Michelle Achlatis spends her days studying life on the ocean floor. As a benthic ecologist, she focuses on creatures that are stuck to the bottom of the sea rather than swimming around. Sponges have been a key part of her work over several years.
In her recent research near the Caribbean island of Curaçao, Achlatis and her team at the University of Amsterdam discovered something surprising. Sponges are not just eating to survive — they can also make their own food using sunlight, just like plants.
Sponges are filter feeders, meaning they pump water through their bodies and trap whatever nutrients they can find. But the team found that half of the sponge species they tested get a significant portion of their energy from tiny photosynthetic microbes living inside them. These microbes use sunlight to produce food and release oxygen.
When the researchers scaled up their findings to the whole reef, they calculated that these photosynthetic sponges account for 11% of gross primary productivity. That means they produce a notable share of the food and energy that supports the entire coral reef ecosystem.
Achlatis said she expected maybe a quarter or a third of sponges might be photosynthetic. Eleven percent from half the species was a welcome surprise.
Jasper de Goeij, associate professor of marine benthic ecology at the university and co-author of the study, said the findings challenge how scientists think about ocean life.
"Together with their symbiotic microbes, sponges can photosynthesize, just like a plant," de Goeij said. "We need to look differently at many organisms living in the ocean, as they do not follow the strict plant-animal distinction as most organisms on land."
The study, published in the journal Functional Ecology, looked at 24 of the most common sponge species in Curaçao. The researchers measured eight of these species in detail and then calculated their role in the wider ecosystem.
Achlatis said the discovery has bigger implications for how we understand coral reefs and climate. Sponges are often left out of food web models and carbon estimates, but this research suggests they deserve a seat at the table.
"The case we are trying to make with this paper is really that people should be aware that sponges are not only recycling carbon and nutrients that they find on the reef, they also have their own ways of making them," Achlatis said.
She argues that sponge-rich ecosystems like coral reefs should be included in future assessments of carbon sequestration — the process of storing carbon dioxide to help counter climate change.