The study, led by Dr. Omri Bronstein from the School of Zoology at Tel Aviv University’s Wise Faculty of Life Sciences and the Steinhardt Museum of Natural History, was published in the prestigious journal Ecology.

The Research Team. Photo credit: Tel Aviv University.

“This is a first-rate ecological disaster”, explains Dr. Bronstein.

She continues: “Sea urchins are vital to the health of coral reefs. They act as the ‘gardeners’ of the reef by feeding on algae, preventing it from overgrowing and suffocating the coral, which competes with algae for sunlight. In 1983, a mysterious disease wiped out most of the Diadema sea urchins in the Caribbean. Unchecked, the algae there proliferated, blocking sunlight from the coral, and the region shifted from a coral reef ecosystem to an algae-dominated one. Even 40 years later, the sea urchin population — and consequently the reef — has not recovered”.

In 2022, the disease reemerged in the Caribbean, targeting the surviving sea urchin populations and individuals. This time, armed with advanced scientific and technological tools to collect and interpret the forensic evidence, researchers at Cornell University successfully identified the pathogen as a ciliate Scuticociliate parasite. A year later, in early 2023, Dr. Bronstein was the first to identify mass mortality events among long-spined sea urchins, a close relative of the Caribbean Sea urchins, in the Red Sea.

Sea urchin mortalities on Reunion Island. Photo credit: Jean-Pascal Quod.

“Until recently, this was one of the most common sea urchins in Eilat’s coral reefs — the familiar black urchins with long spines,” says Dr. Bronstein. “Today, this species no longer exists in significant numbers in the Red Sea. The event was extremely violent: within less than 48 hours, a healthy population of sea urchins turned into crumbling skeletons. In some locations in Eilat and the Sinai, mortality rates reached 100 percent. In follow-up research, we demonstrated that the Caribbean pathogen was the same one affecting populations in the Red Sea”.

Genetic Proof Links Global Sea Urchin Deaths to One Pathogen

Now, using genetic tools, Dr. Bronstein and his international colleagues have shown that the same ciliate parasite is responsible for similar mortality events off the coast of Réunion Island in the Indian Ocean. “This is the first genetic confirmation that the same pathogen is present in all these locations,” he says. “Now it’s a global event, a pandemic. The Caribbean, Red Sea, and the Indian Ocean are critical regions for the world’s coral reefs, and mortality rates for sea urchins in these areas are very high — over 90 percent. As of now, we have no evidence of this pathogen in Pacific Ocean sea urchins, but this is something we are actively investigating. Although we’ve developed genetic tools for the specific identification of the pathogen, it’s difficult to monitor such rapid extinction events in the vast underwater environment. We are terrestrial creatures, and some reefs are located in deep or remote areas. If we miss the mortality event by even a couple of days, we might find no trace of the extinct population”.

 Four healthy sea urchin species on Reunion Island. Photo credit: Jean-Pascal Quod.

To track the progression of the pandemic, Dr. Bronstein has established an international network of collaborators. He provides them with alerts about the likelihood of mortality events in their regions and sends them the necessary equipment to sample and preserve affected sea urchins for comparison with samples from other locations. These kits are then sent back to the laboratory at Tel Aviv University.

“For populations that are already infected, we really have no tools to help them,” says Dr. Bronstein with regret. “There is no Pfizer or Moderna for sea urchins — not because we don’t want one, but because we simply can’t treat them underwater. Our focus must be on two entirely different tracks. The first is prevention. To prevent further spread of the pandemic, we need to understand why it erupted here and now. We’ve developed two hypotheses for this. The first is the transportation hypothesis — that the pathogen from the Caribbean was transported by humans to new and distant regions after being carried in the ballast water of ships, infecting sea urchins in the Red Sea before spreading to the Western Indian Ocean.

The sea urchin Diadema setosum before (left) and after (right) mortality. The white skeleton is exposed following tissue disintegration and loss of spines. Photo credit: Tel Aviv University.

How climate change might be spreading marine diseases worldwide

Incidentally, if this hypothesis is correct, we would expect to see mortality events in West Africa as well — as many cargo ships from the Caribbean stop there on their way to the Mediterranean and then through the Suez Canal to the Red Sea. Indeed, just in the past few weeks, we’ve discovered widespread mortality events in West Africa, as we predicted, and we’ve managed to obtain a limited number of samples collected during these events, which we are currently analyzing in the lab. If ships are indeed the source of the spread, then we could think of mitigation strategies. It’s not simple, and ships will never be completely sterile, but there are measures we can take. The second possibility is even more concerning: that the pathogen has always been present, and climatic changes have triggered its virulence and outbreak. That’s a challenge of an entirely different magnitude, one that we, as marine biologists, have very limited means to address”.

In parallel with global efforts, Dr. Bronstein has recently established a breeding nucleus for the affected sea urchins at the Israel Aquarium in Jerusalem, in collaboration with the Biblical Zoo and the Israel Nature and Parks Authority. This breeding population will serve as a reserve to restore affected populations and advance research into infection mechanisms and possible treatments.

