Why would aliens design their von Neumann replicators to use nitrogen?

Question

I want to contrive a scenario in which 23rd century humanity faces a dire crisis due to an extraterrestrial von Neumann ecology that rapidly consumes Nitrogen as a part of its life cycle. So rapidly in fact that it threatens to deplete Titan's Nitrogen atmosphere in under a few hundred years, potentially starving the millions of orbiting habitats around Earth and Mars which rely on nitrogen imported from the outer Solar System to grow their food. Scientists fear the tiny machines may even adapt to living inside a human digestive system if allowed to spread to human-occupied territory, sucking the Nitrogen out of the food processed by our digestive systems before our bodies have a chance to use it. Effectively, we would all die of malnutrition, starvation, or both if these things are not stopped.

Now the biggest question is, who would build this kind of thing and why? Clearly the answer to the first half is an alien species whose biology is not reliant on Nitrogen in any significant way and who cannot or simply failed to conceive of a species like humanity that is extremely reliant on it. But the second half is the most important part. Why? What advantages does creating an ecology of Nitrogen-eating machines provide you?

My best guess is that nitrogen is extremely abundant in the interstellar medium and a species that is not reliant on it might see no problems in taking advantage of this to help their probes spread throughout interstellar space, because they simply do not see Nitrogen as having any value relative to its abundance. However I am not sure if this is a good enough answer to stand up to scientific rigor. Are there any other possible justifications for this?

Answer 1

Nitrogen has a peculiar property - it is a near-inert when in gaseous form (diatomic form - N2) and quite reactive when in elemental form (single atomic N). In its gaseous form, nitrogen forms roughly 78% of Earth's atmosphere and provides stability by preserving material far longer than it would last in, say, an oxidative environment. In its elemental form, it readily forms compounds with carbon, oxygen and hydrogen. The covalent C=N bond is one of the most abundant bonds in organic chemistry and makes up one of the basic building blocks of life - amino acids and proteins. Also nitrogen compounds like nitrates and ammonia are fundamental to plant growth.

The conversion from diatomic N gas to nitrogen compounds is not easily done at all and, on Earth, is mostly performed by specialized bacteria (with some amount by lightning strikes and now increasingly, human industrial activity). These nitrogen compounds play their part in the food cycle and then get decomposed into ammonia, nitrates and finally diatomic nitrogen gas. This is the vitally important nitrogen cycle for us.

It is possible the aliens that invented these nitrogen-guzzling critters do not have this kind of nitrogen cycle on their planet. Maybe their environment never really needed or evolved this kind of nitrogen-fixing bacteria. Some research posits nitrogen came to be on our planet thanks to the mass CO2 sink provided by the oceans, leaving behind massive amounts of ammonia that decomposed naturally into nitrogen. Maybe that never really happened on this alien planet and their biochemistry doesn't really need nitrogen in any form.

Nitrogen is the seventh most abundant element in the milky way and apart from neon is the only inert gas in the top ten. And neon does not have this very interesting elemental nitrogen reactivity property. Maybe these aliens thought some nitrogen-based solution was the ultimate fuel/power/building-material idea and simply did not consider that there could be organisms out there using it the way we do. Maybe the self-replicators are protein-based forms, maybe they have a nitrogen cycle of their own, maybe that's how they make themselves up. These bots completely disrupt the nitrogen cycle for us, perhaps locking up the nitrogen in some unusable form, perhaps attacking the soil bacteria that converts basically not-very-useful nitrogen gas to nitrogen derivatives, perhaps converting out existing necessary amino acids/proteins into un-usable (for us) forms.

Interesting corollary, all of you reading out there: what if we make self-replicating bots that use up silica and silica-based materials? Would our bots threaten silica-based alien life-forms somewhere out there?