[adrian_b]

Plants feed on oxidized compounds, i.e. water, carbon dioxide and dinitrogen, plus solar energy. Because carbon dioxide and dinitrogen are very volatile they must be taken from atmosphere.

Reduced organic compounds, like hydrocarbons, are not food for plants, but for fungi or similar organisms, which, like humans, need only dioxygen from the atmosphere.

There already are genetically-modified fungi that can produce the complete proteins required by humans (i.e. whey proteins or egg-white proteins) when fed with cheap carbohydrates and ammonia or a simple amino-acid. There are also fungi that can feed on hydrocarbons.

Creating genetically-modified fungi able to feed on hydrocarbons and produce glucose and proteins for human consumption is not far from the already existing technology. With a serious effort in this direction, this could be solved in a decade or so. The glucose and proteins produced by fungi would be used not only for direct consumption, but also for feeding other microbial cultures that are needed for producing vitamins and essential fatty acids, using the techniques that are already in use today for this purpose.

[thaumasiotes]

Do plants take nitrogen from the atmosphere? There's plenty of it available, but my understanding was that molecular nitrogen likes being molecular nitrogen and is difficult to react into a useful form.

If plants took nitrogen from the atmosphere, there would be no issue of fixing nitrogen in the ground. But instead, that's a huge issue. We also provide nitrogen fertilizer, which is another way of getting it into the ground. Why would we do that?

And similarly, I was pretty sure that most plants got their hydrogen and oxygen from water, which they draw out of the ground with special, purpose-dedicated organs called "roots". Their stylized interaction with the air is carbon dioxide in, molecular oxygen out. That's a source of carbon and maybe a little oxygen. The atmosphere doesn't even contain any significant amount of hydrogen.

[adrian_b]

Plants in the strictest sense, i.e. the terrestrial descendants of green algae, or even in some wider senses, e.g. all green plants or all eukaryotes that have chloroplasts whose origin is in a primary symbiosis or even all eukaryotes with chloroplasts, cannot take nitrogen from the atmosphere by themselves.

Nevertheless, some plants have symbionts that are various kinds of bacteria and which can take nitrogen from the air. The most important of these plants are the legumes, i.e. beans, peas, lentils and all their close relatives.

The ancestor of the chloroplasts, which are the parts of the plant cells that capture the solar energy, were free-living blue-green algae a.k.a. cyanobacteria, which had been able to take nitrogen from the atmosphere for billions of years.

However during the integration of the chloroplasts into the nucleate cells of the plants, when they have lost their ability to live independently, the chloroplasts have been simplified and among the lost features was the ability to take nitrogen from air. Thus the plants do not have this ability and they have remained dependent for nitrogen on bacteria that either live independently or in symbiosis with plants.

Before humans have started to convert nitrogen from the air into ammonia, for fertilizers and other applications, all organic nitrogen came ultimately also from the air, through the blue-green algae or through other kinds of bacteria.

The nitrogen content of natural rocks is negligible. All the rocks with nitrogen, e.g. saltpeter, come from the decomposition of bodies or excretions of living beings.

In all planets that are not cold enough for ammonia to become a liquid or an ice, the nitrogen is almost entirely in the atmosphere.

[thaumasiotes]

> The nitrogen content of natural rocks is negligible. All the rocks with nitrogen, e.g. saltpeter, come from the decomposition of bodies or excretions of living beings.

That's fair, but it seems similar to noting that the oxygen content of natural air is negligible. It's not really relevant to where things get their oxygen/nitrogen.

[AngryData]

The plants certainly don't. However there are certain plants that symbiotically harbor nitrogen fixating bacteria. Most notably legumes and the rhizobia bacteria that live inside root nodules. Common legume plants are alfalfa, peanuts, lentils, chickpeas, and bean species.

[thaumasiotes]

> Because carbon dioxide and dinitrogen are very volatile they must be taken from atmosphere.

I don't follow the logic. These two ideas are definitely compatible - oxygen is very volatile, and it's present in significant quantities in the atmosphere, and therefore can be (and is) taken from the atmosphere.

But they seem to me to be in tension rather than supporting each other. Oxygen is present in the atmosphere, but _because_ it is very volatile, it's constantly reacting with stuff on the ground, which is a process that tends to eliminate it from the atmosphere. What's the reasoning that suggests that the volatility of carbon dioxide means it has to be taken from the atmosphere?