Award: OCE-1558580

Sponges are bottom-dwelling animals that dominate Caribbean reefs now that reef-building corals have been declining for decades. Sponges feed by filtering huge volumes of seawater, providing a mechanism for recycling organic material back to the reef. A new theory called the sponge-loop hypothesis proposes that sponges absorb dissolved organic carbon (DOC, including dissolved sugars) released by seaweeds and returns this material to the reef as particles in the form of shed sponge cells. We tested the sponge-loop hypothesis for 9 species of sponges off the Florida Keys and Belize, and for one species in the Red Sea, by analyzing the dissolved and particulate components in seawater samples that were collected before and after passing through the sponge body. We discovered that sponge species that had a high abundance of microbial symbionts consumed DOC as more than 50% (and as much as 90%) of their diet, but those without microbial symbionts consumed mostly particulate food. Strangely, genetic analyses of the microbial symbiont community across sponge species did not help to explain differences in the uptake of DOC. An important outcome of this project was that none of the sponge species were net producers of particles; therefore, this aspect of the sponge-loop-hypothesis was not validated for the sponge species that were studied. The giant barrel sponge, which is now the most abundant animal on Caribbean reefs, was determined to consume the greatest amount of DOC, returning that carbon to the reef as sponge tissue. Because sponges also produce dissolved nitrogenous waste, the results of this project supported another theory, called the vicious circle hypothesis, that sponges produce fertilizer that enhances the growth of seaweeds which produce DOC that enhances the growth of sponges, both to the detriment of reef-building corals. Taken together, the intellectual merit of the outcomes of this project are a better understanding of what may be driving the dramatic changes to Caribbean reef ecosystems. Broader impacts of this project included support for the education and training of 7 undergraduate, 8 MS, 3 PhD, and one postdoctoral students. STEM experiences for these students included field work in the Florida Keys and Belize, as well as attendance and presentations at scientific meetings. Public outreach was provided through the publication of over 25 new videos related to coral reef ecology at the Pawlik Lab YouTube channel, which has over 500 subscribers and >130,000 views. This project addressed a poorly understood component of the carbon cycle, which is central to understanding the most significant threats facing humanity: rising CO2 levels, climate change, and ocean acidification. These threats likely have been responsible for helping to reduce coral cover on Caribbean reefs to a fraction of historical levels, but as a consequence, sponges are now the dominant habitat-forming organisms on these reefs, which remain of vital economic importance for fishing and tourism in the Florida Keys and across the Caribbean. Last Modified: 03/10/2021 Submitted by: Joseph R Pawlik