Bob Yirka, Phys.org, 4 Oct 13;
A team of researchers from the Netherlands has found that material sloughed off by sponges is eaten by organisms that live on or near coral reefs. In their paper published in the journal Science, the team describes experiments they conducted in their lab, then in the field to find out if sponges contribute to the survival of coral reef organisms by providing them with something to eat.
For many years scientists have wondered how it is that there is so much life on and around coral reefs—such areas are a virtual desert which begs the question, what are all the organisms eating? Prior research has found that bacteria in the water produce some nutrients, but not nearly enough to feed the wide variety of organisms found around coral reefs. Suspecting that sponges might provide some answers, the research team ran some experiments to find out if they are actually feeding those around them.
First, the team placed several sponges in a tank full of water, then fed them some sugar and a mixture of other material that held carbon and nitrogen in heavier than normal forms. Then they waited—sponges slough off detritus left over from meals they have consumed. Examining the detritus that covered the sponges, the researchers found evidence of the heavy nitrogen and carbon, indicating that the material the sponges were sloughing off was residue from food they had eaten.
Next, the researchers moved to the ocean, specifically the coral reef off the island of Curaçao in the Caribbean. There they created a trap for several sponges living in cracks in the coral. Doing so allowed for keeping food in and also for collecting and measuring what the sponges were sloughing off. They fed the sponges the same kind of food as in the tank experiment then waited again. And once again, they found that the material sloughed off by the sponges had heavy carbon and nitrogen in it. They then waited a few more days before gathering some specimens of aquatic life living in the crack and found that they had measurable amounts of the heavy nitrogen and carbon in their tissues as well. This meant, of course, that they had eaten the material sloughed off by the sponges, and in so doing revealed at least one source of coral reef sustenance.
Sponge feces as the driving force behind tropical coral reefs
Darwin’s Paradox solved after 171 years, as published in Science
Netherlands Organisation for Scientific Research media release, 4 Oct 13;
How can coral reefs, one of the most diverse and productive ecosystems on Earth, survive in nutrient-poor tropical waters, like an oasis in a marine desert? That question, also known as Darwin's Paradox, seems to be answered after 171 years by researchers Jasper de Goeij from the University of Amsterdam and Dick van Oevelen from the Royal Netherlands Institute for Sea Research (NIOZ). With colleagues from the Dutch Universities of Maastricht, Wageningen and Utrecht, and the research institute Carmabi on Curaçao, they published their findings in the renowned journal Science.
The study, led by De Goeij and Van Oevelen, shows that sponges are the missing link between corals and algae and the other inhabitants of the coral reef. Sponges recycle the waste products from corals and algae and convert these into a food source that is accessible to other reef inhabitants. This recycling pathway, termed the 'sponge loop' by the research team, explains how energy and nutrients are conserved within the coral reef ecosystem instead of leaking to the surrounding waters of the marine desert.
The sponge loop
Sponges are known as impressive filter feeders, feeding on small particles like bacteria, planktonic algae and even virus particles. The majority of their daily diet, however, consists of invisible dissolved organic substances, such as sugars. In fact, these dissolved substances form the largest source of energy and nutrients on coral reefs and are produced by corals and algae. This food source is not available to most other organisms living on the coral reef and may therefore leak to the surrounding tropical seas.
The research team of De Goeij and Van Oevelen has now shown that sponges prevent the leakage of dissolved energy and nutrients and make these resources accessible to other reef inhabitants. They labeled dissolved food, traced it throughout the entire ecosystem and found that sponges took up the labeled food and quickly turned it into detritus, or, in other words: sponge feces. Then, this detritus rains down on the reef where it forms an important food source for other reef inhabitants, like small crabs, snails and worms. These small animals are eaten in turn by larger animals, whereby the originally dissolved food is looped back into the food web. De Goeij and colleagues hereby show that sponges are at the base of a previously unknown recycling pathway – the sponge loop – that plays a pivotal role in the food web of coral reef ecosystems.
The importance of sponges
The study led by De Goeij and Van Oevelen emphasizes the role of sponges in future research and management of coral reefs. To date, their role is severely undervalued, while coral reefs, important socio-economic areas for tropical coastal areas, are threatened worldwide. Sponges help us to understand how coral reefs function, but also how these ecosystems can be highly productive, without leaking energy and waste. This knowledge can be applied in the development of sustainable aquaculture and the construction of so-called Integrated Ocean Farms.
