Plants will grow in lunar regolith, but they don’t like it

Image of plants grown in a plastic sample dish
Enlarge / The plants grown in lunar soil (right) aren’t nearly as happy as those grown in a soil meant to simulate it (left).
Tyler Jones, UF/IFAS

As anyone who has read or watched The Expanse or The Martian knows, growing plants in space has some big advantages. Plants can contribute to the maintenance of a healthy atmosphere, as they recycle water and provide some variety to diets. While they can be grown hydroponically, the process requires a significant amount of water, which might be in short supply. So for missions that will land on a body like the Moon or Mars, growing plants in the local soil might be a better solution.

But local soils on these bodies don’t look like the ones we find on Earth, which have a complicated mix of minerals, organic compounds, and microbial life. Can plants adjust to these differences? A group of researchers at the University of Florida—Anna-Lisa Paul, Stephen Elardo, and Robert Ferl—decided to find out, and they used some incredibly rare material: lunar soil returned by the Apollo missions.

In the mix

The lunar soil exists in a form called regolith, which is basically loose, dusty material created by the constant bombardment of lunar rocks by micrometeorites. When the first samples were returned during the Apollo era, studies of the interactions of this regolith with living things focused on the fear of pathogens that could pose a danger to life on Earth. As a result, plants and seeds were briefly exposed to lunar soil and then tested to see if this exposure altered their growth. There were no attempts to grow anything in the soil.

NASA has since developed an Earth-made material, called JSC-1A, that is meant to simulate lunar soil. But there are some significant differences between it and lunar soil. These include chemical differences, with lunar regolith containing higher amounts of titanium and some trace minerals than JSC-1A. Earth’s oxidizing environment also creates some differences in the chemical state of some of the metals present, including that of iron, a key component of many enzymes, such as those involved in photosynthesis.

Finally, there are some physical differences between the material and the soil. The rapid melting and cooling caused by micrometeorite impacts on the regolith creates small globs of glassy material. JSC-1A uses volcanic glasses to approximate this process, but there are still physical differences.

So the researchers decided to try working with the real thing, using JSC-1A as a control. And with the help of the Johnson Space Center staff, they obtained three different lunar samples returned by Apollo 11, Apollo 12, and Apollo 17. The samples all came from regions with a volcanic origin but differed in their age, with Apollo 11’s material having the longest exposure on the surface and Apollo 17’s having the shortest.

Growth and stress

Given the small size of their lunar regolith samples, the researchers constructed a system in which small sample wells were filled with 900 milligrams of soil and fed water from underneath. For plants, the researchers chose Arabidopsis, a small flowering plant related to mustard that has been used in biology research. Using a well-understood plant allowed the researchers to track which genes were active in different materials.

About a week after seeds were placed in the soil, things sprouted as normal, so the differences in the soil weren’t significant enough to interfere with this process. Several days after, the researchers removed all but one of the plants from each sample well, giving them a look at the developing roots, which were stunted compared to those of seedlings grown in JSC-1A.

On average, growth on all lunar soils was slower and more erratic than growth in JSC-1A. The plants took longer to unfold leaves, they spread to a smaller diameter, they didn’t grow as tall, and they had altered pigmentation. But these phenotypes were variable; some plants grown in lunar soil were clearly defective, while others looked normal, albeit a bit smaller. Problems generally correlated with the age of the regolith, with plants in the Apollo 17 sample (the youngest) doing the best.

The researchers found that the plants grown in lunar soil activated many of the genes involved with stress responses, including those involved with phosphate starvation, metal toxicity, and reactive oxygen problems (the last potentially due to the differences in iron between the soils). The genes accounted for over 70 percent of the genes that were activated compared to plants grown in JSC-1A. The rest were mostly involved with nutrient metabolism.

The researchers also divided the plants into three categories: defective, small, and near-normal. The latter two groups had only 100–150 altered genes compared to the JSC-1A controls, with most of the genes involved in responses to drought and salt stress. But the dwarfs saw over 1,000 genes with different activity levels.

What this tells us

There are a couple of ways to look at these results. The first is that the study represented a serious stress test. While the samples were watered using a nutrient solution, they were dumped into lunar regolith as-is—no mixing with organic material and no microbial growth that could sequester some of the metallic toxins before the plants encountered them. So the experimental setup made things harder than they needed to be.

On the flip side, everything that might better prepare the lunar soil for plant growth would take time and mass, two things that can be in very short supply on space missions. Viewed from that angle, the decision to use lunar soil doesn’t save as much mass as it might, undercutting one of its advantages.

The work suggests that a few chemical treatments might help leach away some of the heavy metals and convert the iron into an oxidation state more similar to that seen in typical Earth soils. But it’s clear that while some plants can tolerate the harsh conditions produced by lunar regolith, they’re not happy about it. That means we probably can’t expect to use this system to grow anything edible, given that plants seem to struggle to grow at all.

The most promising possibility is to use this information as a springboard to study in more detail what is causing the plants to struggle and then start engineering and selecting for strains that can tolerate the regolith better. But if the study tells us nothing else, it’s that we don’t have a very good analog for lunar soil, and we certainly don’t have enough regolith samples to make these sorts of experiments possible. Working on materials that could enable these kinds of studies is the most pressing need going forward.

Communications Biology, 2022. DOI: 10.1038/s42003-022-03334-8  (About DOIs).

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