For centuries scientists, even Darwin, couldn’t make sense of it: Tropical water contains so few nutrients, you can see right through it. And yet coral reefs are oases that support about a quarter of all known species on Earth.
How could that be?
The answer to this paradox, in part, is sugar.
We tend to think the ocean tastes salty. But shaken, stirred and dissolved in seawater are microscopic morsels of sugars and carbs, known as dissolved organic matter. This dissolved substance makes up most of the organic material in the ocean. And it’s especially abundant around coral reefs.
“Imagine all these sugars dissolved into the ocean: If no one can use them, they might as well not be there,” said Michelle Achlatis, a researcher at the California Academy of Sciences.
But then there’s Cliona orientalis.
This filter-feeding sponge lives on coral reefs in the Indian and Pacific Oceans, straining tiny plankton to eat as it sits in the water. But its filtering cells also sip sugars from seawater.
Few organisms can directly take in this bounty hiding in seawater. “But these organisms have evolved ways to feast on that sugar,” Dr. Achlatis said. She and her colleagues refer to it as “cell drinking.”
It had been known that sponges somehow took in dissolved organic matter, but it remained unclear whether they could do it on their own or needed help from their bacterial symbionts. In a study published Wednesday in Proceedings of the Royal Society B that used new imaging technology that could see inside the sponge’s filtration cells, Dr. Achlatis and her colleagues showed that the sponges were capable of taking in these sugars without the bacteria’s help. The group’s study improves understanding of how unusual eating habits help sponges — and coral reef ecosystems — survive on limited nutrients.
In 2013, Jasper de Goeij, a marine biologist at the University of Amsterdam and co-author on the current paper, reported that sponges bathing in seawater containing dissolved organic matter took in the sugars and left the water behind. And they got 90 percent of their carbon from these sugars — much more than previously thought.
He proposed these nutrients continued in the food chain through a “sponge loop.” Sponges sipped dissolved organic matter and turned it into tissue. Other animals feasted on the sponge’s tissue. Scientists had long known that bacteria ran sugar recycling centers on coral reefs, but the sponges provided another way for that matter to enter the food chain.
But because sponges are loaded with bacteria, scientists still wondered: Were bacterial partners inside sponge tissue actually sipping sugar for the sponge?
Finding out for certain began in February 2016, along an island in the Great Barrier Reef. Here, C. orientalis approaches coral from below, slowly dissolving the reef’s skeleton and building its own around it.
To collect samples, Dr. Achlatis dived with a power drill and bore holes into coral 20 feet underwater.
“It might be the hardest sponge to collect — like, literally, the hardest,” she said.
She returned with cylinders of sponge and fed them dissolved organic matter tagged with special ions that she could see beneath a microscope. She looked at the material’s location over time, as the animals fed. And eventually it ended up inside each sponge’s own filtration cells.
Their cells’ membranes appeared to form vesicles that ventured out, intercepted dissolved organic matter and brought it back for processing, as if the cell were drinking. To know for sure, Dr. Achlatis said, they’ll need an even more powerful microscope.
Although the team did not look directly at the sponge’s bacteria in this study, they think those symbionts are taking in dissolved organic matter as well, and plan future studies to see if the bacteria do, and if so, how much.
They also plan to use their methods to test other species: While this was the first sponge to reveal its own cells are sugar-sippers, it likely won’t be the last.