Life is no cake walk at the ocean floor, where carbon is scarce and light nonexistent. At least near deep ocean vents, mineral-rich water bubbles up from magma beneath the crust, providing both heat and a source of energy. In these alien environments, lithotrophs– bacteria that eat minerals instead of organic carbon- have staked out a niche by evolving some creative metabolic strategies.
But minerals are a poor source of energy compared to organic matter. Lithotrophs are slow-growing critters, easily outcompeted when carbon is abundant. They need all the help they can get. And it turns out, they might very well get help… from an unlikely source. A study published last week in Science Express reports how viruses may be helping deep marine bacteria eat sulfur.
Viruses are everywhere- in soils, skies, oceans, plants and animals, even deep beneath the ocean floor. In spite of their ubiquity, it’s not well-known how viruses influence marine bacteria and the nutrient cycles they drive.
To study the impact of viruses on sulfur-eating bacteria, a team of scientists led by Dr. Karthik Anatharaman at the University of Michigan collected samples from five different hydrothermal vents : four in the Western Pacific Ocean and one in the Gulf of California. They used shotgun metagenomic sequencing to look at the genomes of the bacteria and viruses present in these environments.
The researchers found five different virus “types” that carried two genes involved in sulfur metabolism. Known as rdsrA and rdsrC, these genes encode different pieces of dissimilatory sulfite reductase, an enzyme that breaks down elemental sulfur.
How and why did viruses come to acquire these genes? Not technically considered alive, viruses don’t really need enzymes because they don’t perform metabolism on their own. A virus’s sole purpose is to infect a host with its genetic code so that it can turn the host cell into a virus factory. But during viral replication, pieces of DNA can be accidentally snipped out of the host and integrated into the viral genome. The scientists found bacteria in the same samples carrying both the rdsrA and rdsrC genes. It’s likely that accidental incorporation of host DNA is how viruses ended up with sulfur genes in the first place.
While acquiring sulfur genes may have been a complete accident, viruses appear to have been actively maintaining those genes for a long time.
When the scientists compared rdsrA and rdsrC from viral and bacterial genomes, they found something interesting: completely different DNA sequences surrounding the genes. If viruses had acquired sulfur genes during recent replication errors, some of the nearby DNA would probably have come along for the ride as well. That completely different DNA sequences surround the sulfur metabolism genes in bacteria and viruses suggests viruses have been passing along rdsr genes, generation to generation, for a very long time. In other words, sulfur metabolism genes are being maintained in viruses by natural selection.
Why would deep sea viruses bother to maintain sulfur metabolism genes they don’t use? It turns out these viruses may actually have a use for sulfur genes, albeit an indirect one: helping out their hosts. By carrying genes that bacteria can make use of, viruses may assist their hosts in acquiring energy. The ability of sulfur-eating bacteria to acquire energy is ultimately limited by how quickly they can decode their sulfur genes to build enzymes that can do the work. But if a bacteria were to contain viruses with extra copies of the genes it needs, that could help speed the process along. In essence, when a sulfur virus infects a sulfur bacteria, it donates genes that its host can use.
What’s a virus to gain from all of this? Remember, a virus’s sole purpose is replication. Staying in your host’s good graces has its perks. An “infected” bacteria that can acquire energy faster than its neighbors may be able to grow and reproduce faster- leading to more infected offspring, and more viruses. By supplementing host metabolism, viruses may help ensure their continued survival.
This study sheds light on a potentially widespread, mutually beneficial ecological interaction between bacteria and viruses. Lithotrophs in the deep ocean play an important role in the global sulfur cycle and have done so for billions of years. By giving an adaptive advantage to sulfur-eaters and helping them survive, it’s possible viruses have played an equally important role in the geochemical evolution of our planet.
Anantharaman, K., Duhaime, M., Breier, J., Wendt, K., Toner, B., & Dick, G. (2014). Sulfur Oxidation Genes in Diverse Deep-Sea Viruses Science DOI: 10.1126/science.1252229