Sponges help coral reefs thrive in ocean deserts
The mystery of how coral reefs thrive in "ocean deserts" has been solved, scientists say.
James Morgan, BBC News, 7 Oct 13;
Reefs are among Earth's most vibrant ecosystems, yet they flourish in waters lacking nutrients - a phenomenon known as Darwin's Paradox.
A team found that sponges keep the reef alive - by recycling vast amounts of organic matter to feed snails, crabs and other creatures.
Writing in Science, they hope their findings will aid conservation.
Sponges recycle nearly ten times as much matter as bacteria, and produce as much nutrition as all the corals and algae in a reef combined, the scientists calculate.
They are the "unsung heroes" of the reef community, said lead author Jasper de Goeij, an aquatic ecologist at the University of Amsterdam.
"Up until now no-one has really paid sponges much attention. They look nice, but everybody was more interested in corals and fish," he told BBC News.
The scientists tested sponges in a Caribbean reef
"But it turns out that sponges are big players - and they deserve credit for their role.
"If you want a reef which is colourful and biodiverse, you need a 'sponge loop' to maintain it."
It was during his voyage on the Beagle that Charles Darwin famously observed that tropical reefs are like oases in a desert.
They are surrounded by waters lacking nitrogen and phosphorus - the building blocks of life - which ought to prohibit their growth.
And since corals release up to half their organic matter into seawater, reefs need a system to recover these nutrients and recycle them into the ecosystem.
Bacteria do part of the job, but are not abundant enough to service the chemical dependencies of a whole teeming reef community.
Sugar daddies
Sponges (poriferans) are filter feeders which live in rock crevices, sucking up plankton and organic matter released into the sea by corals.
The idea that they could be a missing link in the reef food cycle has been proposed before.
But it was not clear how much nutrition they could supply, nor how exactly they feed their reef neighbours - worms, crustaceans and other sea floor foragers.
On the Caribbean island of Curacao, de Goeij and his team studied four common species of sponges - first in laboratory aquariums, then in a natural reef where the scientists sealed off a cavity.
They fed the poriferans with labelled sugars - and traced these molecules on their journey.
First the sugars were absorbed from the water by the sponges, then quickly shed in dead filter cells (choanocytes) - detritus which fell to the seabed.
Within two days, the same molecules were present in snails and other creatures feeding on the sediment containing sponge waste.
These snails are in turn eaten by larger animals, and so the cycle continues.
It was not only the speed, but the sheer volume of food turnover which took the authors by surprise - about 10 times more than bacteria recycle.
The sponge Halisarca caerulea for example takes up two-thirds of its body weight in dissolved carbon each day, but it barely grows in size - because old cells are shed to the seabed.
In total, the Dutch team estimated this "sponge loop" produced nearly as many nutrients as all the primary producers (corals and algae) in an entire tropical reef.
And other marine deserts, like deep-sea cold-water coral reefs or temperate Mediterranean reefs, may also rely on poriferans to recycle their nutrients.
By recognising sponges as lynchpins - the unheralded heroes of the reef - they hope to aid conservation efforts in these fragile havens.
Surviving in a Marine Desert: The Sponge Loop Retains Resources Within Coral Reefs
Jasper M. de Goeij, Dick van Oevelen, Mark J. A. Vermeij, Ronald Osinga, Jack J. Middelburg, Anton F. P. M. de Goeij, Wim Admiraal
Science, 4 Oct 13;
Abstract: Ever since Darwin’s early descriptions of coral reefs, scientists have debated how one of the world’s most productive and diverse ecosystems can thrive in the marine equivalent of a desert. It is an enigma how the flux of dissolved organic matter (DOM), the largest resource produced on reefs, is transferred to higher trophic levels. Here we show that sponges make DOM available to fauna by rapidly expelling filter cells as detritus that is subsequently consumed by reef fauna. This “sponge loop” was confirmed in aquarium and in situ food web experiments, using 13C- and 15N-enriched DOM. The DOM-sponge-fauna pathway explains why biological hot spots such as coral reefs persist in oligotrophic seas—the reef’s paradox—and has implications for reef ecosystem functioning and conservation strategies.
