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War on tiny giants - do viruses impact Pelagibacterales genotype dynamics in the Western English Channel

Marine bacteria

Completed project

Project start: June 2018  |  Project end: May 2020
Principal Investigator: Dr Susan Kimmance

The oceans, and in particular coastal regions, are responsible for about half of all global photosynthesis as marine bacteria and algae capture sunlight to produce metabolites for growth. Through cell death and the leaky nature of cell walls, products of photosynthesis find their way into the water, where they are consumed by other bacteria (known as heterotrophs), releasing the captured carbon dioxide back to the atmosphere.

Perhaps the most important group of heterotrophs is the Pelagibacterales. These tiny cells dominate global oceans (up to 500,000 in every mL of seawater) and are responsible for converting up to 40% of marine photosynthetic products back to atmospheric CO2. As a result, they have a major impact on global carbon cycling and can be considered global bioengineers.

For over a decade scientists have been studying how nutrient availability in the oceans drive Pelagibacterales ecology and evolution, in a bid to build better models of future global carbon cycling under the influences of climate change. As oceans warm and nutrients become less available, Pelagibacterales abundance, and their importance in carbon cycling, is set to increase further. However, not every member of the Pelagibacterales is equal - they differentiate into distinct ecological niches (known as 'ecotypes') with different capacities to take up resources and release important climate-changing gases such as methane and dimethylsulfide. Therefore, understanding which conditions favour which type of Pelagibacterales is of major importance for climate modelling.

Perhaps the only organisms on Earth more important to global carbon cycles than the Pelagibacterales are the viruses that infect them. Virus numbers are staggering - they are by far the most abundant and diverse organisms on Earth. Of these, the vast majority are viruses that infect and kill bacteria (known as 'phages'), and of these, around a quarter are thought to infect Pelagibacterales. Yet, until 2013, the existence of viruses that infect Pelagibacterales was entirely unknown. Virus-induced cell death releases the contents of the bacterial cell into the water column and this soup of dissolved organic matter provides nutrients to surviving cells. Furthermore, bacteria and viruses co-evolve to produce resistance and counter-resistance mechanisms. In some cases, this co-evolution can lead to the emergence of new types of host, resistant to viruses and capable of thriving despite high viral abundance. Therefore, both nutrients and viral predation can influence the abundance and diversity of marine bacteria.

This project is the first attempt to evaluate the impact of the viruses infecting Pelagibacterales on their diversity and abundance over seasonal timescales. The findings will enable us to build better models of future carbon biogeochemistry by accurately incorporating viral predation of the Pelagibacterales in global carbon cycling.


The focus of this project, the Pelagibacterales and their viruses, are the most abundant organisms on Earth. The amount of carbon encapsulated within members of the Pelagibacterales is similar to that of all marine fish, and it is responsible for converting up to 20% of global primary production back to atmospheric carbon dioxide. As oceans warm and become more stratified, further limiting nutrient availability, the dominance of Pelagibacterales is set to increase further. Yet, understanding of how viruses should be incorporated into models such as ERSEM remains unknown. Thus, it is difficult to imagine an organism for study that would that would have greater impact on the accuracy of our models.

Furthermore, development of novel methods during the project to measure viral host range in situ in complex communities will be of significant benefit to those both directly and indirectly involved in developing phage therapy as an alternative means of controlling infection in the face of increasing antimicrobial resistance.