So how do boundary layer processes scale to the geographic extent of SSWD in 2013-2014? And other future ideas.

It’s been a very busy start to 2021… notwithstanding a global pandemic, an administrative pivot, and the star of teaching, the team’s also been busy since the publication of our work on sea star wasting disease and the boundary layer oxygen diffusion limitation hypothesis. For the last few weeks, news media outlets from AAAS/Sciencemag through to news sites in Italy have covered the article’s findings, many of which were re-hashes of Cornell’s College of Agriculture and Life Sciences news release, but some were based on independent interviews. It’s gratifying to see many years of work communicated to the wider public!

Since the publication we’ve also been thinking about new ways that others could look at this phenomenon in the future. Our funding to work on sea star wasting disease has run out, but we are continuing to work on virology of densoviruses, and will be excited to see others take up this important topic! Some of these ideas may fit well with a summer REU project; others may take years to resolve and may be more suited to graduate study. I hope that by putting these ideas out there it may foster new science and new perspectives. And watch out soon for other work on this topic from the lab – there are still a few stories to tell even if our research is over!

  1. What was different in 2013 – 2014 from an environmental standpoint?

In Aquino et al. (2021) we provide support for the hypothesis that sea star wasting is driven by boundary-layer processes which stem from primary producer-derived dissolved organic matter. We removed much of the discussion about what was different in 2013 – 2014 compared to years before and after because of space limitations (though please stay tuned for an opinion piece on this topic). However, it would be very interesting to look at historical trends in chlorophyll a in regions affected. One of the strongest tools to do this is satellite-derived chlorophyll a / remote sensing. We’ve pulled some satellite images from NASA (MODIS-Aqua) to compare years at affected sites and it looks like there was something different about coastal chlorophyll a at the time, but it’s difficult to know whether this was the case without quantitative information – so if you’re into remote sensing – this could be a very interesting project!

The same could be said for upwelling and physical oceanography. We roughly plotted upwelling indices through data pulled from the Columbia River DART program to compare 2013-2014 with other years (as it relates to onset of wasting) and it does look like there is some correspondence between upwelling that occurred several months before onset, but the relationship is far from clear. It would be really interesting for physical oceanographers to see if models of stratification, upwelling, light intensity, water and air temperature, and river flow from nearby catchments resolve – or not. Since wasting appears to be a result a combination of these – and its synergistic effects on chlorophyll-a – it is likely that a full picture of wasting disease could come from such an analysis- and enable it’s prediction in the future.

2. Lability and composition of DOM from sea star- and algal-derived DOM

We posited in Aquino et al. (2021) that DOM released from adjacent stars and from primary producers may fuel bacterial production in the diffusive boundary layer; however, the identity and lability of these compounds is not known. It would be really exciting, for example, to determine the composition of DOM released by sea stars on decomposition and exuded from both phytoplankton and macroalgae, and then look at its lability to microorganisms inhabiting near-sea star heterotrophic microorganisms. We only did this for a few cultures – Altermonads and a Ruegeria relative – but this could be extended to other isolates and the natural assemblage using radiotracer approaches.

3. Secretome of the diffusive boundary layer microbiota

While we found evidence for oxygen depletion within the sea star diffusive boundary layer, we cannot exclude that disease process was the result simply of having so many bacteria or highly active bacteria near their tissues; bacteria often produce enzymes like nucleases and proteases to break down complex molecules before they can be taken up. A large amount of these enzymes may cause irritation/inflammation to outer tissues of sea stars. It would be really interesting in our opinion to look at the diversity of enzymes produced by bacteria inhabiting the diffusive boundary layer, and if there is a single or class of enzymes which dominate this pool, see whether they cause damage to animal tissues.

4. Micro-scale oxygen, bacterial respiration and abundance

Patchiness in aquatic microbiology is brought about by hotspots of OM availability – this has been well documented in bacterioplankton and especially in OM-rich environments, like algal blooms. At the same time, oxygen also can show micro-scale patchiness in OM aggregates (e.g. marine snow and zooplankton carcasses, etc). We tried to measure actual oxygen concentrations on sea star surfaces; the problem was the sensors (microsensors) are fragile and easily broken, and sea stars do actually move quite a lot. But it would be really interesting to look at both the composition, abundance and activity of microorganisms on the microscale, and oxygen concentrations within the diffusive boundary layer, using something like an optode approach. We hope to do the latter in our new work on sea cucumbers, but we suspect it will be really obvious on sea stars.

5. Macroalgae and sea stars

While Aquino et al. (2021) hypothesized that boundary layer oxygen diffusion limitation was a consequence of phytoplankton exudates, it is equally plausible that macroalgal-excreted DOM may result in the same phenomenon. Yet, in conversations with phycologists, there have been almost no estimates of DOM exudation and it’s lability over seasonal cycles in affected areas. It would be really interesting to know, for example, if the seasonal breakdown of macroalgal biomass coincides with sea star wasting, and whether long-term trends in macroalgal density correspond with the onset of mass wasting in 2013-2014.

So there remain many important questions even if the boundary layer oxygen diffusion limitation model is correct. Oceans are very complex habitats, and there are far more parameters and other things to look at in aquatic disease events than can be shouldered on a single funding grant. We hope these ideas provide fodder for future investigation!

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