Integrating Knowledge on the Distribution and Maternal Transfer of Organic Pollutants to Advance Sea Turtle Conservation

The following post is one of a series previewing the research that will be presented at the SETAC North America 42nd Annual Meeting (SciCon4), 14–18 November 2021.

A guest post by Cynthia C. Muñoz and Peter Vermeiren, Radboud University, The Netherlands

Knowledge regarding the internal distribution and subsequent maternal transfer of organic pollutants—such as polychlorinated biphenyls (PCBs), once used in electical equipment and plastics; organochlorine pesticides (OCPs), and polybrominated diphenyl ethers (PBDEs), which make materials nonflammable—within organisms is of critical importance to scientists who interpret tissue-specific biomonitoring results and refine risk assessments. Although the manufacture and/or use of most of these chemicals were banned decades ago, they persist in the environment and accumulate in wildlife. This is particularly true for long-lived organisms, such as sea turtles, where pollution burdens build up over time and affect health later in life. Moreover, organic pollutants can accumulate over many years before being transferred from the mother to its offspring, where they can interfere with critical development processes. For example, maternal transfer of organic pollutants into yolk, through the placental barrier, or during lactation has been linked to decreased survival rates and impaired embryo and juvenile development in several long-lived vertebrate species. Yet knowledge on the internal and maternal distribution of organic pollutants remains limited for many such species, due to ethical, economic, and logistical restrictions on sampling them, as many are threatened or endangered. Additionally, a diverse chemical universe of legacy, new, and emerging organic pollutants is present in the environment, of which the behavior within the environment and upon contact with long-lived species is largely unknown. In short, the challenge of refining risk assessments specific to the characteristics of long-lived species, such as sea turtles, is complex, without an easy solution.

Several processes lead to a selective maternal transfer of organic pollutants in sea turtles, and varying concentrations among internal tissues as captured in our conceptual model. (From Muñoz and Vermeiren [2020])

To improve our understanding of the physiology underlying the internal and maternal distribution of organic pollutants in sea turtles, we conducted a meta-analysis of relevant data across the scientific literature. Our synthesis of 40 years of research on the internal concentrations of organic pollutants across all species of sea turtles supported the concept that concentrations of organic pollutants per volume of lipid in a tissue are comparable among the heart, kidney, muscle, and lung of an individual sea turtle—tissues characterized by high blood flow and perfusion. By contrast, the brain, fat, and blood plasma show lower concentrations per volume lipid, which could be due to physiological features such as the blood-brain barrier, poor blood perfusion in fatty tissues, and the high protein content of blood plasma. The deviating patterns could have important toxicological implications. The brain, for example, is a target for neurotoxic effects, while fatty tissues are important for buffering short-term variations in internal pollutant concentrations in long-lived species. Patterns of internal distribution also differ among juvenile and adult life stages. Such life-stage differences could reflect the more dynamic internal distribution of organic pollutants in juveniles due to their higher growth rate compared to the more stable internal physiology and build-up of fatty reserves in adults. These tissue-specific patterns and differences in life stages must be accounted for to make toxicity risk assessment relevant to the most sensitive tissues and life stages for sea turtles and other-long-lived species.

Individuals that died naturally at different embryo stages shed valuable insights into development processes and their interaction with pollution. Photo credit: Cynthia Muñoz

For some individuals, their first exposure to pollutants begins even before they come into contact with the environment. Maternal transfer of organic pollutants confronts developing embryos with a legacy of pollution during one of the most sensitive life stages. For protected species—like all sea turtles—maternal transfer of pollutants poses a conservation threat due to its potential negative impacts on new generations. Our research suggested a selective maternal transfer in sea turtles where compounds that are less soluble in fatty tissues (lower lipophilicity) are more easily transferred to eggs. Additionally, the length of time between nesting events and the relative use of dietary versus stored lipid reserves to fuel egg development also affect transfer rates. These outcomes underscore the importance of chemical as well as physiological and ecological characteristics in determining maternal transfer rates.

Long-lived migratory species like sea turtles are unique sentinels of environmental pollution (i.e., “canaries in the coal mine”) as they integrate environmental effects over long periods and across locations that might be hard to monitor otherwise. Consequently, improved knowledge on the processes that determine concentrations in different tissues benefits the information that can be gained from biomonitoring data. After harmonizing and standardizing concentrations in different egg compartments, we showed temporal patterns in concentrations of organic pollutants related to trends in usage, production, release, and persistence in the environment. Such large-scale meta-analyses complement local and regional analyses of biomonitoring data, conducted for instance in Mexico and across the USA. Moreover, large-scale meta-analyses can improve the understanding of differences among similar species. For instance, the higher tropic level of loggerhead turtles compared to other sea turtles was reflected in the generally higher concentrations of several OCPs in loggerhead eggs, although there were temporal and geographic differences.

Synthesis and meta-analysis of scattered data from published records can improve our understanding of processes determining how pollutants are internally distributed and expand the use of biomonitoring data for sentinel species. Yet, there are critical challenges to optimizing the information retained within the scientific literature. Based on the experiences throughout our research, we highlight several ATTAC issues: data Access, Transferability and TrAansparency among studies with different methods and analytical approaches, and wise use of Conservation-sensitive materials. We propose best practice guidelines for these issues and provide a concrete example where the development of a lipid content database allowed us to homogenize data on concentrations of organic pollutants across studies even when lipid contents were not reported. Similar fundamental physiological research and the development of associated databases would improve everyone’s ability to combine knowledge. Other best practice guidelines also echo broader calls in the scientific community for increased open access to detailed rather than aggregated summary data and the careful consideration of lethal sampling in light of statistically meaningful results.

Despite global efforts to respond to environmental pollution over the past few decades, an increasingly complex chemical universe is present in the environment. The threats posed by environmental pollution, including organic pollutants, remain difficult to integrate quantitatively in global conservation assessments, such as those conducted by the IUCN Marine Turtle Specialist Group. Furthermore, our knowledge is lagging, with up to a decade between sampling, analysis, and publication of research results. This delay in knowledge generation limits the inclusion of pollution in vulnerability assessments and weakens the urgency to speed up the often long, slow process of regulating environmental pollutants at a global scale, critical to the habitat usage of long-lived migratory species such as sea turtles. The comprehensive tissue partitioning and maternal transfer patterns analyzed in our research improves the accuracy of sea turtle conservation assessments by enabling a quantitative integration of threats posed by legacy and other organic pollutants.

Presentation information for SciCon4:

03.03.06: Internal Distribution and Maternal Transfer of Emerging and Legacy Persistent Organic Pollutants in Sea Turtles: A Meta‐Analysis

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