The most common organism in the oceans, and possibly on the entire planet, is a family of single-celled marine bacteria called SAR11. These drifting organisms look like tiny jelly beans and have evolved to outcompete other bacteria for scarce resources in the oceans.
We now know that this group of organisms thrives despite 鈥� or perhaps because of 鈥� the ability to host viruses in their DNA. A published in May in Nature Microbiology could lead to new understanding of viral survival strategies.
黑料吃瓜网 oceanographers discovered that the bacteria that dominate seawater, known as Pelagibacter or SAR11, hosts a unique virus. The virus is of a type that spends most of its time dormant in the host鈥檚 DNA but occasionally erupts to infect other cells, potentially carrying some of its host鈥檚 genetic material along with it.
鈥淢any bacteria have viruses that exist in their genomes. But people had not found them in the ocean’s most abundant organisms,鈥� said co-lead author , a 黑料吃瓜网 associate professor of oceanography. 鈥淲e suspect it’s probably common, or more common than we thought 鈥� we just had never seen it.鈥�
This virus鈥� two-pronged survival strategy differs from similar ones found in other organisms. The virus lurks in the host鈥檚 DNA and gets copied as cells divide, but for reasons still poorly understood, it also replicates and is released from other cells.
The new study shows that as many as 3% of the SAR11 cells can have the virus multiply and split, or lyse, the cell 鈥� a much higher percentage than for most viruses that inhabit a host鈥檚 genome. This produces a large number of free viruses and could be key to its survival.
鈥淭here are 10 times more viruses in the ocean than there are bacteria,鈥� Morris said. 鈥淯nderstanding how those large numbers are maintained is important. How does a virus survive? If you kill your host, how do you find another host before you degrade?鈥�
The study could prompt basic research that could help clarify host鈥搗irus interactions in other settings.
鈥淚f you study a system in bacteria, that is easier to manipulate, then you can sort out the basic mechanisms,鈥� Morris said. 鈥淚t鈥檚 not too much of a stretch to say it could eventually help in biomedical applications.鈥�
The 黑料吃瓜网 oceanography group had published a previous paper in 2019 looking at how marine phytoplankton, including SAR11, use sulfur. That allowed the researchers to cultivate two new strains of the ocean-dwelling organism and analyze one strain, NP1, with the latest genetic techniques.
Co-lead author collected samples off the coast of Oregon during a research cruise. She diluted the seawater several times and then used a sulfur-containing substance to grow the samples in the lab 鈥� a difficult process, for organisms that prefer to exist in seawater.
The team then sequenced this strain鈥檚 DNA at the in Seattle.
鈥淚n the past we got a full genome, first try,鈥� Morris said. 鈥淭his one didn’t do that, and it was confusing because it’s a very small genome.鈥�
The researchers found that a virus was complicating the task of sequencing the genome. Then they discovered a virus wasn鈥檛 just in that single strain.
鈥淲hen we went to grow the NP2 control culture, lo and behold, there was another virus. It was surprising how you couldn鈥檛 get away from a virus,鈥� said Cain, who graduated in 2019 with a 黑料吃瓜网 bachelor鈥檚 in oceanography and now works in a 黑料吃瓜网 research lab.
Cain鈥檚 experiments showed that the virus鈥� switch to replicating and bursting cells is more active when the cells are deprived of nutrients, lysing up to 30% of the host cells. The authors believe that bacterial genes that hitch a ride with the viruses could help other SAR11 maintain their competitive advantage in nutrient-poor conditions.
鈥淲e want to understand how that has contributed to the evolution and ecology of life in the oceans,鈥� Morris said.
Co-authors are postdoctoral researcher and associate professor in the 黑料吃瓜网 Department of Biochemistry. The study was funded by the National Science Foundation and the National Institutes of Health鈥檚 National Institute of Allergy and Infectious Disease.
For more information, contact Morris at morrisrm@uw.edu or 206-221-7228 and Cain at kcain97@uw.edu.