Pyrosomes are cylindrical, bioluminescent (glowing) sea creatures that host thousands of microorganisms. When pyrosomes bloom, their population grows at a rapid rate. The impact of these blooms on marine microbial ecosystems remains largely unknown. In the field, there is a gap in research on pyrosome grazing (feeding) preferences and the interactions between pyrosomes and microorganisms. Our study is the first to explore how pyrosomes—specifically, P. atlanticum—interact with microbial communities.

 

During the 2018 pyrosome bloom, we collected pyrosomes and seawater off the Oregon Coast. We analyzed the samples using high-throughput sequencing (technology that rapidly sequences DNA and RNA), flow cytometry (a process that detects cells based on their size and light properties), and microscopy (use of microscopes). The sequencing process involved extracting DNA, identifying sequence variants, and then categorizing and analyzing those variants. We used flow cytometry to measure the number of pigmented cells in the pyrosomes and seawater. Additionally, we used microscopes to capture images of the cells of preserved pyrosome samples.

 

Key Findings:

• Pyrosome microbial communities have a distinct structure and are significantly less diverse in richness and evenness compared to seawater microbial communities.

• Alphaproteobacteria and Gammaproteobacteria dominate the pyrosome microbiome, though only the former is part of the pyrosome core microbiome.

• Pyrosome symbionts are distinct from symbionts of other marine invertebrates.

• Microbial taxa (groups of organisms) can be pyrosome-specific, seawater-specific, or shared (found in pyrosomes and seawater).

• Pyrosomes are living hosts to complex marine biofilm (bacteria that stick to surfaces) communities.

• Bioluminescent bacteria are likely responsible for pyrosomes’ iconic bioluminescence.

• Pyrosomes graze a variety of microbial prey but prefer large eukaryotic phytoplankton.

 

Our findings suggest that microbial symbionts (microbes that live on pyrosomes) and microbial prey (microbes grazed by pyrosomes) are both central to pyrosome biology. We identified several symbionts known to be related to bioluminescent microbes, suggesting that they are the source of pyrosome luminescence. Altogether, these findings are evidence of the complex microbial ecosystems that exist on and around the surface of pyrosomes. From these findings, we deduce that pyrosome blooms can impact marine food webs and the structure of microbial ecosystems.

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Pyrosomes in the ocean

Photo by Dr. Kelly Sutherland