Nitrous Oxide Cycling Study Points to Microbial Networks Key to Maintaining the Balance of Marine Nutrients

Oceans are critical to stabilizing the world’s climate, absorbing a quarter of all carbon dioxide emissions and capturing 90 percent of the excess heat they generate. By some estimates, the ocean also accounts for up to half the annual emissions of another greenhouse gas – nitrous oxide (N2O), the third most important climate emission after carbon dioxide and methane. With the climate changing amid record high atmospheric levels of all three gases, the importance of the marine nitrogen cycle is ripe for study.

Against this backdrop, researchers from the Bermuda Institute of Ocean Sciences, a unit of the Julie Ann Wrigley Global Futures Laboratory at Arizona State University and the University of Victoria have been examining relationships between marine microbial community structure and potential impacts on water column N2O cycling – with an eye toward examining what impact environmental change might have on those relationships and, potentially, on the N2O budget of the world’s oceans.

A paper published in Nature Communications Biology presents the results of one such study based on six months of field work in Saanich Inlet, a glacial fjord on Canada’s Vancouver Island that lacks oxygen for most of the year. Lead author, Postdoctoral Scientist Brett Jameson, noted that Saanich Inlet is “one of the most well studied marine environments in the world.” With water flow into its main 220-meter-deep basin restricted by a shallow mouth, the deep waters are devoid of oxygen as the microbial decomposition of biological matter consumes and depletes it. The study looked at the inlet’s diverse microbial community, including taxa that can switch to alternative substrates in the absence of oxygen – using nitrogen instead and potentially producing N2O gas.

Through sequencing of DNA in bimonthly water samples taken over six months in 2018, the study found initial evidence that the inlet’s keystone microbial taxa – many of which also underpin nutrient cycles in other oxygen deficient aquatic ecosystems around the world – exert potential control on water column N20 cycling, effectively mediating metabolic interactions between numerous species that produce and consume N2O.

“We study Saanich Inlet because it’s a really accessible environment that’s relatively easy to sample and that sort of mirrors what we see in terms of oxygen gradients in some of our larger open ocean oxygen deficient zones (OZD),” said Jameson, a former undergrad student and teacher at BIOS who performed the research with his PhD advisor, BIOS Marine Nitrogen Cycling Lab Director Damian Grundle. The paper was published as part of Jameson’s PhD thesis, earned at the University of Victoria, Canada.

Although agriculture and the application of nitrogen-rich fertilizers is the main source of increased N2O in Earth’s atmosphere, the potent greenhouse gas is also released from the ocean when oxygen depleted waters are brought in contact with the surface.

“We get a lot of nitrous oxide production right at the borders of those oxygen deficient zones, and, as you go deep where oxygen actually reaches zero…, then we tend to get net N2O consumption,” Jameson said. “We’re really interested in, as a set of processes, what controls the balance between nitrous oxide production and consumption because the balance between those two processes really determines what is going to be emitted to the atmosphere.”

Jameson noted that N2O produced in marine environments is released to the air when it rises to the surface through physical processes – “mixing events” such as coastal upwelling and winter overturning. He added that a key finding of the research was how the inlet’s microbes act on the nitrogen cycle together, rather than individually, functioning as “hubs of ecological interaction.”

In addition to Jameson and Grundle, authors of the published paper (“Network analysis of 16S rRNA sequences suggests microbial keystone taxa contribute to marine N20 cycling”) included Qixing Ji, a former postdoctoral researcher in Grundle’s group, Sheryl A. Murdock, a current postdoctoral researcher in Grundle’s group, and two University of Victoria faculty members: Catherine J. Stevens, and S. Kim Juniper. While pursuing his PhD, Jameson worked in Juniper’s and Grundle’s labs, studying the importance of microbes in driving global nutrient cycling and ecosystem functioning. That set the stage for his further research into “how microbes turn the wheels to allow life in the ocean to go on in perpetuity, how they facilitate the cycles of life and death through nutrient turnover in the ocean.”

The Saanich Inlet study involved collecting and sequencing the genetic material in seawater samples – basically taking an inventory of the microbial community to discover “what’s present and what the metabolic potential is,” Jameson said. Through sequencing, the team found a series of “statistical relationships…sets of correlations and counter-correlations” that link specific groups of organisms to the production and consumption of N2O in the water column.

The next step, he said, involves experiments “to verify that these keystone taxa are actually doing what we think they are doing.” To do this, Jameson and Grundle, along with colleagues from the University of Victoria, are examining and comparing N2O production and consumption by microbial communities in marine sediments in a Bermudian mangrove and on the Continental Shelf off Vancouver Island.

“The next step is to start to see if some of the relationships we’re seeing are robust across different environments by taking this approach to a vastly different ecosystem,” Jameson said, adding a paper presenting results of this follow-up study should be complete in the next few months. “We are already seeing some really interesting consistencies between the two studies in terms of the keystone taxa that are showing up as potentially important. This will really add some robustness to our results if we can demonstrate that this is happening across really diverse ecosystems.”

He added, “This could be very useful information to have, looking at this question: what is going to happen to the nitrous oxide budget as our surface oceans warm and as our oceans continue to lose oxygen…can we make some predictions based on these types of studies and further work down the line? What keystone taxa and what communities might take over as the environment shifts? Then maybe we can gain a little more confidence about what is going to happen in terms of our nitrous oxide budget going forward.”