The secret lives of sponges: Solving mysteries on a coral reef

This month's Aquarius mission is part of a larger research effort to understand the role of sponges in the Florida Keys, from near shore (including Florida Bay) to the offshore coral reefs. Sponges are an important part of the coral reef ecosystem, yet surprisingly little is known about their biology and role in the cycles of nutrient elements in reef ecosystems. A major focus of this work is to understand how fast sponges respire oxygen and recycle nitrogen to the reef. In other words, how fast do they breathe, how much nitrogen do they release to surrounding waters while they breathe, and what forms of nitrogen do they release? Sponges are animals that make their living on the reef by filtering massive amounts of water to extract bacteria and other fine particles for food. Importantly, recent discoveries have shown that large populations of bacteria live inside about half of the sponge species under study. Some of these bacteria are hypothesized to make a living by turning ammonium created during sponge respiration into nitrate while others may be able to take dissolved nitrogen gas in seawater (note: the composition of our atmosphere is mostly nitrogen gas and thus there is a lot of dissolved nitrogen gas in seawater) and convert it to forms of nitrogen that can be used to help support sponge nutrition. Still other bacteria in these sponges may help convert ammonium into nitrogen gas and send it back up to the atmosphere thus ridding the reef of nitrogen. The consequences of these different processes for the nutrient budget of the larger reef system are potentially large. The sponges may be bringing in new nitrogen or they may be helping to rid the reef of an excess nitrogen supply that can lead to algal growth and other undesirable ecosystem changes.

Dr. Martens and Dr. Niels Lindquist and their team of experienced students plus co-investigators Dr. Brian Popp and Jan Riechelderfer from U. Hawaii and Dr. Susanne Schmidt from Germany, will conduct experiments during their Aquarius mission to evaluate the importance of each of these different processes. In particular, underwater chambers will be used to contain sponges in ways that keep the animals happy while the scientists measure respiration rates and nitrogen uptake and release under light and dark conditions. The scientists will also seek evidence from variations in the natural stable isotopic composition of sponge tissues and dissolved ammonium and nitrate. In another series of experiments, 15N labeled nitrate and ammonium will also be provided to the sponges to see if these forms of nitrogen, which are readily available in seawater, can be used or stored by the sponges to support growth.

There are three potentially important and exciting discoveries that could result from these experiments. First, if nitrogen gas that is dissolved in seawater is being converted to forms of nitrogen that are useful to other organisms on the reef, for example seaweeds, Dr. Martens and his team will have discovered a new and potentially important source of nitrogen affecting the biology and ecology of coral reefs. Second, if sponges and their bacterial colonists are able to capture and store dissolved nutrients, such as nitrate and ammonium, then the recently discovered pulses of nutrients that frequently occur throughout the Keys from upwelling (see Leichter mission for details) become even more important. Thirdly, if sponge-hosted bacteria can convert nitrate and ammonium to nitrogen gas for export back to the atmosphere an important pathway for removing excess nitrogen from the reef will have been discovered. Together, results from these experiments have the potential to fundamentally reshape how scientists and managers think about nutrient dynamics on coral reefs.

Additional experiments will be conducted to learn more about the bacteria that live inside the sponges. Questions exist about where the bacteria come from; are they filtered from seawater and captured by sponges or are they resident populations that are cultured by the sponges? What is it like to live inside a sponge? Preliminary data suggest that some sponges are devoid of oxygen and are the habitat for bacteria capable of living in extreme environments. Needle oxygen sensors will be inserted in sponges and used to study their oxygen content. Experiments will measure bacterial populations outside the sponges in the surrounding water, inside the sponges, and in the water filtered and expelled by the sponges. That way, the dynamics of the sponge and bacterial relationship will become better understood. This dynamic bacteria-sponge relationship may also affect oxygen levels inside the sponge. Although the significance of low oxygen in sponges is not known, the possibility exists for many unexpected chemical reactions that occur only under low concentrations. A number of laboratory experiments will be conducted during the Aquarius mission to characterize how specific bacterial populations influence oxygen levels and nitrogen reactions in sponges. This work includes identifying species of bacteria present and molecular techniques to identify genes that code for the production of special enzymes necessary to use atmospheric nitrogen.

Finally, the work conducted this month in Aquarius represents an extension of topside-based research conducted over the last three years in the Florida Keys. Drs. Martens and Lindquist and colleagues are working to understand how environmental factors, such as near shore to offshore gradients in light, depth, and potentially land-based factors related to nutrient pollution, affect the carbon and nitrogen chemistry of sponges that exhibit the different nitrogen cycling strategies described above. Ultimately, it is hoped that these studies will provide a means to monitor short- and long-term changes in water quality - an issue of major significance in the Florida Keys.