Charles Darwin asked himself the same question: how can coral reefs survive so well in nutrient-poor tropical oceans? Marine biologist Jasper de Goeij discovered that cave sponges play an important role in recycling nutrients on  coral reefs.

‘The coral reef is one of the most productive ecosystems in the world’, explains De Goeij. ‘You can compare it to a tropical rainforest. This high production is extraordinary because the tropical ocean is very nutrient poor. That’s why the water is so clear and blue. There’s nothing in it. That’s why coral reefs are also called oases in a desert. It’s still a mystery how coral reefs can survive in such a nutrient-poor environment. This is also called the Darwin Paradox.

Figure 1: Sponges have been found to be highly important nutrient recyclers on coral reefs (photograph: Erik van Bommel). 

Darwin was one of the pioneers of coral reef research and was also the first to describe this problem.’ In order to gain more insight into the food cycle, De Goeij studied the role of sponges in coral caves.

"The coral reef is one of the most productive ecosystems in the world; this high production is extraordinary because the tropical ocean is very nutrient-poor".

Colourful painting

A coral reef is created through an accumulation of the skeletons of corals, tiny animals that live in colonies. During the accumulation process, all kinds of hollows, holes and caves are created too. ‘Very little attention is ever paid to these caves because they are so dark and appear to be uninhabited. If you bring a light there, though, you can see that the walls resemble a huge, colourful painting’, says De Goeij, who has seen it with his own eyes. This wonderful colour palette is mainly due to cave sponges, which are between one and five millimeters thick and cover the walls in thin layers.

A coral reef is created through an accumulation of the skeletons of corals, tiny animals that live in colonies. During the accumulation process, all kinds of hollows, holes and caves are created too. ‘Very little attention is ever paid to these caves because they are so dark and appear to be uninhabited. If you shine a light in there however, you can see the walls resemble a huge, colourful painting’, says De Goeij, who has seen it with his own eyes. This wonderful colour palette is mainly due to cave sponges, which are between one and five millimeters thick and cover the walls in thin layers.

"Very little attention is ever paid to these caves because they appear to be uninhabited. If you shine a light in there however, you can see the walls resemble a huge, colourful painting".

Dissolved organic material

De Goeij discovered that these cave sponges play an important role in the food cycle of the coral reef. Algae and corals produce a great deal of organic material. Because a great deal of this is dissolved in the water and is very difficult to break down, very few organisms can use it as food.

Figure 2: Cellular division in sponges is extremely high; these animals continuously release dead cells into the water, which serve as a food source for reef inhabitants. This biological recycling helps the reefs to reuse precious nutrients (photograph: Erik van Bommel).

If no organisms use this food, it will wash out of the reef and into the ocean. This would equal energy leaking out of a system. De Goeij, however, has proved that much of the dissolved organic material is taken up by the cave sponges. He was able to determine this by measuring the amounts of carbon, nitrogen and phosphorous – the basic elements of all life on earth – that went into the caves and how much came out again.

"It seems that 90% of a cave sponge's diet consists of dissolved organic carbon. Previously, it was believed sponges mainly fed on particles". 

De Goeij discovered that these cave sponges play an important role in the food cycle of the coral reef. Algae and corals produce a great deal of organic material. Because a great deal of this is dissolved in the water and is very difficult to break down, very few organisms can use it as food. If nobody uses this food, it will wash out of the reef and into the ocean. Energy would leak out of the system. De Goeij, however, has proved that much of the dissolved organic material is taken up by the cave sponges. He was able to determine this by measuring the amounts of carbon, nitrogen and phosphor – the basic elements of all life on earth – that went into the caves and how much came out again. It seems that 90% of a cave sponge's diet consists of dissolved organic carbon (DOC); previously, it was believed sponges mainly fed on particles.

Intestines

But what exactly do the sponges do with all that food? They can’t invest in growth because the space is limited on a coral reef. De Goeij discovered that they mainly use the food to rejuvenate their cells. ‘Sponge cells divide incredibly fast. By comparison, cell division in a mouse’s intestine takes at least half a day. Sponges take between 5 and 6 hours. According to De Goeij, sponges need to do this because they come into contact with so much dirt. ‘Sponges filter their food out of the water. Because they live in a nutrient-poor environment, they sometimes have to pump 100 liters of water a day. This increases the chances of a sponge coming into contact with all kinds of viruses, bacteria and poisonous substances. These things can cause permanent damage. In order to avoid this, the sponge constantly rejuvenates its cells.’ Remarkably, the structures the sponges use to take up their food are very similar to the human intestinal tract. ‘Sponges are about 700 million years old and are our oldest multicellular ancestors. It looks like our digestive tract has not changed so much during evolution.’

 Sponge cultivation

Thus cave sponges constantly produce a huge stream of dead cells which – unlike the dissolved organic material – can be easily eaten by the other organisms on the reef.

Figure 3: By using a flow cell, the uptake and usage of dissolved carbon by the sponge can be determined (photograph: Dr. Jasper de Goeij).

Has De Goeij’s research resolved the Darwin Paradox? ‘No, it hasn’t. But at least we now understand better how such an ecosystem can function – by recycling efficiently.’ He hopes that his results can be used in sponge cultivation. ‘That’s not really successful at the moment. Perhaps we’ll get better at it now we know exactly what they eat and how cells grow and die.’

Tiny medicine factories

De Goeij sees a lot of applications for cultivated sponges. Because they are outstanding filters, they could be used to purify water. But sponges also contain substances that could be used as medication. ‘Sponges are tiny chemical factories, but the substances they produce are so complicated that it is much too expensive to synthesize them and the medicines would be unaffordable. It is therefore important to develop good cultivation methods for sponges. At the moment, natural medicines mainly come from plants and animals that live on dry land. However, 70% of the earth is under water and the sea contains a wealth of interesting substances.’

Curriculum vitae

Jasper de Goeij (1977) graduated as a biologist in 2001 at Wageningen University. In 2003 he started his PhD research at the Royal Netherlands Institute for Sea Research. His supervisor was Prof. G.J. Herndl of the University of Groningen. The research was financed by NWO. His thesis is entitled Element cycling on tropical coral reefs: The cryptic carbon shunt revealed. He and Dr Ronald Osinga will be continuing research with their own company Porifarma (www.porifarma.com).

"De Goeij sees a lot of applications for cultivated sponges. Because they are outstanding filters, they could be used to purify water. Sponges also contain substances that could be used as medication."

References:

www.rug.nl

de Goeij, J.M. (2009). Element cycling on tropical coral reefs: The cryptic carbon shunt revealed. 192 pp. No. ISBN: 978-90-367-3664-0.