscores the need for continued monitoring and management of PCBs and other persistent pollutants, even after their ban, to prevent further environmental contamination and protect marine ecosystems from their adverse effects. It also highlights the importance of adopting sustainable practices and reducing the use of harmful substances to prevent long-term impacts on our oceans and the broader environment.

According to Professor Ronnie N. Glud, the concentrations of PCBs in hadal sediments, unlike coastal areas, are highest in the upper layers of sediment, suggesting that PCBs have relatively recently reached the deeper trenches and concentrations have not yet peaked. This indicates that the hadal trenches may still see increasing concentrations of PCBs in the coming years. The findings suggest that these remote and isolated areas are not immune to the impacts of human-induced pollution, and there is a continued risk of contamination even in the deepest parts of the ocean. This highlights the need for ongoing monitoring, research, and efforts to mitigate the spread and impact of persistent pollutants like PCBs, as well as the importance of adopting sustainable practices to protect the health and integrity of marine ecosystems for future generations.

The abyssal trenches of the deep sea are teeming with a diverse array of microorganisms and animals that have evolved to thrive in the extreme and harsh living conditions. Among them, there may be organisms that have developed unique abilities to metabolize the pollutants that are often deposited in these remote and inaccessible areas.

The Danish Center for Hadal Research has made it a priority to investigate the potential for pollutant metabolism in the deep-sea trenches. To support this research, the center has amassed a substantial collection of frozen sediment samples obtained from recent expeditions to various deep-sea trenches in 2021 and 2022. These invaluable samples serve as a crucial resource for ongoing studies aimed at understanding the unique adaptations and capabilities of microorganisms and animals residing in these extreme environments. The center's dedication to advancing our understanding of the deep-sea trenches and their inhabitants may lead to important insights into the potential for natural bioremediation of pollutants in these remote and pristine ecosystems.

Ronnie N. Glud, a leading researcher at the Danish Center for Hadal Research, explains that their investigations are driven by a curiosity to uncover whether polychlorinated biphenyls (PCBs), a type of persistent organic pollutant, are not only found in the Atacama trench but also in other deep-sea trenches around the world. Additionally, the team aims to shed light on the functions of the bacteria that inhabit these extreme environments. By studying the presence of PCBs and understanding the roles of deep-sea bacteria, the center seeks to expand our knowledge of the ecological impacts of pollutants in the deep-sea trenches and the potential for microbial-mediated remediation. This research underscores the center's commitment to advancing our understanding of these remote and pristine ecosystems and their response to environmental stressors.

The deep-sea trenches are located in the hadal zone of the ocean, which lies at depths of 6-11 km. There are about 27 deep-sea trenches, also called hadal trenches, named after the Greek god Hades, who ruled the underworld.

Published in journalNature Communications

Source/CreditUniversity of Southern Denmark

Reference Number: en041723_02

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