Award: OCE-1632348

Coral reefs are well known biodiversity hotspots of considerable interest to the public and scientific community. Reefs around the world are currently under threat from multiple factors such as pollution, coastal development, overfishing and climate change where both the warming and acidification of tropical waters contributes to the loss of coral reefs and the many ecoservices they provide such as protection from hurricane damage, and sustainable food resources. Many studies are focused on corals, the conspicuously dominant group of organisms on many coral reefs, but other organisms are also important. One group, sponges, are essential for healthy reef function as they provide food and homes for many other reef organisms, they dramatically effect the nutrient cycles on reefs, and they synthesize important compounds of interest to the biomedical community. Sponges are ubiquitous members of Caribbean coral reef communities, where they have multiple roles the most important being their ability to filter the ambient seawater through their tissues for food and oxygen. There is evidence that sponge populations are actually increasing as corals decline due to climate change. The role of food plays a crucial role in controlling the distributions of species, especially sponges. However, the relative importance of predation and nutrient availability on the dynamics of sponge populations remains a hotly debated topic. Recently, it has been proposed that sponges filter out large amounts of dissolved organic carbon from the oceans and release large numbers of dead particles, known as detritus, to fuel a "sponge loop" pathway of significance to other members of the coral reef community. Sponges also consume large amounts of particulate organic matter, mainly bacteria presumably as an important source of nitrogen. An emerging area of coral reef science is the study of deep reefs at depths greater than 30 meters (>100 feet). These coral reef systems, known as mesophotic coral reef ecosystems, were largely inaccessible until the transfer of technical diving approaches using mixed gases to the scientific community. In this project we studied sponge populations from 3 meters to over 100 meters to examine their ability to utilize both dissolved and particulate food sources that may help explain the increasing sponge biodiversity and growth rates with increasing depth. During this study we provided training opportunities for undergraduate and graduate students as well as post-doctoral researchers, especially from underrepresented groups. In this project we accomplished several important tasks. First, we quantified, on a seasonal basis, the depth-dependent concentrations of particulate and dissolved organic matter, both their carbon and nitrogen components. We found that in all cases particulate sources of food increased with increasing depth while the dissolved components decreased with increasing depth. Second, we measured the growth rates of sponges from 3-91 m and found that sponges were always bigger as depth increased and that rates of growth increased with increasing depth. Third, we conducted a transplant experiment between different depths to show the depth dependency of growth rates and its association with differences in food supply, as well as measuring feeding rates and the biochemical make-up of the sponge tissues. During these experiments we saw that sponges from deep populations, or transplanted to deeper depths, always exhibited higher rates of growth compared to their shallow-water counterparts, and that there was no evidence of predation by either invertebrates or fish that are known to consume sponges that could affect these experiments. Finally, we examined in detail the role of the sponge microbiome, the microbes that reside symbiotically in sponge tissues, on the ability to take-up and utilize the food resources filtered by sponges. With continuing changes in the community structure of both shallow and mesophotic reefs, understanding whether we can predict, using models of ecosystem function, which reefs will undergo transitions to sponge dominated communities and what factors contribute to these transitions, will be of use to local marine resource managers. These data will also inform the broader field of marine ecology, as well as provide new insights into mesophotic reef structure and function. Last Modified: 08/22/2020 Submitted by: Michael P Lesser