A new study from Tel Aviv University (TAU) found that sponges in the Gulf of Eilat have developed an original way to keep predators away.

The researchers found that the sponges contain an unprecedented concentration of the highly toxic mineral molybdenum. In addition, they identified the bacterium that enables sponges to store such high concentrations of this precious metal and unraveled the symbiosis between the two organisms.

The study was led by PhD student Shani Shoham and Professor Micha Ilan from the School of Zoology at TAU’s George S. Wise Faculty of Life Sciences. The paper was published on July 17, 2024, in the journal Science Advances.

Sponges are the earliest multicellular organisms known to science. They live in marine environments and play an important role in the earth’s carbon, nitrogen, and silicon cycles. A sponge can process and filter seawater 50,000 times its body weight every day. With such enormous quantities of water flowing through them, they can also accumulate various trace elements, including toxic amounts of materials like arsenic and molybdenum. Scientists are trying to understand how the sponges cope with these materials.

“Twenty to thirty years ago, researchers from our lab collected samples of a rare sponge called Theonella conica from the coral reef of Zanzibar in the Indian Ocean and found in them a high concentration of molybdenum,” Shoham says. “Molybdenum is a trace element, important for metabolism in the cells of all animals including humans and widely used in industry. I wanted to see whether such high concentrations are also found in this sponge species in the Gulf of Eilat, where it grows at depths of more than 27 meters [about 89 feet]. Finding the sponge and analyzing its composition, I discovered that it contained more molybdenum than any other organism on earth.

“Like all trace elements, molybdenum is toxic when its concentration is higher than its solubility in water,” Shoham adds. “But we must remember that a sponge is essentially a hollow mass of cells with no organs or tissues. In Theonella conica, up to 40% of the body volume is a microbial society: bacteria, viruses, and fungi living in symbiosis with the sponge.

“One of the most dominant bacteria, called Entotheonella sp., serves as a ‘detoxifying organ’ for accumulating metals inside the body of its sponge hosts. Hoarding more and more molybdenum, the bacteria convert it from its toxic soluble state into a mineral. We are not sure why they do this. Perhaps the molybdenum protects the sponge, by announcing ‘I’m toxic! Don’t eat me!’ In return for this service, the sponge does not eat the bacteria and serves as their host.

“Future research should focus on the ability of Entotheonella sp. bacteria to accumulate toxic metals. A few years ago, our lab discovered huge concentrations of other toxic metals, including arsenic and barium, in a close relative of Theonella conica called Theonella swinhoei, which is common in the Gulf of Eilat. In this case, too, Entotheonella sp. was found to be largely responsible for hoarding the metals and turning them into minerals, thereby neutralizing their toxicity.

“Continued research on the bacteria can prove useful for treating water sources polluted with arsenic, a serious hazard which directly affects the health of 200 million people worldwide,” Shoham concludes.