Deep beneath the soil of a Massachusetts forest, an international team of researchers has uncovered a multitude of mysterious, gigantic viruses of unprecedented ecological diversity.
These colossal parasites are comparable in size to some bacteria and have genomes larger than those of more complex organisms.
Studying viruses in soils
Investigations of viruses in freshwater environments have primarily dominated the landscape of virus research. These include algae and amoeba-infecting Mimiviruses. But there’s a whole different story beneath our feet.
Recent genomic sequencing has pointed to the presence of diverse and abundant viruses in the sediments and soils of the Earth. Scientists believe that our soils host an astounding 97% of all viral particles on our planet.
This extraordinary research was led by virologist Matthias Fischer of the Max Planck Institute for Medical Research in Germany. His team delved into the soils of Harvard Forest, a sprawling area west of Boston covering approximately 16 square kilometers. Here, they discovered an astonishing variety of giant viruses.
The team managed to visualize these enormous viruses using electron microscopy, a technique previously used only for sequencing.
The images they captured revealed an unexpected diversity of forms. While the scientists expected to find 20-sided icosahedral shapes, the discovery of myriad modifications amazed them.
These included tails, altered points, multi-layered or channeled structures, and even viruses with long tubular appendages, which the team playfully named “Gorgon” morphology.
What’s even more fascinating, the researchers found many of these viral particles covered with almost hairlike projections of varying lengths, thicknesses, densities, and shapes.
The viruses exhibited tentacle-like “electron-dense inner tube” appendages, icosahedral protein shells, and a variety of other shapes with unknown biological purposes. Some even resembled ornate perfume bottles from centuries past, leading the researchers to dub them “Flacon.”
Focus of the giant soil virus study
The study has been posted to bioRxiv and is currently awaiting peer review. It focuses on the microbes’ morphology rather than their packed genomes, as had been the focus of past studies.
The researchers used a transmission electron microscope to create detailed images of the giant viruses and nearby virus-like particles.
One particular group of viruses, which they named the “Gorgon” viruses after the snake-haired women of Greek mythology, displayed between eight to eleven tubular, tentacle-like appendages.
The researchers estimated each of these appendages to be about 500-650 nanometers long and 30-65 nanometers wide. The researchers suspect that these appendages might be hollow, and some may have evolved to release the virus’s infectious genetic payload.
Implications of the study
Speaking to Live Science, Fischer said, “What we found is a whole new diversity of shapes that we have never seen before. I would bet that many of those, if not the majority, are completely new and first sightings of viruses that we have never seen before.”
As of yet, these gargantuan viruses have not shown any threat to humans. These viruses mostly infect other single-celled microbes, such as amoebas and paramecia. But the implications of these discoveries, and the broadened understanding of the role of these giant viruses in ecological and evolutionary contexts, are undoubtedly profound.
This research opens the door to a whole new world of virology, with countless secrets about these giant viruses waiting to be unearthed.
More about soil viruses
The soil beneath our feet is a complex and diverse ecosystem teeming with countless microorganisms, including bacteria, fungi, and yes, viruses. These viruses, known as soil viruses or soil phages, play a crucial role in the soil ecosystem, influencing microbial populations, nutrient cycles, and overall soil health.
Just as viruses infect human and animal cells, soil viruses, the majority of which are bacteriophages (or simply phages), infect bacterial cells. They can drastically influence bacterial population dynamics by lysing or breaking apart bacterial cells.
This process results in the release of nutrients back into the soil. The viral predation involved can alter the composition of microbial communities and contribute to biodiversity in the soil.
Horizontal gene transfer
In addition to their role in regulating bacterial populations and nutrient cycling, soil viruses also facilitate horizontal gene transfer among bacteria. In this process, one organism transfers genetic material to another organism that is not its offspring. The transfer can lead to rapid genetic diversification and adaptation in bacterial populations.
Challenging to study
One of the challenges of studying soil viruses is their sheer diversity. Estimates suggest that quadrillions of virus particles reside in the Earth’s biosphere, with many inhabiting the soil.
This extreme variety, combined with the fact that many soil viruses don’t easily fit into existing taxonomic categories, makes it difficult for researchers to identify and classify them.
Moreover, the inability to grow viruses in pure culture, unlike bacteria or fungi, complicates their study. This is where metagenomics comes in. This approach involves sequencing the collective DNA from soil samples. It has allowed researchers to uncover a vast diversity of previously unknown soil viruses.
Recent research into soil viruses has also begun to reveal their potential applications. For example, certain phages are being explored as potential alternatives to antibiotics due to their ability to kill specific bacterial strains. Phages could also be used in agriculture to control bacterial pathogens or to promote the growth of beneficial bacteria.
The study of soil viruses is a rapidly expanding field. It is reshaping our understanding of the microbial world and revealing new possibilities for medical and agricultural applications. However, much remains to be discovered about the diversity of viruses in soil, their interactions with other microorganisms, and their impacts on the soil ecosystem and beyond.