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Nitrogen Fixation


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Fritz Haber

Fritz Haber

I’m haunted by one of the stories in the latest episode of Radiolab, can’t get it out of my head. Like everyone else, I love Radiolab and often sprinkle stories I learned from the show into cocktail party conversation (do I go to nerdy cocktail parties or do I make cocktail parties nerdy?), but the Bad Show was especially gripping, in particular the story of Fritz Haber. Haber was a German chemist working in the early 20th century, but his name is well known in Chemical Engineering departments (he is, after all, one of The Most Popular Chemical Engineers Ever.) I’ve even looked him up in wikipedia recently, focusing on the details of the chemical process he invented and never scrolling down to learn more about his life. It’s in this scrolling down that Radiolab is amazing, bringing his complicated and tragic story to life.

Haber invented a process that sustains one third of the population on earth: the production of ammonia fertilizer from nitrogen gas. Nitrogen is required for life, a crucial component of both DNA and proteins, but even though nitrogen is the most abundant gas in our atmosphere, our cells can’t use it in its atmospheric form, relying on other processes to “fix” that nitrogen into a biologically available form. A few microorganisms possess nitrogenase enzymes that can perform this chemical reaction, and about half of the nitrogen in your body comes from these microorganisms. The other half comes from the Haber-Bosch process (Carl Bosch scaled up Haber’s process to large-scale industrial levels).

Azolla caroliniana by Kurt Stüber

Biological nitrogen fixation and utilization in agriculture is a fascinating, multi-layered symbiosis. Just as we live in a kind of symbiosis with our domesticated crop plants, depending on their growth for our survival, so too are many species dependent on “domesticated” nitrogen-fixing microbes that live in or near them and provide the nutrients they need. One of the best-known examples is the rhizobia bacteria that form nitrogen-fixing nodules inside of the roots of legumes. Crop rotation alternating between legumes and crops that can’t form symbiosis with rhizobia keeps the soil nitrogen-rich and able to support plants that need an external source of fixed nitrogen. Similarly, the small fern Azolla forms a symbiosis with the nitrogen-fixing cyanobacteria Anabaena. Azolla are then used as nitrogen-rich fertilizer for growing rice.

The Haber-Bosch process uses high pressure and temperatures to do what the microbial nitrogenase enzymes can do inside the cell, producing fertilizer at an immense scale and making industrial agriculture possible. Artificial fertilizers have prevented perhaps billions of people from dying of starvation, but they also contribute to significant environmental degradation that now threatens many many lives as well. The Haber process alone contributes significantly to greenhouse gas production: first, hydrogen is produced through steam-reformation of natural gas, which is then combined with nitrogen gas from the atmosphere at incredibly high temperature. The industrial Haber-Bosch process uses about 3-5% of the world’s total natural gas production for the production of hydrogen, as well as approximately 1-2% of the total yearly energy supply. Artificial fertilizers also cause significant pollution due to agricultural run-off of excess nitrogen into bodies of water.

An interesting robo-ecological installation that was part of the “synth-ethic” art and synthetic biology exhibition in Vienna last year explores some of the complex symbioses of mechanized industrial agriculture. Andy Gracie’s Autoinducer_Ph-1 is a self-sustaining system where a robotic arm will scoop Azolla in symbiosis with Anabaena onto growing rice plants in response to nitrogen conditions monitored by electronic sensors. This piece explores how these organisms evolved to live together, and how they can further evolve in symbiosis with artificial, bioengineered or electro-mechanical systems. Perhaps such an evolution will enable high-yield agriculture at lower impact, without the need for artificial ammonia fertilizer.

Autoinducer_Ph-1 by Andy Gracie

But the ethics of industrial agriculture aren’t the only thing that make Fritz Haber’s story complicated. Ammonia is not only good fertilizer, but its high-energy bonds also make an excellent explosive. Chemistry has other uses in war, however, and Fritz Haber was one of the leaders in the development of chemical warfare during World War I, even going to the front to oversee the deployment of chlorine gas against the soldiers in the trenches. I won’t spoil the tragedy of the story because Radiolab does it better, but Haber was proud of his country, and is quoted as saying that “During peace time a scientist belongs to the World, but during war time he belongs to his country.” These are stories that have particular resonance today, as bioengineers debate the ethics of using their research directly for defense-related projects and the potential for research on deadly diseases to be misused by those who would want to wage biological warfare. It’s difficult to think about the ways that technologies can be used for both good and evil, it’s impossible by definition to think about the unexpected consequences of any technology, and stories like Haber’s are difficult to hear but necessary (seriously, go listen to it now) to reminds us that it’s important to keep ethics as part of any discussion of science and technology, not just as a discussion of possible risks and downstream consequences after it’s done.

Christina Agapakis About the Author: Christina Agapakis is a biological designer who blogs about biology, engineering, engineering biology, and biologically inspired engineering. Follow on Twitter @thisischristina.

The views expressed are those of the author and are not necessarily those of Scientific American.



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  1. 1. Binkley 6:34 pm 01/14/2012

    Good article, I’d like to point out an error in the second paragraph however,

    “… even though nitrogen is the most abundant gas in our atmosphere, our cells can only use it in its atmospheric form …”

    I believe you meant to say ‘atomic’ form, as the atmospheric form, N2 cannot be used? Although ‘atomic’ is not quite correct either, as it never exists as simply N but as NOx compounds.

    Link to this
  2. 2. Christina Agapakis in reply to Christina Agapakis 8:05 pm 01/14/2012

    Yup, definitely a typo, thanks for pointing that out! I meant to say that most cells can’t use it in atmospheric form (N2) but need it either as ammonia or as other reduced molecules.

    Link to this

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