Isolation and characterization of a virus infecting the freshwater algae Chrysochromulina parva
The major result of our study was the discovery of a new virus, CpV-BQ1, which infects the freshwater algae Chrysochromulina parva. Until now, the only known freshwater algal viruses were viruses that infect Chlorella-like algae that were once symbionts of protozoans and metazoans, so our work provides evidence that viruses infect diverse freshwater algae.
The CpV virion is a 150 nm-diameter, naked icosahedron with a 485 kb dsDNA genome that potentially encodes hundreds of genes. Its sensitivity to chloroform suggests that CpV, like chloroviruses, has a lipid component, yet its taxonomy is still unresolved. Different marker gene sequences from CpV suggest close relationships with species of either Mimiviridae or Phycodnaviridae, related families of large (giant!) dsDNA viruses. CpV replicates in cytoplasmic virus factories leading to the release of around 100 particles upon cell lysis. In nature, the virus can be found at high abundances coexisting with its host suggesting a particularly interesting ecology; that the host is a major primary producer in freshwaters around the world highlights the importance of viruses in ecosystem processes.
Close up TEM image of virions within C. parva cytoplasm. The virion in the upper right side of the image shows the encapsidation of tiny human heads within the maturing particles. The scale bar at the bottom of the image is 100 nm.
This study was one of the most fulfilling research projects I have ever conducted. It was not only wonderful that so many members of my lab team including both undergrads and grad students were able to collaborate and participate in the entire publication process, but this project was also a blast because it was initiated based on an educated guess that turned out to be true. Our discovery of a new virus may have been a lucky fluke, but it made me feel (momentarily) quite clever anyway.
We decided to try to isolate a virus that infected the prymnesiophyte algae Chrysochromulina parva because of a couple of key observations. My lab’s earlier cell-free, PCR-based work on algal virus diversity in freshwater environments revealed a single gene sequence that was very closely related to genes from viruses that infect the marine prymnesiophyte algae Chrysochromulina brevifilum. Also, historical records of algae observed in Lake Ontario and the other North American Great Lakes showed that Chrysochromulina parva, then classified as a chrysophyte algae (thankfully I remembered this from my undergrad algal systematics class and recognized the older classification), was prevalent and one of only a few species of chrysophytes that were commonly observed in these lakes. Because our unidentified environmental virus sequence was most closely related to marine Chrysochromulina viruses, and there is some evidence for virus-host coevolution among algal viruses, it seemed reasonable to grow cultures of C. parva and screen Lake Ontario water samples for lytic activity against them. Needless to say, it was absolutely thrilling when a water sample from Lake Ontario repeatedly lysed our C. parva cultures. Who knew killing algae could be so much fun!
Another thrilling moment was seeing the first TEM images of the virions forming inside the cells, and exploding out of recently lysed cells; the fact that the cell’s ultrastructure was so well preserved in our thin sections was the cherry on top. This has all felt like a research slam dunk, but this story is even more intriguing as there are huge holes in our knowledge of these viruses and their hosts. For example, the gene sequence that we eventually obtained from CpV is not the same as the environmental sequence that prompted the work in the first place. Clearly, we have only just begun to tap into the diversity of algal viruses in freshwater environments. Now that we can easily grow this virus and its host in our lab, we have a clear path to develop this pair as a model system for detailed studies of algal virus physiology, ecology, and evolution. We’re excited about our future efforts and are certain that our next steps with CpV and C. parva are going to bring more amazing discoveries and surprises.
About the authors