From mesenteric lymph nodes to the rest of the body: Microplastic translocation and accumulation in subsistence harvested eastern Chukchi Sea beluga whales

In the remote, icy waters of the Arctic, the beluga whale serves as a living barometer for the health of its environment. These long-lived, intelligent mammals travel vast distances and eat a wide variety of prey, making them excellent indicators of what is happening in the food web. For decades, scientists have used them to track traditional pollutants like heavy metals and industrial chemicals, which tend to build up in the bodies of animals at the top of the food chain. However, a newer and more visible threat has emerged: microplastics. These are tiny fragments of plastic, ranging from the size of a grain of sand to a small bead, that have broken down from larger items or were manufactured that way. They are now found everywhere, from the deepest ocean trenches to the highest mountain peaks. Because these particles are so small, they can be eaten by tiny sea creatures and move up the food chain, eventually reaching large predators. The critical question for scientists is not just whether these animals eat plastic, but what happens to it once it is inside their bodies. Does it stay in the gut, or does it travel to other organs, potentially causing harm over a lifetime?

A team of researchers set out to answer this question by studying male beluga whales harvested by Alaska Natives for subsistence. These whales, which live in the eastern Chukchi Sea, provided a rare opportunity to examine internal tissues from healthy, free-ranging animals. The scientists focused on five specific parts of the whale's body: the liver, the kidneys, the muscle, the thick layer of blubber, and a set of small glands called mesenteric lymph nodes. These lymph nodes are part of the immune system and sit near the intestines, acting as a sort of checkpoint where the body filters substances from the digestive tract. The researchers wanted to see if microplastics were simply passing through the gut or if they were crossing into the bloodstream and settling in different tissues. They also wanted to know if older whales carried more plastic than younger ones, which would suggest that the plastic accumulates over time, and whether the type of food the whales ate influenced how much plastic they carried.

To find the plastic, the team had to carefully break down the whale tissues. They dissolved samples of muscle, liver, kidney, and blubber in a chemical solution that removed the organic material but left the plastic behind. They then filtered the liquid through a fine mesh to catch any tiny particles. To ensure they were not counting dust or fibers from the lab itself, they ran strict controls, including empty filters exposed to the air and filters processed without any whale tissue. Once they isolated the particles, they counted them and measured their size and color. Most of the particles they found were fibers, likely from clothing or ropes, and they came in various colors, with clear, black, and blue being the most common.

The results revealed a clear pattern in where the plastic ended up. The mesenteric lymph nodes contained the highest concentration of microplastics, significantly more than the muscle, kidneys, or blubber. This suggests that these nodes act as a primary site where the body captures plastic particles after they are eaten, possibly as part of an immune response. While the lymph nodes had the most plastic, the other tissues still contained measurable amounts. The liver, which is responsible for filtering toxins from the blood, held more plastic than the muscle or kidneys, though the difference was not statistically significant. The blubber, which is fatty tissue with less blood flow, had the lowest amount of plastic. This distribution hints that the plastic travels through the bloodstream to reach these organs, rather than just sitting in the gut.

Perhaps the most significant finding was related to the age of the whales. The researchers looked at the muscle tissue of male whales of different ages, using their body length and skin color as a proxy for how old they were. They found that older whales had higher concentrations of microplastics in their muscles than younger ones. This indicates that microplastics are not just passing through the body and being excreted; they are staying behind and building up over the whale's life. This process, known as bioaccumulation, suggests that as a whale gets older, its body becomes a reservoir for these particles. However, the study also found that the type of food the whales ate did not seem to matter. By analyzing the chemical makeup of the muscle, the researchers could tell what the whales were eating and where they were feeding, but these factors did not correlate with how much plastic was in their bodies. This implies that the accumulation is driven more by the passage of time and the whale's physiology than by specific feeding habits.

When the team tried to identify exactly what kind of plastic they had found, the task proved difficult. Only a small fraction of the particles they isolated were large enough or clear enough to be identified with certainty. Among those they could identify, the most common type was polyethylene terephthalate, a plastic often used in clothing fibers and bottles. They also found traces of polyethylene, polypropylene, and nylon. The fact that they found so little identifiable plastic is likely because the particles were so small and had been altered by the digestive process and the environment, making them hard to match to known plastic types. Despite the difficulty in identification, the presence of these specific plastics is notable because they are often associated with textile fibers, which are a major source of microplastic pollution in the ocean.

The study concludes that microplastics are not just a surface-level problem for Arctic whales; they are penetrating deep into the body's tissues and staying there. The high concentration in the lymph nodes suggests these organs play a key role in capturing the particles, while the buildup in the muscles of older whales shows that the body cannot easily get rid of them. While the researchers did not find evidence that the plastic is moving up the food chain based on the whale's diet, the fact that it accumulates over a lifetime raises concerns about long-term health effects. If these particles are interfering with the whale's immune system or metabolism, it could have serious consequences for the population. The findings add to a growing body of evidence that microplastics are a pervasive contaminant in the Arctic, one that is quietly settling into the bodies of its most iconic inhabitants.