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Nitrogen for Mars Life

January 17, 2016


Does Mars have a nitrogen cycle? Another great story from last year, which microbiologists need to explain better.

We here about carbon all the time–a carbohydrate (sugar) gets oxidized to CO2 (problem) or else fermented and reduced to methane, CH4 (a worse problem). That’s the carbon cycle in a nutshell.

But the nitrogen cycle is far more complicated. The main form of nitrogen in the atmosphere (N2) gets fixed into ammonia (NH3), or as some prefer the ionized form, ammonium ion (NH4+). Ammonia/ammonium gets fixed into our amino acids and DNA bases. In the soil, however, bacteria just see ammonia as another thing to oxidize for energy, analogous to carbohydrates, ammonia gets oxidized. Instead of just one oxidation state, there are several, such as nitrous oxide (N2O), a serious greenhouse gas–one of the worst. Marine dead zones (of which there are several hundred worldwide) put out lots of nitrous oxide. Luckily, still other bacteria oxidize nitrous oxide to nitric oxide (NO), nitrogen dioxide (NO2), nitrite (NO3-), and nitrate (NO4-). Plants can take up nitrate, if they get it before it runs off in groundwater. Along the way still other bacteria pull off the oxygens releasing N2–completing the cycle for still other bacteria to fix the N2 back down into hydrogenated forms that make up living bodies.

If you haven’t fallen asleep by this point, you can see why microbiologists wake up at the news that Curiosity Rover may have found NO2 and nitrate (NO4-) in Martian soil. The implication is that Mars has a nitrogen cycle–perhaps abiotic (not conducted by life) and perhaps limited to small amounts, but still, if various oxidation states of nitrogen exist, they could provide a basis for microbes to get a start on metabolism providing the basis of proteins and chromosomes.

Nitrogen cycles are so complex that even here on Earth, we are still discovering new connections. Holger Daims reports a new find of “comammox” bacteria that complete the entire leg of nitrogen oxidations from ammonia through nitrate. Here they are in this micrograph, labeled yellow by fluorescent DNA probes that base-pair with just the right gene sequence.

We used to think that two or three kinds of bacteria had to get together each contributing one of the conversions in the nitrogen chain. That still happens, no doubt, but there are certain species that can actually run the whole way. Such a self-sufficient microbe might be the likeliest to get the first foothold on Mars.

One Comment
  1. January 17, 2016 10:35 pm

    Fascinating–good news and bad–that’s science, I guess.

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