Curaçao Sponges Also Make Food From Sunlight

For decades, sponges were assigned a straightforward role on coral reefs: remaining fixed in place and feeding on whatever drifted by in the water.

New research from Curaçao’s reefs suggests that view is incomplete. Many sponges can also produce their own food using sunlight, through the same fundamental process used by plants.

The animal that behaves like a plant

Michelle Achlatis researches organisms attached to the seabed, rather than the creatures that move through the water above. Sponges have been a central focus of her work for many years.

“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,” Achlatis said.

She conducted the research while working as a postdoctoral researcher at the University of Amsterdam’s Institute for Biodiversity and Ecosystem Dynamics (IBED).

Specialised cells draw seawater through a sponge’s body, capturing edible particles as it passes through.

For a long time, this filtering function shaped biologists’ understanding of the whole group. Sponges were essentially regarded as mouths with no other role.

Not just a consumer

This plant-like capacity is supplied by minute resident organisms.

Certain sponges contain photosynthetic microbes in their tissues, chiefly cyanobacteria, which use sunlight to make food and give off oxygen.

“Together with their symbiotic microbes, sponges can photosynthesize, just like a plant,” said Jasper de Goeij, associate professor of marine benthic ecology at IBED and a co-author of the study.

Scientists already knew that this ability existed. However, earlier studies had documented it in only a limited number of species.

A range of light responses

Achlatis and her team investigated 24 of the most widespread sponge species found around Curaçao. Eight of them underwent the most detailed testing.

The researchers monitored responses to oxygen and light, both in laboratory conditions and directly on the reef.

They then extrapolated those figures across the wider ecosystem, allowing sponges to be compared with other reef organisms that generate food from light.

Among the eight closely examined species, reactions ranged from virtually no response to light to a strong dependence upon it.

By combining oxygen measurements with chlorophyll fluorescence, the researchers could determine how intensively each sponge functioned as light levels increased.

Sponges contribute to the food supply

The findings surprised the researchers. One-half of the species tested reacted to light and carried out photosynthesis.

Achlatis had expected perhaps a quarter or a third of them to do so. She had also not anticipated such a substantial role in the reef’s food supply.

“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.

Most of these sponges nevertheless consume more energy than they produce. Sunlight provides a meaningful share of their daily energy needs, ranging from roughly 10 percent for some species to 40 percent for others.

“But they are quite flexible feeders and can supplement with photosynthesis,” said Achlatis.

Sponges hidden in a reef’s framework

The structure of the reef helps account for the unexpected result. Earlier surveys estimated sponge abundance using flat images photographed from above the seabed.

That method overlooks a substantial amount. Nearly half of a reef’s living matter is concealed in crevices, caves and the framework below the coral.

“A previous study found that when you look at the three-dimensional reef, there are so many more sponges there than you expect,” Achlatis noted.

“By doing more detailed measurements, you can calculate their volumes.”

Measuring productivity by biomass

Once this more complete estimate was applied, sponges climbed considerably in the rankings of reef producers.

One-half of the tested species represented approximately 11 percent of the reef’s gross primary production, placing them fourth overall.

Only large seaweeds, hard corals and gorgonians produce more. Sponges also slightly surpass crustose coralline algae, which have long been recognised for generating food from light.

The way productivity is assessed has a major effect on the results. When calculated from flat surface area, sponges seem to make only a minor contribution.

When assessed by biomass, however, which more accurately represents their actual size, their contribution rises many times over.

Why carbon budgets should shift

The measurements were collected at a depth of about 10 metres.

Some photosynthetic sponges occur much farther down, in low-light waters approaching 150 metres, while still retaining their light-harvesting partners.

This matters when accounting for ocean carbon. The authors say that habitats rich in sponges should be included when evaluating how reefs capture and retain carbon.

On Curaçao’s leeward reefs alone, these sponges could fix carbon at almost one-half the rate of all the island’s mangrove forests.

“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, even if they’re small amounts, comparatively,” Achlatis said.

A new look at ocean life

The study examined one tropical area only. However, sponges in similar environments frequently host the same partners, meaning the pattern could extend far beyond Curaçao.

Professor de Goeij considers the results to point towards a broader lesson about marine life.

“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,” he said.

Sponges have survived hundreds of millions of years of oceanic change. Evidence of species that feed using light already spans much of the world’s coastline, from tropical waters to cold polar seas.

According to this study, categorising them simply as consumers has underestimated them all along.