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Understanding microbial competition for nitrogen


Interactions among microorganisms account for nitrite accumulation just below the sunlit zone, with implications for oceanic carbon and nitrogen cycling.

Press Contact

Lauren Hinkel
Email: hinkell@mit.edu


About 200 meters down, just below the ocean’s sunlit zone, resides a layer of nitrite, an intermediate compound in the nitrogen cycle. Scientists have found this robust feature, called the primary nitrite maximum, throughout the world’s oxygenated oceans. While several individual hypotheses have been put forward, none have convincingly explained this marine signature until now.


Regulating the biological pump


Somewhere along the line, nitrite has been accumulating at the base of the sunlit zone, which has implications for ocean biogeochemistry. “Broadly, we’re trying to understand what controls the remineralization of organic matter in the ocean. It’s that remineralization that is responsible for forming the biological pump, which is the extra storage of carbon in the ocean due to biological activity,” says Zakem. It’s this strong influence that nitrogen has on the global carbon cycle that captures Follows’ interest. “Growth of phytoplankton on nitrate is called ‘new production’ and that balances the amount that’s sinking out of the surface and controls how much carbon is stored in the ocean. Growth of phytoplankton on ammonium is called recycled production, which does not increase ocean carbon storage,” Follows says. “So we wish to understand what controls the rates of supply and relative consumption of these different nitrogen species.”

Battle for nitrogen 

The primary nitrite maximum resides between two groups of microorganisms in most of the world’s oceans. Above it in the sunlit zone are the phytoplankton, and in the primary nitrite maximum and slightly below that rest an abundance of nitrifying microbes in an area with high rates of nitrification. Researchers classify these microbes into two groups based on their preferred nitrogen source: the ammonium oxidizing organisms (AOO) and nitrite oxidizing organisms (NOO). In high latitudes like the Earth’s subpolar regions, nitrite accumulates in the surface sunlit zone as well as deeper.




Comparing the fundamental physiologies of the nitrifiers revealed that differences in metabolisms and cell size could account for the nitrite build-up. The researchers found that the second step of the nitrification process that’s carried out by the nitrite oxidizers requires more nitrogen for the same amount of biomass being created by these organisms, meaning that the ammonia oxidizers can do more with less, and that there are fewer nitrite oxidizers than the ammonia oxidizers. The nitrite oxidizing microbes also have a higher surface to volume constraint than the smaller and ubiquitous ammonium oxidizing microbes, making nitrogen uptake more difficult. “This is an alternative explanation for why nitrite should accumulate,” Zakem says. “We have two reasons that point in the same direction. We can’t distinguish which one it is, but all of the observations are consistent with either of these two or some combination of both being the control.”

The researchers were also able use a global climate model to reproduce an accumulation of nitrite in the sunlit zone of places like subpolar regions, where phytoplankton are limited by another resource other than nitrogen like light or iron. Here, nitrifiers can co-exist with phytoplankton since there’s more nitrogen available to them. Additionally, the deep mixed layer in the water can draw resources away from the phytoplankton, giving the nitrifiers a better chance at survival in the surface.

“There’s this long standing hypothesis that the nitrifiers were inhibited by light and that’s why they only exist at the subsurface,” Zakem says. “We’re saying that maybe we have a more fundamental explanation: that this light inhibition does exist because we’ve observed it, but that’s a consequence of long-term exclusion from the surface.”

Thinking bigger

“This study pulled together theory, numerical simulations, and observations to tease apart and provide a simple quantitative and mechanistic description for some phenomena that were mysterious in the ocean,” Follows says. “That helps us tease apart the nitrogen cycle, which has an impact on the carbon cycle. It's also opened up the box for using these kind of tools to address other questions in the microbial oceanography.” He notes that the fact that these microbes are shunting ammonium into nitrate near the sunlit zone complicates the story of carbon storage in the ocean.

Two researchers who were not involved with the study, Karen Casciotti, associate professor in the Stanford University Department of Earth System Science, and Angela Landolfi, a scientist in the marine biogeochemical modeling department at the GEOMAR Helmholtz Centre for Ocean Research Kiel, agree. “This study is of great significance as it provides evidence of how organisms’ individual traits affect competitive interactions among microbial populations and provide a direct control on nutrients' distribution in the ocean,” says Landolfi. “In essence Zakem et al., provide a better understanding of the link between different levels of complexity from individual- to community up to environmental level, providing a mechanistic framework to predict changes in community composition and their biogeochemical impact under climatic changes,” says Landolfi.

This research was funded by the Simons Foundation’s Simons Collaboration on Ocean Processes and Ecology, the Gordon and Betty Moore Foundation, and the National Science Foundation.

Topics: Research, Microbiology, Chemistry, Biology, Oceanography and ocean engineering, Ocean science, Microbes, Carbon, Ecology, EAPS, School of Science

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