Award: OCE-1536368

The biological fertility of the ocean is an important part of global environmental conditions. Ocean life relies on the density structure of the ocean, which keeps phytoplankton in the sunlit surface water supplies nutrients, as well as ocean circulation, which supplies the needed nutrients from deep water. In turn, ocean life can affect Earth?s physical conditions in important ways; for example, it modifies climate through its effects on the atmospheric concentration of the greenhouse gas carbon dioxide. An understanding of these interactions is difficult to achieve simply from experiments in the laboratory or monitoring of modern ocean conditions. The past provides a vast library of natural experiments of ocean-climate-life interactions, from which we can learn if we are able to develop tools (?paleo-proxies?) to reconstruct past environmental and biological conditions. Because corals lay down layers of calcium carbonate skeleton over time, scientists can measure chemical and physical properties of a core through a coral that has recorded hundreds of years of history. The nitrogen isotopic composition (δ15N) of the organic matter bound in the coral skeleton is a new tool in development that has the potential to tell us about past nutrient cycling in coral reefs and the nearby open ocean. As corals feed and grow, they record the δ15N patterns in the environment, which reflect regional and local N cycling processes. However, interpreting δ15N measurements from coral skeletons might be complicated if changes in the corals? internal nutrient cycling also affect their δ15N. Corals, like other animals, produce metabolic waste (rich in nitrogen (N) and phosphorus) from the breakdown of the food that the coral polyps capture from the water; this animal waste, however, is much-needed fertilizer for plant organisms. In a farm-like symbiosis, corals house and provide nutrients from their waste products to algae living inside them (?zooxanthellae?), which then return useful organic matter, produced through their photosynthesis, to the coral host. It is a possibility, and indeed one that has been suggested, that changes in the nature of this symbiosis may change the δ15N relationship between environment and coral. On the one hand, this potential variability in the environment-coral δ15N relationship might be useful for studying modern corals. On the other hand, it would complicate the use of corals to reconstruct past changes in nutrient cycling of coral reefs and the nearby ocean. To address this question, we conducted experiments in the laboratory and studied modern corals in their natural environment. Our laboratory results indicate that even a 4-fold change in feeding rate had very little effect on corals? δ15N. From these data and a simple model for the corals? internal nutrient cycling, we found that corals? internal N cycling is unexpectedly stable. In our field survey, we measured the δ15N of various organisms and N pools (corals, macroalgae, feather duster worms, and food sources) along a transect of Bermuda reefs, across which there is a significant range in food availability. The δ15N changes across the Bermuda reefs were similar among the measured organisms and N pools, indicating that the corals are able to capture δ15N patterns in the environment. Together, the experiments and field results give us confidence in the use of δ15N measurements from coral cores to reconstruct past changes in nutrient cycling in the coral?s environment. Training and education were an integral part of the project. Two graduate students and five undergraduate students were deeply involved in the project, working closely with one another and with more senior scientists. Research results were communicated with 14 local New Jersey elementary and middle school teachers attending QUEST, a weeklong professional development workshop to further their personal science understanding. During this workshop, a hands-on coral feeding activity was developed that can be used in grades 4-8 classrooms to help students learn about different feeding strategies used by corals, including the coral symbiosis with zooxanthellae. In addition, at the Bermuda Institute of Ocean Sciences for the 8-week duration of the experiment, weekly tours through the tanks of the coral feeding experiment were given to the public, which included both Bermudians and international tourists. Last Modified: 12/11/2018 Submitted by: Daniel M Sigman