The more we learn about giant viruses, the more questions we have. Unlike most viruses, these giant strains are so large that they could even be mistaken for bacteria, and the size and complexity of their genomes exceed expectations.
Giant viruses were first discovered only in this century, but numerous similar discoveries since then have challenged our long-standing assumptions about what viruses really are, including whether they should really be considered living things.
Some of these giant viruses seem to even create their own genes; others have a genetic code that we have never encountered before.
It seems that we are increasingly finding types with features only visible in living cells, and a startling new discovery made by scientists at Virginia Tech has revealed other surprising genetic similarities between giant viruses and cellular life.
“In terms of their genomic repertoire, they have much more in common than we might expect,” explains microbiologist Frank Aylward.
In a new study of viral diversity, scientists are sifting through publicly available metagenomic databases containing a variety of genetic code from which they have assembled putative genomes for 501 different types of giant viruses in the proposed order of large DNA nucleocytoplasmic viruses (NCLDVs). Mainly from the aquatic environment (where algae contamination occurs).
In addition to looking for anticipated genes for processes such as capsid construction and viral infectivity, the team found that giant viruses carry a huge variety of genes involved in aspects of cellular metabolism, including processes such as nutrient uptake, light harvesting, and nitrogen metabolism.
The researchers say metabolic genes have been found in viruses before, but this is something different.
Previous research by NCLDV has identified genes thought to be acquired in cell life through lateral gene transfer – the movement of genetic material between organisms, as opposed to how it is passed from parent to offspring. In a viral context, this suggests that viruses can accidentally acquire genes from infected hosts.
The team found evolutionary lines of viral metabolic genes that went much deeper, suggesting a long-standing relationship between pathogens and hosts whose symbiotic significance we cannot yet fully disclose.
“This implies that viruses have had these genes for millions of years, even billions of years, and they are virus-specific metabolic genes,” explains Aylward.
'Once viruses infect a cell, we can no longer think of it as our own autonomous entity. Viruses rearrange fundamental aspects of cellular physiology after infection. '
In other words, giant viruses and their ancient ancestors could live together with cellular organisms for eons, not only multiplying in the cells of living beings, but also exerting an invisible effect on their metabolic processes all this time.
Like many other discoveries that scientists are making about giant viruses, it requires a double approach, if not a direct paradigm shift.
“Viruses have historically been viewed as accessories for cell life, and as such their effect on biogeochemical cycles has been largely viewed through the prism of their effect on host mortality, rather than through any direct metabolic activity,” the authors write in their paper.
“The large number of cellular metabolic genes encoded by genomes that we are uncovering in this study highlights an alternative view in which virus-specific enzymes play a direct role in shaping viral cell physiology.”
Then, scientists want to conduct experimental work, investigating how host metabolites can be affected by giant viruses and viral genes, supposedly carried over to alter metabolic processes.
Regardless of what answers we find, given that we are dealing with giant viruses, you can bet there will be many new unknowns.
“They're just a storehouse of mysteries,” says microbiologist Mohammad Moniruzzaman. “They are like a big forest, and you are standing in front of the forest and you do not know what is in it.”
The results are presented in Nature Communication.
Sources: Photo: (Chuan Xiao and Yuejiao Xian / University of Texas at El Paso)
