Over the last couple of years, I have become fascinated by the movement of energy and nutrients through ecosystems. During my PhD at the Center for Tropical Marine Ecology in Bremen, Germany, I was given the opportunity to work on a Red Sea coral reef on the Jordanian coast. My work was part of a project which investigated carbon and nitrogen fixation in the coral reef as it experienced the seasonality of the northern Red Sea. This seasonality is exceptionally high for coral reefs as these reefs are some of the most northern in the world. My focus in the project was on carbon. The vast majority of carbon fixation in these highly sunlit ecosystems is done by photosynthesis. A coral reef is a vast bank of photosynthetic organisms which harvest the energy from the sun by photosynthetic pigments and these pigments are what give coral reefs their fascinating colours.
One of the main research topics for coral reef biologists is the fact that a high percentage of the reef’s photosynthesis is performed by the corals. Many of these cousins of anemones and jelly fish have formed a symbiosis with single celled algae (zooxanthellae) in their tissues. The algae are the actual photosynthesizers and give a large fraction of the sugars they produce to their animal host. In return, they have a lovely stable environment in the coral tissues and the coral provides them with nutrients. The availability of nutrients is another major research topic in coral reef biology. Coral reefs have adapted to thrive in extremely nutrient poor waters. You can see this by the clear blue waters which surround them (available nutrients in the water column would lead to more phytoplankton, turning the water green). To maintain their high productivity in nutrient depleted surroundings, coral reef organisms have many ways of capturing, generating, and recycling essential nutrients. The most obvious way coral reefs capture extra nutrients is that corals like anemones and jelly fish are predators. Katharina Fabricius described coral reefs as a “wall of mouths”, sieving particles and plankton from the water that moves across them. In addition, many other reef organisms like giant clams, tunicates, and basket starfish also filter their food supply from the water column.
In recent years, research on coral reefs has moved towards microbiology. The more we look, the more we find that corals are not just working together with their algae, but that they also have a community of bacteria which play a symbiotic role (the coral, algae, and microbes together are known as a holobiont). The other part of our project, nitrogen fixation, focused on some of these bacteria. Nitrogen fixation turns atmospheric dinitrogen into ammonia which can be used by organisms on the coral reef as a source of much needed nitrogen. Working together with my colleagues Ulisse Cardini and Vanessa Bednarz, we found that pretty much every part of the reef seafloor community investigated (corals, macroalgae, the sediment between the corals, the hard substrate on which the corals grow), contained microbes capable of nitrogen fixation. What we also found was that in summer, when bio-available nitrogen levels in the water drop even lower than during the rest of the year, nitrogen fixation was far more active in all substrates investigated. This strongly indicated that nitrogen fixation was an omnipresent process in this nitrogen limited coral reef, and that the organisms could increase their ammonia production when ambient bio-available nitrogen levels dropped.
Having collected a giant dataset on carbon dynamics in many of the coral reef’s functional groups, I decided to try and link all these values together into a reef-wide ecosystem model. To expand the picture even further, I joined forces with Laura Rix, a colleague who was focusing on carbon and nitrogen dynamics in sponges. Sponges are another massive component of the reef community. However, their vast majority lives in cavities within the reef. Coral reef cavity sponges were recently found to be able to access an energy source which was previously thought to be off limits to multi-cellular organisms. The majority of organic carbon released in the oceans dissolves. This process was always thought to make it unavailable to larger organisms. The only way in which this carbon was recycled was through the microbial loop where microbes grew on the dissolved carbon and they themselves were then grazed by zooplankton. In that way, the carbon came back up the food web. What Jasper de Goeij and others recently postulated was that sponges were also capable of accessing this dissolved source of energy. This so-called sponge loop was the focus of Laura Rix’s work and it provided her with a large database of carbon dynamics in coral reef sponges. We are now working on linking all energy flows between the different functional groups of coral reef organisms to see if we can make them work together in an overarching picture of coral reef energy dynamics taking the new role of sponges into account.