September Aquarius Mission Project Summary

The Secret Life of Sponges: Solving mysteries on a coral reef

This month's Aquarius mission is part of a larger research effort to understand the feeding biology of sponges in the Florida Keys, from nearshore (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 ecology. A major focus of this work is to understand how sponges secure carbon and nitrogen to meet their basic needs of metabolism and growth. In other words, what and how do they eat? 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 some sponge species. The bacteria are hypothesized to have the ability 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. The consequences of these different feeding strategies - filtering or using the products of bacteria - are significant for individual species and for the nutrient budget of the larger reef system.

Dr. Martens and his team (with co-Principal Investigators Dr. Niels Lindquist and Dr. Brian Popp who are working topside) will conduct experiments during their Aquarius mission to evaluate the significance of these two feeding strategies on the biology of different sponge species. In particular, underwater chambers will be used to contain sponges in ways that keep the animals happy while the scientists measure how the sponges use special forms of nitrogen that will be added to the chambers. The special forms of nitrogen include a stable isotope (15N) that can be tracked and measured as it is converted from nitrogen gas to a variety of potential nitrogen compounds produced by the bacteria living inside the sponges. Stable isotopes are naturally present in the environment and are sometimes used by scientists to conduct feeding experiments and also to identify and trace nutrients as they move through an ecosystem. 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 two 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 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. Together, results from these experiments - producing new nitrogen and efficiently capturing pulses of nutrients - have the potential to fundamentally reshape how scientists and managers think about nutrient dynamics on coral reefs.

An additional experiment 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? 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. Oxygen measurements will also be made on water around sponges and filtered by sponges. Interestingly, sponges remove substantial amounts of oxygen from the water they filter and thus have the ability to substantially alter their immediate surroundings. Although the significance of this phenomenon is not known, the possibility exists for many unexpected chemical reactions that occur only under low oxygen conditions. A number of laboratory experiments will be conducted after the Aquarius mission to characterize how 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 two years. Drs. Martens and Lindquist are working to understand how environmental factors, such as nearshore 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 feeding 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.