Analysis of viral signatures from Marine Microorganisms by Single-cell Amplified Genomes and Metagenomic Assembled Genomes

Abstract

70% of the world’s surface is covered by oceans; its impact on the global carbon cycle, climate change, and acid-base biochemistry remain crucial to our understanding of the natural world. The oceans act as important buffers against climate change, absorbing 25% of anthropogenic carbon and over 90% of rising temperatures. 90% of the ocean’s biomass is composed of marine microorganisms and their impact on global systems, particularly in the face of anthropogenic climate change, remains an active area of research. Marine microorganisms are critical in the energy cycle and are the foundation for marine life. Warmer waters have led to increasingly stratified and nutrient-depleted water masses at the ocean surface, favouring low-nutrient microbial specialists. One group of these, known as the SAR11 clade, comprise up to 40% of the microbial community and are estimated to convert up to 20% of all global primary production back to atmospheric CO2 as well as being an important biological source of methane. Increasing SAR11 abundance in warming oceans and concomitant increases in remineralisation of CO 2 and methane may create a positive feedback loop for global warming. A potential brake on the influence of SAR11 carbon remineralisation is their associated viruses, which are predicted to lyse up to 20% of cellular biomass daily. These viruses also encode an enormous array of genetic diversity and its relationship with both physical and biological factors is key to understanding the marine biome’s population dynamics. Predation of cells by viruses is a major driver of carbon export to the deep ocean, but our knowledge of these interactions in the SAR11 clade is limited, in part due to the paucity of host-virus model systems for this clade. However, studying these microorganisms remains challenging since only a few SAR11 strains have been isolated and cultured for in vitro experimentation. Alternative study methods include obtaining genomes via metagenomics studies and Single-cell Amplified Genomes (SAGs). Therefore, the goal of this project is to extract and explore SAR11 host and associated phage genomes from metagenomic and SAG data. Here, I present a study of 451 SAGs collected from the Tara Ocean expeditions and twelve prokaryotic metagenomic samples from the Bermuda Atlantic Time Series (BATS). Overall, I summarise the difficulty of obtaining contiguous and high-quality SAR11 genomes from metagenomic data. I conclude possible reasons why existing bioinformatics tools are ineffective at recovering such sequences and suggest improvements through long-read technology. Through SAG data, I identified and evaluated genomic regions associated with phage defence to improve our understanding of SAR11-associated viral dynamics in the oceans. Additionally, I characterised two previously undescribed clades of SAR11, both phylogenetically and ecologically. Our 451 SAGS contained fewer phage sequences than SAGs from other taxa, indicating the SAR11 clade does not conform to the expected statement that 20% of all marine microorganisms are infected at any given time. Lastly, I confirmed that a hypervariable region identified as a putative site for host-virus Red Queen dynamics is present within all clades of SAR11, and concluded these regions are enriched in genes related to cell wall biosynthesis. I hypothesise that these genes are related to phage defence, altering the cell wall receptors and preventing recognition of a host by SAR11 phages, therefore resisting infection. These findings together increase our understanding of additional host-phage interactions SAR11 has and impact current models when calculating SAR11 phage carbon-sequestering via the viral shunt.