Kyoto, Japan -- As tiny as they are, marine plankton play pivotal roles in biogeochemical cycles and in sustaining aquatic ecosystems, producing around half of the oxygen on Earth and also acting as carbon sinks. When they die, their decomposing cells produce dissolved organic carbon that can be transformed and stored in the ocean for thousands of years, so understanding plankton mortality is essential for uncovering the mechanisms behind marine ecosystems.

However, assessing the death of a particular plankton population out of a community consisting of hundreds of coexisting species poses a complex challenge. A team of researchers at Kyoto University addressed this by focusing on viral infection and cell lysis, the process through which a cell's membrane breaks down and releases its genetic information into the environment.

First, the team developed a method for measuring plankton lysis by growing cultures of two phytoplankton species, diatoms and raphidophytes, in a seawater-based medium, and then extracting rRNA and quantifying it using digital PCR. Since rRNA released into seawater degrades over time, the researchers added a culture of spike-in ribosomes to the medium and measured the degradation rate constant. The resulting flux model incorporated both changes in host cell-free rRNA and the degradation constant, making it possible to accurately assess the rate of cell lysis.

Antidepressants and their metabolites remain biologically active after they leave the human body, persisting in aquatic environments and potentially affecting fish. However, the sensitivity of fish to these substances at the molecular level remains poorly understood. Now, researchers from Japan have characterized fish serotonin, dopamine, and norepinephrine transporters, revealing that fish versions of these proteins are more sensitive to antidepressants than their human counterparts. This highlights an overlooked risk to aquatic wildlife from pharmaceutical pollution.

Many of the drugs that target human brain chemistry, such as antidepressants, are still biologically active when they leave the body. These substances and their metabolites can persist in wastewater and eventually end up in the sea, rivers, or other aquatic environments. Because fish also use the same neurotransmitters as humans, including serotonin, dopamine, and norepinephrine, pharmaceuticals designed to interact with these chemical signals in humans can also interfere with fish’s biological processes, potentially affecting both behavior and physiology.

 

However, observing behavioral changes in exposed fish does not by itself reveal how a given drug caused that effect. Antidepressants often act on proteins called monoamine transporters, which help clear serotonin, dopamine, and norepinephrine from the synaptic cleft. The three main transporters are known as SERT for serotonin, DAT for dopamine, and NET for norepinephrine. Although fish possess versions of these proteins, it remains unclear whether they respond to human-targeted pharmaceuticals in a similar way to their human counterparts. Moreover, most mechanistic research has focused on a small number of fish species, so scientists are not sure whether pharmaceutical sensitivity is broadly shared across fish.

 

To address these questions, a research team led by Professor Shinichi Miyagawa of the Department of Biological Science and Technology, Faculty of Advanced Engineering at Tokyo University of Science, Japan, characterized monoamine transporters from two evolutionarily distinct fish species: medaka (Oryzias latipes) and ayu (Plecoglossus altivelis). The study, published in Environmental Science & Technology journal on August 24, 2026, was co-authored by Professor Masaru Ihara of the Faculty of Agriculture and Marine Science at Kochi University, Japan.

 

The researchers first identified and cloned genes encoding DAT, NET, and two types of SERT (SERTa and SERTb) from both fish species. The team produced these transporters in the lab by culturing genetically engineered human cells and then exposed them to a range of commonly used antidepressants. By measuring how effectively the transporters took up a fluorescent marker, the researchers could determine how strongly each drug inhibited transporter activity.

 

The results showed that SERTa was consistently much more sensitive to antidepressants than SERTb in both fish species. This lines up with the team’s genetic analysis, as human SERT belongs to the SERTa lineage, whereas SERTb is absent in mammalian species. SERTb also contains substitutions at several residues associated with antidepressant interactions.

 

Surprisingly, when the researchers compared fish SERTa with human SERT, the fish transporter often responded to lower drug concentrations, sometimes more than ten times lower than the human transporter. The team also found that some drugs not typically thought to act on these particular proteins in humans still affected the fish versions, hinting at side effects that could not be predicted from human pharmacology alone.

 

Another important finding was that drug concentrations needed to block fish SERTa in these lab tests overlapped with the concentrations of antidepressants already measured in polluted waterways. Duloxetine, fluoxetine, citalopram, and paroxetine, for example, inhibited medaka SERTa at concentrations ranging from a few hundred to roughly 1,300 nanograms per liter. “By demonstrating that key molecular targets in fish can be more sensitive than their human equivalents, our work offers crucial insights into the potential risks of pharmaceutical exposure to aquatic wildlife,” remarks Prof. Miyagawa.

 

Because the study included two fish species that are only distantly related, the shared sensitivity pattern observed suggests this heightened vulnerability to antidepressants may extend beyond a single fish species. The team suggests these findings could help guide regulators, as Prof. Miyagawa explains: “Our research provides a vital scientific basis for prioritizing specific pharmaceuticals in environmental monitoring programs and for deriving more protective, species-specific risk thresholds in water quality guidelines.” Future in vivo studies examining environmentally realistic exposures and pharmaceutical mixtures will be needed to determine how these molecular effects translate into biological and ecological outcomes.

 

 

 

 

 

 

 

 

Reference                    

Title of original paper: Characterization of fish serotonin, dopamine and norepinephrine transporters as a potential target for environmental pharmaceuticals

Journal:  Environmental Science and Technology

DOI: https://doi.org/10.1021/acs.est.6c04982

 

 

 

About The Tokyo University of Science

Tokyo University of Science (TUS) is a well-known and respected university, and the largest science-specialized private research university in Japan, with four campuses in central Tokyo and its suburbs and in Hokkaido. Established in 1881, the university has continually contributed to Japan's development in science through inculcating the love for science in researchers, technicians, and educators.

 

With a mission of “Creating science and technology for the harmonious development of nature, human beings, and society," TUS has undertaken a wide range of research from basic to applied science. TUS has embraced a multidisciplinary approach to research and undertaken intensive study in some of today's most vital fields. TUS is a meritocracy where the best in science is recognized and nurtured. It is the only private university in Japan that has produced a Nobel Prize winner and the only private university in Asia to produce Nobel Prize winners within the natural sciences field.

 

Website: https://www.tus.ac.jp/en/mediarelations/

 

 

About Professor Shinichi Miyagawa from Tokyo University of Science

Dr. Shinichi Miyagawa is a Professor at the Department of Biological Science and Technology, Faculty of Advanced Engineering of Tokyo University of Science, Japan. His research area includes tumor biology, developmental biology, and morphology/structure. He serves on the editorial board of the Journal of Applied Toxicology and is an active member of several international scientific committees focused on endocrinology and environmental science. He has been felicitated with several awards including the NIBB Young Scientist Award in 2015 and the National Institutes of Natural Sciences Young Scientist Award in 2012.

 

 

Funding information

This work was supported by the Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) (20H006301, 21H02522, 25H01183): Environment Research and Technology Development Fund of the Environmental Restoration and Conservation Agency provided by Ministry of the Environment of Japan (5-1952 and 5-2204): UK-Japan Research Collaboration Grants from the Ministry of the Environment, Japan, the Department for Environment, Food and Rural Affairs (Defra), UK: the cooperative research program of Leading Academia in Marine and Environment Pollution Research (LaMer) (26-23).

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