An international research team, co-led by the University of Tennessee, Knoxville, and the University of Maryland, has published findings detailing how viral infection of blue-green algae in the Sargasso Sea enhances ecosystem productivity and contributes to increased oxygen levels in the water. The study, published in Nature Communications, identifies a significant ecological role for marine viruses in nutrient cycling and ocean oxygenation, particularly within the ocean's food web.
Research Overview
The research investigates the behavior of marine viruses within the oxygen-rich waters of the subtropical Atlantic Ocean. Historically, the ecological relevance of marine viruses, which are typically microscopic, was not fully understood. However, advances in the late 1980s revealed a significantly higher abundance of viruses in seawater than previously recognized, with most infecting microorganisms such as bacteria and algae. These microorganisms form the base of the ocean food web and contribute to approximately half of the planet's oxygen generation.
Methodology and Key Findings
The study was conducted during a National Science Foundation research cruise to the Sargasso Sea in October 2019. The team performed around-the-clock RNA sequencing surveys of the microbiology at the Bermuda Atlantic Time-series Study (BATS) site, which has been collecting oceanographic data for nearly four decades.
Researchers collected samples from a meters-thick band of oxygen in the Sargasso Sea, an area dominated by Prochlorococcus cyanobacteria. By sequencing community RNA, the team assessed viral activity and observed widespread viral infections within Prochlorococcus populations. The rate of virus infection in this oxygen-rich band was approximately four times higher than in surrounding ocean areas.
Mechanism of Oxygen Generation
The study demonstrates that the infection of Prochlorococcus cyanobacteria by viruses leads to the lysis (breaking open) of these cells, releasing organic matter and nutrients into the water. This released organic matter is subsequently taken up by other bacteria. These bacteria then respire carbon and release nitrogen in the form of ammonium. This freed nitrogen appears to stimulate further photosynthesis and growth of Prochlorococcus cells, thereby contributing to the generation and maintenance of the oxygen band.
This activity is suggested to drive a meters-wide band of oxygenated water, approximately 50 meters below the ocean surface, which is present for several months annually.
Connection to the "Viral Shunt" Model
The paper establishes a direct connection between the 'viral shunt' — a model first described by Steven Wilhelm and Curtis Suttle in 1999 — and the microbial loop within the ocean's food web. The viral shunt model proposes that marine viruses lyse microorganism cells, releasing carbon and nutrients back into the water. This process is hypothesized to increase nutrient availability for marine phytoplankton, which support krill, fish, and larger marine life, thereby underpinning global fisheries and aquaculture. The current study provided direct observation of this viral shunt in action.
Expert Perspectives
Steven Wilhelm, a professor at the University of Tennessee's Department of Microbiology and a senior author of the study, noted that viral activity can stimulate growth and production within microbial processes. Joshua S. Weitz, a biology professor from the University of Maryland, indicated that the analysis of large-scale data on cellular and viral activity, including infection status and viral abundances, helped identify the system-scale impact of viral infections. Weitz stated that viral infection appears to enhance the recycling of carbon and nutrients by other microbes, thereby driving productivity and clarifying the link between viral activity and ecosystem functioning below the surface.
Broader Implications
The research contributes to an increasing body of evidence indicating that viruses are central to ecosystem function, including carbon storage in deep oceans. Understanding these microscopic mechanisms is considered relevant for monitoring and responding to environmental changes on a global scale.
Research Team and Support
The lead author of the study was Naomi Gilbert (PhD ’22). Daniel Muratore was also identified as a lead biologist. Steven Wilhelm and Joshua S. Weitz served as senior authors and co-led the research. Other University of Tennessee authors included Professor Alison Buchan, Assistant Professor Gary LeCleir, Professor Jennifer DeBruyn, and former students Helena Pound and Shelby Cagle.
Collaborating institutions included the Georgia Institute of Technology, Ohio State University, and Technion Institute of Technology in Israel. RNA sequencing and additional analyses for the study were conducted at the University of Tennessee.
The study received funding primarily from a National Science Foundation Collaborative Research grant, with additional support from the Simons Foundation and other organizations.