Long Read Viromics Time Series from the Western English Channel
One often hears that the rainforests are the 'lungs of the Earth', producing the oxygen that we breathe through photosynthesis and drawing down atmospheric carbon dioxide. However, perhaps less well-known is the fact that the oceans, and in particular coastal regions, are responsible for about half of all global photosynthesis as marine bacteria and algae (known as phototrophs) 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 we 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. Based on their extraordinary abundance and diversity, J.B.S. Haldane once quipped that 'The Creator would appear as endowed with a passion for stars, on the one hand, and for beetles on the other'. In comparison, the Creator's zeal for viruses would make stars and beetles appear to be a side-project performed with perfunctory indifference. Virus numbers are staggering - they are by far the most abundant and diverse organisms on Earth. If one assumes that there are 10 trillion galaxies in the universe and each one is similar to our Milky Way and contains 100 billion stars, then the oceans are populated with a million viruses for every star in the universe. 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! Similar to interactions between lions and wildebeest in the Serengeti, predation is rather unfortunate for the host, but provides benefits to the scavengers of the ecosystem. Phage-induced cell death releases the contents of the host cell into the water column and this soup of dissolved organic matter provides nutrients to surviving cells. Furthermore, like the arms-race between lions and wildebeest (bigger claws, horns etc.), 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.