How Sponges Use Bacteria to Ward Off Predators

A recent investigation at Tel Aviv University has revealed that sponges in the Gulf of Eilat have developed a novel defense mechanism against predators. Researchers discovered that these sponges contain an unusually high concentration of the highly toxic mineral molybdenum (Mo). They also identified the specific bacterium responsible for enabling the sponges to accumulate such high levels of this metal and explored the symbiotic relationship between the two organisms.

The study, led by Ph.D. student Shani Shoham and Prof. Micha Ilan from TAU's School of Zoology, was published in the journal Science Advances.

The researchers point out that sponges are some of the oldest known multicellular organisms. They reside in marine environments and are crucial for the Earth's carbon, nitrogen, and silicon cycles. Sponges have an impressive ability to filter seawater at a rate 50,000 times their body weight daily, which allows them to accumulate various trace elements. This raises questions about how they manage to handle and detoxify large amounts of harmful substances such as arsenic and molybdenum.

Shani Shoham comments, "Several decades ago, our lab's researchers found elevated levels of molybdenum in a rare sponge species, Theonella conica, from the coral reefs of Zanzibar in the Indian Ocean. Molybdenum is an important trace element for cellular metabolism in animals, including humans, and is used in various industrial applications.

"In my research, I aimed to determine if similarly high levels of molybdenum are present in this sponge species in the Gulf of Eilat, where it is found at depths greater than 27 meters. My analysis showed that this sponge contains the highest recorded concentration of molybdenum in any organism: 46,793 micrograms per gram of dry weight."

Shoham further explains, "Molybdenum, like many trace elements, can be toxic at high concentrations. However, sponges are essentially simple cellular structures without complex organs or tissues. In Theonella conica, up to 40% of the sponge's volume is taken up by a microbial community—including bacteria, viruses, and fungi—that exists in a symbiotic relationship with the sponge.

"One dominant bacterium, Entotheonella sp., functions as a 'detoxifying organ' by accumulating and converting molybdenum into a less toxic mineral form. The exact reason for this process is unclear, but it might serve as a warning to predators that the sponge is toxic. In exchange, the sponge provides a habitat for these bacteria without consuming them."

Despite the high demand for molybdenum, especially for making high-strength steel and other alloys, extracting it from sponges is not feasible. Shoham explains, "Even though the concentration is very high, the actual amount of molybdenum we can extract from each sponge is minimal, and the sponge itself is relatively rare. Cultivating sponges in marine agriculture for pharmaceutical uses is complex due to their delicate nature and specific habitat requirements.

"Future research should explore how Entotheonella sp. bacteria accumulate toxic metals. Our lab previously discovered high levels of other toxic metals, such as arsenic (As) and barium (Ba), in a related sponge species, Theonella swinhoei, found in the Gulf of Eilat. Entotheonella sp. was again crucial in managing these metals. Investigating these bacteria further could be beneficial for addressing water pollution issues, particularly with arsenic, which poses a serious health risk to 200 million people worldwide."