Scientists have gained key new insights into how marine microbes and viruses interact on a global scale, revealing complicated secrets of Earth's oceans.
Ocean microbes are not only vital to the health of our oceans, but they also produce half of the oxygen we breathe and drive chemical reactions and energy transfers that influence major ecological processes, the University of Arizona reported. The work is part of the Tara Oceans Expedition, which has spent the past decade using research vessels to take samples across 180,000 miles of ocean.
Just like humans, these tiny marine microbes are susceptible to viral infections that have the potential to harm the ecological processes they influence. One group of ocean viruses, for example, invade the algae that control photosynthetic process that replenish the world's oxygen.
A recent study looked at viral genome data to gain a better understanding of how marine viral communities maintain their impressive regional diversity, which could help scientists create models of how these virus-microbe interactions influence the Earth's ecosystems.
"We established a means to study viral populations within more complex communities and found that surface ocean viruses were passively transported on currents and that population abundances were structured by local environmental conditions," said Matthew Sullivan, associate professor in the Department of Ecology and Evolutionary Biology and a member of the BIO5 Institute. The work was completed with the assistance of a Gordon and Betty Moore Foundation Investigator grant, a highly prestigious award given to researchers focused on environmental science and conservation.
The researchers looked at double-stranded DNA viral genomic sequence data (viromes), of the entire viral community from 43 Tara Oceans expedition samples representing diverse environments. They focused of the viruses geological diversity in hopes of gaining information that sheds light on previous observations suggesting viral diversity at any given site was as high as what is observed globally. To gain this valuable insight, the team proposed a "seed-bank" hypothesis, which suggests high local genetic diversity comes about by drawing from variations seen in common and limited global gene pools. Local communities are made up of viruses that influence the surrounding environmental conditions, which have an effect on microbial hosts that in turn can change the structure of the viral communities. The phenomenon means the communities serve as a "bank" for nearby ecosystems, and can be transferred through the movement of waves. To identify global community patterns, the researchers started by looking at viral particles and comparing their morphological features.
"This is the low resolution way to do things - viruses that appear identical may have completely different genomes," Sullivan said. "The fact that all viruses don't share a single common gene calls for some clever approaches to investigating viral diversity."
The team then cataloged the viral populations by common proteins between them in a process dubbed "protein clustering." This technique allowed them to pinpoint core genes these viruses had in common. In the last step, the scientists looked at how the viral communities were distributed, and found the direction they moved in was closely linked to ocean currents.
"Ocean virus-microbe interactions have a huge impact on global biogeochemistry," Sullivan said. "As they destroy microbial cells, they change the forms of nutrients available to other, larger organisms in ocean ecosystems. This recycling of nutrients through viral lysis is an important pathway that regulates how the oceanic ecosystem functions. Viral infections simultaneously reduce the amount of nutrients and materials available to larger organisms by killing microbial cells, but also stimulate microbial activity through the release of organic matter and nutrients, which provides increased biomass available for larger organisms including fish."
The findings are an important breakthrough in the methodology used to look at microbial communities in the world's oceans, coming close to allowing for a quantitative analysis.
"Up until recently, the methods used to study virus-microbe interactions were often qualitative," Sullivan said. "With this study, we have made great quantitative advances. The goal now is to determine how our quantitative estimates can be used to build predictive models."
The findings were published in a recent edition of the journal Science.
