Award: OCE-1537111

Intellectual Merit "Sea star wasting disease" describes a suite of abnormal symptoms that have affected starfish since at least 1898 and led to their widespread disappearance from coastlines. In 2013 ? 2014, millions on starfish from Alaska to Baja California were afflicted by this condition and since that time some species, including the Sunflower Star, have almost disappeared from some places (e.g. the Salish Sea) entirely. The cause of sea star wasting disease was initially believed to be a virus called the Sea Star associated Densovirus (SSaDV). We hypothesized that sea star wasting disease resulted in changes in the starfish microbiome, starting with infection by this virus, followed by induction of starfish self-destruction (called apoptosis) and ending with the rapid growth of bacteria that decompose the dying animals. Work very early in this project failed to confirm a connection between this virus and sea star wasting, so we instead searched again for possible causes of the condition. This project found that sea star wasting is a consequence of a complex interaction between microorganisms (primarily bacteria) living near the animals surface and phytoplankton-derived material. Starfish are surrounded by a layer of relatively stagnant water that is rich in nutrients. Because bacteria thrive on these nutrients, this layer generally contains a much larger number of these organism than surrounding waters. When these bacteria consume nutrients, they deplete oxygen within this layer. These bacteria also readily consume nutrients provided by algae, so when there is a lot of phytoplankton around, these bacteria thrive and drive the layer above animals to become suboxic. Starfish breathe through their skin. Hence, when this layer becomes suboxic they cannot breathe as well, and so many deleterious processes are triggered, including a form of self-destruction known as apoptosis. Ultimately, through this project, we have linked the occurrence of sea star wasting disease to the timing of algal blooms, their proximity to decomposing carcasses of adjacent starfish afflicted by the disease, and warmer water conditions (which further reduce oxygen availability). We showed this phenomenon to occur during experiments where we depleted available oxygen in aquaria, where we added sugars and other analogs of algal exudates, and by looking at the types of bacteria that occur prior to, at the time of, and after the animals start to show sea star wasting disease signs. Furthermore, we found a significant relationship between the surface roughness ? which dictates the width of this layer ? and the extent of sea star wasting disease in natural populations. Finally, we looked at elemental signatures of starfish from 2013 ? 2014 and found strong evidence that animals that died from sea star wasting had tissues enriched with an isotope that indicated the prevalence of anaerobic conditions. Hence, we rejected our initial hypothesis (that viral infection triggered apoptosis and bacteria proliferated on the dying animal) and accepted the alternate hypothesis that phytoplankton stimulated bacteria, which led to suboxic conditions that triggered apoptosis. Furthermore, we found that numerous viruses became very abundant after wasting began, so it is likely that our initial observation of SSaDV associated with sea star wasting was wrong because it is a symptom, not cause, of the condition. The finding that sea star wasting disease is likely not caused by a microorganism (i.e. it is not transmissible or pathogenic), but may relate to the density of organisms within a population, has important consequences for biological oceanography. For example, other organisms that breathe through their skin (including sponges and corals) may face challenges in the future through this phenomenon. Broader Impacts This project trained two graduate students, seven undergraduate researchers, and a high school student, including 4 under-represented minority students. The project results have been communicated to the public through engagement with popular media outlets, through public talks at museums, and through the PIs website/blog. In addition, we have engaged the public through several social media accounts to report significant findings. All 12 publications to date have been published in open-access peer-reviewed scientific journals, and data has been deposited at publically available databases. Last Modified: 09/01/2020 Submitted by: Ian Hewson