Microbial Ecosystems - Pelagic Marine Viruses

7 important questions on Microbial Ecosystems - Pelagic Marine Viruses

What is the estimated ratio of prokaryotic cells to viruses in seawater, and why is this ratio significant in marine ecosystems?

The estimated ratio of prokaryotic cells to viruses in seawater is about 1:10, meaning there are approximately 10 times more viruses than prokaryotic cells. This ratio is significant as it highlights the high abundance of viruses in marine ecosystems and their potential ecological impact on prokaryotic populations.

How do viruses impact bacterial mortality in seawater, and what are the consequences of these interactions on marine food webs?

Bacteriophages (viruses that infect bacteria) contribute significantly to bacterial mortality in seawater. It is estimated that 5-50% of bacteria in seawater are killed by bacteriophages daily. This interaction, along with grazing by protists, contributes to bacterial death. Unlike grazing, viral lysis releases cytoplasm as dissolved organic matter (DOM), amounting to approximately 3 gigatons of carbon per year. This decreases the transfer of carbon to higher trophic levels, affecting marine food webs.

Describe the Autolykiviridae, and explain their significance in marine virology.

Autolykiviridae is a newly discovered lineage of non-tailed double-stranded DNA bacteriophages. They have a more-or-less icosahedral morphology and exhibit a broad host range, infecting both Bacteria and Archaea. Their discovery has filled a major gap in our understanding of environmental virology. They were identified through plaque assays using marine Vibrio cells as hosts, and subsequent metagenomic analyses revealed their presence in 13 bacterial and archaeal phyla, including abundant marine groups like Alphaproteobacteria and Thaumarchaeota.
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What role do cyanophages play in marine ecosystems, specifically regarding the transfer of photosynthesis genes among strains of Synechococcus and Prochlorococcus?

Cyanophages are phages that can transfer certain photosynthesis genes among strains of Synechococcus and Prochlorococcus, which are major primary producers in the ocean. When cyanophages infect these cyanobacteria, they may incorporate host genes that encode key components of oxygenic photosynthesis. This gene transfer influences host metabolism, allowing the host to better adapt to changing environmental conditions. For example, changes in light intensity or quality can be accommodated, ultimately leading to increased production of both the host and the phage.

Explain the concept of Auxiliary Metabolic Genes (AMGs) in the context of viral infection, and provide an example of their role in marine environments.

AMGs are genes present in viral genomes that, when expressed during viral infection, alter host metabolism toward pathways that maximize the production of new viral particles. In marine environments, cyanophages, which infect cyanobacteria like Synechococcus and Prochlorococcus, may carry AMGs that influence photosynthetic electron transport and redirect energy from carbon fixation to pathways like the pentose phosphate pathway. This alteration enhances the production of ribose 5-phosphate, a nucleotide precursor crucial for virus replication.

How do lysogenic bacteriophages contribute to genetic transfer in the marine environment, and what is the significance of lysogeny when host numbers are low?

Lysogenic bacteriophages can integrate into the genomes of their bacterial hosts, conferring new properties to the host cell. They can also facilitate the transfer of bacterial genes through transduction, a process of horizontal gene transfer. When host numbers are low, lysogeny may be favored as it allows the virus to survive as a prophage within the host until host numbers increase again. This suggests that lysogeny serves as a survival strategy for viruses during periods of low host availability.

What role does viral metagenomics play in understanding viral diversity, and what are the challenges associated with identifying hosts infected by specific viruses?

Viral metagenomics involves sequencing DNA isolated from virus particles to study the diversity of viruses in a particular environment. It has revealed immense viral diversity, with approximately 75% of gene sequences showing no similarity to known genes. Challenges in identifying hosts infected by specific viruses include the lack of universal marker genes in viruses and the need for improved methods, such as matching CRISPR spacers or identifying patterns of co-occurrence between viral populations and putative host bacteria. Despite these challenges, viral metagenomics is expanding our knowledge of the virosphere and viral diversity.

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