Pyrosomes, or "fire bodies", are enigmatic gelatinous zooplankton that capture the public eye for their massive blooms in coastal ecosystems and stunning constellations of bioluminescence. When they bloom, these pickle-like gelatinous animals sink to the sea floor, alter sea-floor communities, and become a new food source for animals on the sea floor. In spite of their profound effects on marine food webs, we still do not know much about what drives blooms, and because the distribution of pyrosomes is patchy across the globe, the biology of these creatures retains a sense of mystery.

 In 2017, in our backyard off the Oregon coast, pyrosomes bloomed in numbers that far exceeded any sightings in recent memory. Millions of these small pink cylindrical animals fascinated beach-goers as they washed up at the high tide mark. The same creatures overwhelmed fishers’ nets from California to the Gulf of Alaska. While this massive bloom was underway, our labs were at sea collaborating on a different project to understand the food web of this coastal system. With loads of pyrosomes filling our nets, we became inspired to combine the Sutherland lab’s expertise on pyrosome biology and the Thompson lab’s expertise on microbial ecology to ask how pyrosomes impact marine microbial communities and how marine microorganisms contribute to the biology of pyrosomes.

The results published in our recent paper show that pyrosomes interact intimately with marine microbial communities. We found that pyrosomes prey on marine microbes across all domains of life but prefer large eukaryotic phytoplankton above other microbial prey. This result means that pyrosomes can alter the structure of the very foundation of the food web when they bloom, shifting microbial communities towards smaller cells. We also found that pyrosomes were dominated by 1-2 microbial sequences, suggesting that pyrosomes contain dense populations of some symbionts. The functional role of these dominant symbionts in pyrosome biology intrigues us and will be the subject of future work.

In addition to these dominating symbionts, we also discovered several potentially bioluminescent microbial symbionts from three Bacterial genera, which were not present in our background seawater samples. At the time we were publishing, two other studies came out that made this topic really exciting. Berger et al 2021 discovered a Photobacterium in the light organ of pyrosomes – supporting the idea that pyrosome bioluminescence is the product of a bacterial symbiosis and matching one of the several bioluminescent microbes we discovered. However, Tessler et al 2020 discovered that pyrosomes themselves make a bioluminescent enzyme. Together, these three papers set the stage for interesting future work to tease apart the source, and potential host-symbiont collaboration, in creating the pyrosome’s iconic bioluminescence.

Altogether, this recent paper shows that pyrosome biology is closely tied to marine microbes, both as food, but also as symbionts involved in some of the pyrosome's characteristic properties. Plans are now underway to compare the pyrosome microbiome across pyrosome species, and to other related pelagic tunicates, to explore host-symbiont interactions.

Full details can be found in our recent publication: https://rdcu.be/ciGjk