“The pathogen is transmitted through water, so even sea urchins raised for research purposes in aquariums at the Interuniversity Institute for Marine Sciences and the Underwater Observatory in Eilat became infected and died. That is why we established a breeding nucleus with the Israel Aquarium, whose aquariums are completely disconnected from seawater. We genetically test the urchins transferred to the nucleus to ensure they are not carriers of the disease and that they genetically belong to the Red Sea population, enabling us to rehabilitate the population in the future. At the same time, we are using them to develop sensitive genetic tools for early disease detection from seawater samples — essentially creating ‘underwater COVID tests’ for sea urchins”.

The research was conducted by PhD student Eden Harel of the School of Zoology in the Wise Faculty of Life Sciences, Prof. Noa Shenkar of the School of Zoologyand the Steinhardt Museum of Natural History, and Prof. Ines Zucker of the School of Mechanical Engineering and the Porter School of Environment and Earth Sciences, all at Tel Aviv University. The study was published in the journal Chemosphere.

 Prof. Shenkar during a research dive (Photo credit: Dr. Tom Shlesinger).

Prof. Shenkar explains: “About a decade ago, when awareness of the plastic pollution problem in the marine environment began to grow, many researchers focused on identifying the location and scale of microplastic particles. Recently, the research focus has shifted to the effects and damage caused by microplastics. However, many experiments in this field are conducted using clean, purchased plastic, whereas in the sea, plastic particles are exposed to a wide range of influences and pollutants. We aimed to examine whether and how plastic changes after passing through the digestive system of a marine organism and how this process affects the presence of plastic and its availability to other organisms”.

How Marine Organisms Process Microplastics

The researchers created an experimental system in the lab simulating seawater containing ascidians — marine animals that feed by efficiently and indiscriminately filtering tiny particles from the water. They exposed the ascidians to two types of plastic particles: conventional polystyrene (PS), which is widely used, and polylactic acid (PLA), marketed as a biodegradable, environmentally friendly bioplastic. They then examined the impact of the ascidians’ filtration process on the concentration and distribution of plastic particles in the water at four intervals: at the time of exposure, after two hours (when the ascidians had filtered all available water and ingested the microplastic particles), after 24 hours, and after 48 hours (following digestion and the excretion of feces into the water).

The laboratory at Tel Aviv University where the experiment was conducted (Photo credit: Eden Harel).

The findings showed that approximately 90% of the polystyrene particles were removed from the water after two hours of filtration, but all the particles returned to the water after 48 hours, following passage through the digestive system. In contrast, the concentration of PLA particles in the water significantly decreased and remained low for 48 hours, larger PLA particles likely broke down during digestion and returned to the water as smaller undetectable nano-sized particles.

In the second phase of the study, the researchers examined what had happened to the microplastic particles that were filtered, digested, and excreted back into the water column. To do so, they isolated microplastic particles from the ascidians’ feces and analyzed them using Raman spectroscopy, an advanced device that identifies the chemical composition of materials by scattering a laser beam.

Eden Harel explains: “We found that the sensitive spectroscopy device could not identify the material as plastic at all and instead identified the particle as another type of organic material. Our findings revealed that microplastic particles are excreted from the ascidian’s digestive system coated with a fecal layer, and it is likely that the marine environment also identifies these particles as this organic material. Since many marine animals feed on feces, they may well ingest plastic that has changed its properties, identifying it as food. In this way, they are also exposed to microplastics and spread them further within the marine food web. The fecal coating may serve as a substrate for bacterial colonization and increase the adhesion and accumulation of pollutants such as heavy metals and residual organic substances (like antibiotics) on the plastic particles”.

Prof. Zucker adds: "This phenomenon also affects bioplastics marketed as 'biodegradable': unless conditions are met for their complete breakdown, they remain as particle pollution that changes properties during passage through the digestive system. The many transformations plastic particles undergo in the environment — from weathering to digestive processes, as investigated in this study — turn them into carriers of pollutants and diseases within the food web”.

 The researchers analyzing the secretions of marine animals (Photo credit: Eden Harel).

What’s the Impact of Microplastics on Marine Life?

In the third phase of the study, the researchers examined the reverse effect: how microplastic particles affect feces, an organic material that plays a vital role in marine ecology. Eden Harel explains: “We found that plastic changes important physical properties of feces. Normal feces sink very slowly through the water column, serving as food for many organisms along the way. In contrast, feces containing microplastic particles sink rapidly to the seafloor. This removes an important nutrient source from the water column. Additionally, the faster sinking rate decreased the dispersion of the feces causing accumulation of feces and plastic particles near where the animals are settled. This accumulation can increase carbon and nitrogen levels on the seafloor and trigger algal blooms, representing another critical impact of microplastics on the balance of the marine food web”.

The researchers conclude: “In this study, we uncovered significant aspects of the influence of filter-feeding animals on the characteristics of microplastic particles in their environment and within the marine food web. The most alarming conclusion is that the microplastic problem is far more complex than initially thought. Plastic pollution in the marine environment has many unexpected dimensions, and its complexities continue to grow. Sometimes, neither we nor the environment can even recognize it as plastic. As time goes on, plastic continues to harm more and more marine ecosystems. We must develop new technologies to mitigate this dangerous phenomenon”.