Fueled by microbiome results from Summer 2018 (Santa Cruz), the team hypothesized that SSW was associated with some kind of bacterial activity at the boundary between sea star respiratory surfaces and seawater. This was borne by data showing a proliferation of some bacterial taxa with facultative and strictly anaerobic metabolisms. Why did these appear, sometimes before SSW had even started? We hypothesized that organic matter may stimulate bacteria at this interface; in doing so, they may use up dissolved oxygen within mm of the animal surfaces, resulting in a suboxic microlayer, which may in turn result in animal stress responses that led to tissue loss and death. To test this hypothesis, Chris, Ian and REU Ryan Besemer (current undergraduate at UNC Wilmington) traveled to the Bodega Bay Marine Laboratory in July 2019 to test the hypothesis that orgaic matter enrichment resulted in SSW and microbiome shifts consistent with wasting.

Staff at the BBML collected 20 Pisaster ochraceus and incubated them for a month prior to the start of the experiment in their high-flowthrough aquariums. We focused the experiments on 3 organic matter substrates: 1) peptone, which is a common ingredient in bacterial culture media; 2) an algal culture (Dunaliella); and 3) particulate matter – most of which is phytoplankon- collected from Bodega Bay. The idea here was to test whether: 1) organic matter generally; 2) phytoplankton; and 3) natural phytoplankton, respectively, affected SSW. Specimens were placed into flow-through baskets in larger flow-through sea tables, and were dosed daily with substrate. We collected specimens for microbiome analyses, bacterial counts, and monitored them for the appearance of lesions. The results showed that all three organic matter substrates resulted in faster wasting than controls.

We were also joined by Thierry Work (USGS Honolulu), a histopathologist and disease biologist, to examine on a cellular level wasting process. In addition to samples from the organic matter experiment, Thierry examined coelomocyte (which are small immune-like cells that circulate within sea stars) density and composition, and we also performed a wound-healing experiments to examine cellular/tissue-level changes in response to physical scarring.

The experiments performed in Summer 2019, together with oxygen depletion experiments performed earlier in 2019 in Ithaca, further emphasized the key role of boundary-layer processes in SSW.

Publications from this Expedition:

Aquino CA, Besemer RM, DeRito CM, Kocian J, Porter IR, Raimondi P, Rede JE, Schiebelhut LM, Sparks JP, Wares JP, Hewson I (2021) “Evidence that microorganisms at the animal-water interface drive sea star wasting disease” Frontiers in Microbiology. DOI: https://www.frontiersin.org/articles/10.3389/fmicb.2020.610009/