Diazotrophs are microbes that can "fix" nitrogen gas in the atmosphere (N2) into biologically available nitrogen (N). N2 fixation is an important process in ecosystems such as the open ocean where fixed N is not already available for primary production. Because the supply of N from diazotrophs in the oceans is so important to how much carbon can be produced in the nutrient-derived ocean, it is also important to understanding how much carbon can be stored. Therefore, the marine N-cycle has a significant impact on our global climate.
However, measuring N2 fixation over such large expanses can be methodologically challenging, particularly as incubating microbes in smaller, more manageable volumes can lead to "bottle effects" when diazotrophs are isolated from their environment. There are also large discrepancies in how much N is fixed when comparing smaller experiments and larger scale models which typically use isotopic measurements. These differences can be on the order of multiple Tg (10^12 grams!) N per year. A new review paper from Frontiers in Marine Science argues that between these exists a gap in N2 fixation knowledge at "fine scales."
However, measuring N2 fixation over such large expanses can be methodologically challenging, particularly as incubating microbes in smaller, more manageable volumes can lead to "bottle effects" when diazotrophs are isolated from their environment. There are also large discrepancies in how much N is fixed when comparing smaller experiments and larger scale models which typically use isotopic measurements. These differences can be on the order of multiple Tg (10^12 grams!) N per year. A new review paper from Frontiers in Marine Science argues that between these exists a gap in N2 fixation knowledge at "fine scales."
The authors define "fine scales" specifically as "submesoscale" meaning 1-10 km distances between sampling stations and measurements at the time scale of days. This includes important ocean processes such as nutrient advection and export which can impact the microbial community and may not be overserved at the scale of most ocean cruises which traverse larger transects and only sample at a station for several hours.
One example of fine scale ocean processes affecting diazotrophs is ocean eddies. In these currents, the authors point out that microbes can be isolated with nutrients but also trapped with predators, They theorize that initially, non-diazotrophs would dominate the community under these conditions, but as nutrients are depleted, diazotrophs would be more competitive. These broader predictions are complicated by the fact that while diazotrophs are a relatively small group of microbes, there are several distinct groups that may behave differently in these marine environments.
The paper concludes with several recommendations for future research but note that technology is limited and not standardized to measure "fine scales" at high-resolution. This will require not only better methods for measuring N2 fixation activity and community structure but also technology to capture physical, chemical, and biological conditions. One hopes these methods will also continue to be used not just in the open ocean, but perhaps in coastal regions as well, where a surprising amount of N2 fixation activity has also been observed. These gaps are ultimately a significant research opportunity for oceanographers, one which will hopefully increase our understanding of the marine N cycle, primary production, and our climate.
One example of fine scale ocean processes affecting diazotrophs is ocean eddies. In these currents, the authors point out that microbes can be isolated with nutrients but also trapped with predators, They theorize that initially, non-diazotrophs would dominate the community under these conditions, but as nutrients are depleted, diazotrophs would be more competitive. These broader predictions are complicated by the fact that while diazotrophs are a relatively small group of microbes, there are several distinct groups that may behave differently in these marine environments.
The paper concludes with several recommendations for future research but note that technology is limited and not standardized to measure "fine scales" at high-resolution. This will require not only better methods for measuring N2 fixation activity and community structure but also technology to capture physical, chemical, and biological conditions. One hopes these methods will also continue to be used not just in the open ocean, but perhaps in coastal regions as well, where a surprising amount of N2 fixation activity has also been observed. These gaps are ultimately a significant research opportunity for oceanographers, one which will hopefully increase our understanding of the marine N cycle, primary production, and our climate.