A novel virus lineage is abundant in metaviromes from Dehalococcoides-containing mixed cultures

In the dark, oxygen-free depths of groundwater and river sediments, a specialized group of bacteria works tirelessly to clean up human-made pollution. These microscopic workers, known as Dehalococcoides, consume toxic chlorinated chemicals that have seeped into the earth, breaking them down and rendering them harmless. They are essential for restoring contaminated sites, yet they are also fragile and difficult to study in isolation. Like all living things, these bacteria are not just static cells; they are constantly interacting with a hidden world of genetic material that moves between them. This mobile genetic material can be thought of as a library of instructions that bacteria can swap, borrow, or carry with them, sometimes changing how they function or helping them survive in harsh conditions. Understanding what lives inside and around these bacteria is crucial, because the tools they use to survive and clean our environment are often hidden within these tiny, shifting genetic packages.

Scientists have long known that Dehalococcoides carry a surprising amount of this mobile genetic material, despite having very small genomes. A new study has now focused on a specific, abundant family of these elements, which the researchers have named IME1. By examining samples from a mixed culture of bacteria known as KB-1, where Dehalococcoides thrive, the team discovered that these elements are not just rare curiosities but are actually the dominant form of genetic material found in the viral fractions of the sample. The researchers analyzed twenty-one of these elements, twenty from Dehalococcoides and one from a related bacterium called Dehalogenimonas alkenigignens. They found that each of these elements is a substantial chunk of genetic code, ranging in size from 20,930 to 28,058 base pairs. These elements exist in two forms: sometimes they are stitched directly into the bacteria's own DNA, and other times they float freely as circular loops outside the main genome.

When the team looked closely at the proteins these elements create, they found a highly consistent structure. Every single one of the twenty-one elements contained the same fourteen groups of related proteins, suggesting a shared, ancient blueprint. However, these elements do not look like the typical viruses that scientists are used to seeing. Most bacterial viruses are shaped like tiny tadpoles with a head and a tail, but the proteins encoded by IME1 lack the specific markers that define these tailed viruses. Instead, the genetic code points toward a different shape entirely. The researchers found that one of the key proteins in these elements is very similar to a specific energy-producing machine found in filamentous bacterial viruses, which are long, thin, and flexible. Furthermore, the elements encode several small proteins that are designed to poke through cell membranes, a feature also seen in filamentous viruses and in certain viruses that infect archaea, a different domain of single-celled life.

The evidence for what these elements actually are came from looking at the physical world, not just the genetic code. When the researchers took samples from the KB-1 culture and filtered out the viral particles, they found that these IME1 elements were the most abundant genetic material present. Under an electron microscope, the viral fractions revealed a sea of long, thread-like particles. This visual confirmation, combined with the genetic clues, led the researchers to propose that IME1 represents a completely new lineage of viruses. They describe them as likely being double-stranded, budding viruses that take on a filamentous shape. While the study confirms that these elements are widespread within the Dehalococcoides bacteria and a few related species in the Chlorofexota phylum, they appear to be restricted to this specific group of bacteria and have not yet been found elsewhere. This discovery expands our understanding of the viral world, showing that even in the most specialized environments, nature has evolved unique and abundant forms of viral life that operate differently from the familiar models.