Billions of years ago, Earth was a different place. Intense volcanism, meteorite impacts, and toxic gases like methane made it hostile to modern life.


Source: Mariiantoniietta

The PSB Chromatium okenii

Most notably, it lacked the oxygen-rich environment that makes our planet as luscious and green as it is today. Within it, photosynthetic bacteria, such as purple (PSB) and green (GSB) sulphur bacteria thrived, playing a vital role in producing organic matter through photosynthesis.

In a recent work published on, ’Viruses of sulfur oxidizing phototrophs encode genes for pigment, carbon, and sulfur metabolisms’, Dr. William Gilhooly III, professor of Earth Sciences at the IUPUI School of Science, and a team of researchers made a trip to northeast Washington to study PSB and GSB within oxygen deficient lakes, where these bacteria mostly reside. What they discovered could rewrite the history of anoxic bacteria and play a key role in understanding what’s affecting modern lake ecology.

Sunlight and hydrogen sulphide

Traditionally, geologists believed purple bacteria need more sunlight, and struggle to tolerate excess hydrogen sulphide, directing them more toward the surface. Green bacteria are the opposite, needing less sunlight, but can tolerate more hydrogen sulphide, pushing them below purple bacteria.

However, the bacteria in these Washington lakes didn’t live according to these rules. Sometimes they were mixed, other times only one bacterium dominated. So, the group tested a theory posed by lead-author Alice Bosco-Santos of the University of Lausanne, Switzerland: What if the bacteria had a viral infection? Sure enough, they did.

“All had genes in their metagenome that come from viruses,” Gilhooly said. “The viruses are going into the bacteria, tinkering with the genetic code, and getting the bacteria to make things for them. It changes things because in the past we’d see a bunch of PSB and they’re producing lots of chlorophyll, which gives them the purple colour.

”Now, knowing that there can be infections, it could be the viruses that are driving them to do these kinds of things. This means that in addition to environment and climate, hidden biological factors may influence the activity of living things.”

Viral influence

The successful findings, while reshaping the way scientists look at bacteria such as purple and green bacteria, raise more questions for the team to discover. How long have viruses changed these bacteria? What other bacteria might be affected by viral infection? What is the impact, positive or negative, of infection?

More importantly, how does it translate to our daily lives? Gilhooly’s belief is that the viral infections might be present in more than just PSB. Problematic bacterium like cyanobacteria could be under viral command as well. Thus, knowing what’s at play is the first step in solving issues relating to water quality. A recently funded $1.5 million NASA Exobiology grant, based on these findings, will help them continue exploring these questions.

“It could have important relevance for modern lakes, or even Eagle Creek Reservoir (Indianapolis), what are the things controlling it?” Gilhooly said. “If you take northeast Indiana for example, lakes up there, it’s known that cyanobacteria blooms are a huge nuisance and can be toxic to animals. If PSB and GSB are affected by viral infection, then more than likely there are other aquatic bacteria that are being affected by viral infections.”

Contributing researchers include Dr. Bosco-Santos, Drs. Poppy Hesketh-Best, Sofia Garcia and Cynthia Silveira of the University of Miami, and Drs. Molly O’Beirne and Josef Werne of the University of Pittsburgh.