The mystery of the reef: where does all the stored energy in the earth’s most productive ecosystem go? IBED researcher Jasper de Goeij found the answer during his doctoral research, a discovery that launched his scientific career with a flying start.
De Goeij has just returned from a field work trip to Curacao. Armed with a laptop full of new photographs of sponges, he talks about his research. ‘Through photosynthesis, corals and algae store energy from sunlight in biomass. However, we are able to calculate that half of that stored energy is lost again in the form of substances dissolved in the water, such as sugars and amino acids. The central question for me was: where does it go?’
Previous studies had already shown that energy may ‘disappear’ in underwater caves, De Goeij informs us. These caves, some of which are no more than a few litres in volume, contain many kinds of organisms, but the majority are sponges. ‘There are fifteen thousand different types of sponges – from bath sponges, pipe sponges to sponges that grow in crusts just a couple of millimetres thick.’ Initially, De Goeij’s assumption that the sponges absorb a large portion of the dissolved nutrients was not that well received by his colleague researchers. ‘At a congress, my idea was received with scepticism. Everyone was convinced that bacteria were responsible; no one had ever demonstrated how other life forms could absorb dissolved substances in such amounts and with the speeds we found. But I had already conducted tests to find out the quantity of nutrients bacteria could process, and that quantity was too low to plausibly explain the matter of the missing energy.’
It then turned out that De Goeij’s sponges could process sufficient quantities: using labelled nutrients, he was able to prove that large quantities of nutrients originating in the reef ended up in sponges. But now another question needed to be answered: what do sponges use all that energy for? De Goeij calculated that the amount of energy they convert into the production of new sponge cellswould mean that they double in size every three days. ‘Sponges are able to grow very rapidly. I once built an artificial cave that was full of sponges within six months. But afterwards, there is basically hardly any more room to grow.’
The secret of the sponge's high energy consumption lay in its filtering system. De Goeij shows me some photos he has on his laptop of a purple star-shaped sponge. He points out the veins that pump water into the sponge, water that then exits through craters. Viewing a cross-section of the sponge, it is not difficult to see that the sponge has a large number of filter cells that it uses to filter its food out of the water. It turns out that the filter cells divide faster than any other type of cell in any other multi-cellular organisms, according to De Goeij. ‘A sponge can replace its entire filtering system in half a day.’
De Goeij thinks this rapid division protects the animal from possible damage. ‘A sponge pumps hundreds of litres of water through its body every day. A millilitre of seawater contains a million bacteria and ten million virus particles. Before any of these can cause a sponge to become diseased, it will already have replaced its filter cells. So the sponge does not grow in size, but advances in age.’
Using a colouring for newly formed DNA, De Goeij proved the extremely rapid formation of new filter cells. He calculated that each day sponges shed about one-third of their body weight in old filter cells.
The sponge excrement also serves a purpose. For example, fish or other reef organisms may feed off it, while they have no use for the dissolved sugars and amino acids that the sponge absorbs. De Goeij: ‘Sponges play a key role in food chains. The question now is whether other sponges also serve the same function. Sponges also live in the Amsterdam canals, for example. Do they work in the same way?’
Thus far, De Goeij’s research has focused primarily on sponges in the Caribbean. Next summer he intends to go looking for sponges in the Mediterranean Sea, where unlike the tropics, there is a change of seasons. He is still also yet to study sponges found in mangrove swamps.
De Goeij points out that the discarded cells are visible on cross-sections of sponges viewed through a microscope. ‘At first I thought that those small points were artefacts, but they really are in every channel.’ He clicks on his laptop to bring up photographs of sponges in their entirety. ‘Here you see the old cells as brown excretions around the craters. You could think of it as the sponge’s poo. Sponges have to be cleaned every day if you cultivate them.’
This summer will also see construction begin on a complex aquarium system in which De Goeij and other IBED researchers will cultivate both fresh-water and salt-water species of sponge. Cultivating sponges is no sinecure, according to De Goeij. ‘If you put a sponge in an aquarium one day, it may very easily be dead the next. We need to understand the kinetics of sponges a lot better.’ Water flow, for example, is extremely important: there must be a strong current but no air bubbles. De Goeij uses a current that is far stronger than you would normally find in aquaria. ‘The water flow is currently 3 litres a minute, but I need to increase that even further. In a cave of 100 litres in volume, the water is replaced in 3 to 6 minutes.’ In trials in Curacao, De Goeij has since been able to keep sponges alive for six months. ‘We are able to keep a whole range of sponges alive and they are even growing.’
In his quest to learn more about sponges, one of the things De Goeij wants to know is which cells are involved in the three types of growth observed in sponges: regular growth, the stable phase and recovery following damage. He also wants to know which factors – light, temperature, food – are important during the transitions between these phases. In another experiment, De Goeij cuts away pieces of sponges in order to study re-growth. ‘Is a sponge like this a single organism? Do sponges communicate?’
The knowledge yielded by this type of research should eventually lead to a whole series of applications. For example, physicians are interested in the sponge as an animal model for intestines. ‘The way in which sponges regenerate can be compared to intestines, which also regularly discard their epithelium cells. Using a sponge as a model for intestines would provide more possibilities than a cell system, while at the same time it is much simpler than cultivating an entire laboratory sponge. Prospective applications include research into the origin of intestinal cancer and the healing of wounds.
De Goeij also thinks sponges could be useful as bio filters. This is because they not only retrieve dissolved particles from the water; they can also absorb micro-organisms, such as bacteria, and viruses with great efficiency. The same may be true for heavy metals, such as arsenic, or oil remnants. Together with Wageningen researcher Ronald Osinga, De Goeij has set up the company Porifarma, an enterprise whose central idea is using the understanding of the coral reef ecosystem as a concept to produce future substances: it is one of the most productive environments and produces zero net waste. With Porifarma, the two researchers are focusing on possible applications of the knowledge they have gained about sponges. ‘You see, I am a scientist in heart and soul. But I want to see action. As there is so little money available, as a scientist you often cannot do what you want to do. With Porifarma we hope to device sustainable and useful concepts that will make money. And that money we want to invest in fundamental research conducted by first-class scientists. You mustn’t make people who are good scientists chase after money. You must allow them to excel in what they are good at: doing science. We hope to be able to let them do just that.’
De Goeij estimates that it will certainly be five years before the company makes a profit: we do not yet have any products on the market. In the meantime he will need other sources of funding, including subsidies. In November 2010, the Netherlands Organisation for Scientific Research (NWO) awarded De Goeij a Veni grant, which he suspects came about in part because of the media attention his research garnered. ‘The article about cell division research was the final part of my doctorate and was published in the Journal of Experimental Biology. That journal focused in even more on De Goeij’s work in its feature Inside JEB. Subsequently, Nature and Science also reported his work in their news sections and even BBC World picked it up. While, unlike other researchers, I have not published much in prestige journals, people are intrigued by my story.’
He views a few more photos of sponges on his laptop screen. Enthusiastic: ‘Beautiful, just look at all those vivid colours. I had sat at a desk for three years before I started with this field work project. And for me this is the most fun: being underwater, seeing the fish. That is the perk of my job.’