Sun-powered sponges may generate 11% of tropical coral reef productivity Stephanie Baum Scientific Editor Andrew Zinin Chief Editor In marine environments, sponges tend to eat other organisms to get their nutrients. But a study published in Functional Ecology by researchers at the University of Amsterdam's Institute for Biodiversity and Ecosystem Dynamics (IBED), demonstrates how sponges may also use photosynthesis, just like plants. This phenomenon can help with productivity—the amount of energy and food produced—in tropical coral reefs, and perhaps in other ecosystems where sponges are also common.

Sponges under the sea As a benthic ecologist, Michelle Achlatis likes to say she's studying the sea life that is stuck to the bottom of the ocean, rather than swimming around. Sponges have been a key part of her work over the past several years, as she researches their role in the ecosystem. "We call them a textbook example of filter feeding and suspension feeding because they are not only filtering, but also absorbing all sorts of edible material from the seawater," said Achlatis, who conducted this research as part of her postdoctoral research at IBED.

Specialized cells pump water through their bodies, trapping whatever is edible in the water and "drinking it" to absorb dissolved nutrients. But sponges also have symbiotic partners. Some of them are tiny microbes that are photosynthetic.

They use the energy in sunlight to produce their own food and release oxygen. While sponges' photosynthetic properties had been researched before, they had been tested in only a few species. As part of the new study, researchers wanted to look at more sponge species and also scale up the measurements to the ecosystem level.

Sampling sponges Achlatis and her colleagues assessed 24 of the most common sponge species from reefs around the island of Curaçao. They conducted extensive measurements for eight of these species. They scaled up their findings to the ecosystem level to compare the sponges with other photosynthesizing organisms and to see how much oxygen they were producing at the ecosystem level.

To assess the abundance of organisms on the reef, they used a more holistic dataset that looks at the reef in all its three dimensions and accounts for its richness. "A previous study found that when you look at the three-dimensional reef, there are so many more sponges there than you expect," Achlatis said. "By doing more detailed measurements, you can calculate their volumes." The researchers found that half of the sponge species tested accounted for 11% of gross primary productivity, meaning they were important producers of organic compounds for the ecosystem.

This was a surprise: Achlatis thought that maybe a fourth or a third would be photosynthetic. She also did not expect how much the sponges contributed to productivity. "It's a group that in previous food web studies or big coral reef modeling were always put aside as pure consumers and not producers," she said.

"But they are quite flexible feeders and can supplement with photosynthesis." The importance of considering multiple ecosystems and sponges' multitasking abilities This study was conducted in one tropical region.