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Nitrogen-fixing bacteria could make farming possible even in Martian soils

New research is investigating the role bacteria could play in future efforts to grow food on planets such as Mars. While such an approach has been shown to boost the growth of clover plants, more work needs to be done to determine exactly how to proceed with off-world farming.

Image credits Kathleen Bergmann.

Nitrogen is a key nutrient for plant growth, one which typically acts as a bottleneck here on Earth. Nitrogen itself cannot be directly assimilated by plants or animals, despite it being available in the atmosphere. Nature has found a workaround to this issue through the formation of symbiotic relationships between the roots of plants and nitrogen-fixing bacteria. These supply essential compounds to the roots that, in turn, feed the bacterial nodules.

Martian soil, or regolith, also lacks essential nutrients, including nitrogen compounds, which would severely limit our ability to grow food in space. In a bid to understand whether we could enrich alien dirt with the aid of Earth-born bacteria, a new study reports on efforts to grow clover in simulated regolith.

Clover for good luck

“Nodule forming bacteria Sinorhizobium meliloti has been shown to nodulate in Martian regolith, significantly enhancing the growth of clover (Melilotus officinalis) in a greenhouse assay. This work increases our understanding of how plant and microbe interactions will help aid efforts to terraform regolith on Mars,” the study reads.

For the study, the team planted clover plants in a man-made regolith substitute that closely resembles that found on Mars. Some of the plants were inoculated with nitrogen-fixing, nodule-forming bacteria, while the others were left to fend for themselves. Sinorhizobium meliloti is a common bacterium on Earth that naturally forms symbiotic relationships with clover plants. Previous research has shown that clover plants can grow in regolith substitutes, the authors explain, but didn’t explore the effects of nitrogen-fixing bacteria on their growth rate.

One of the key findings of the study was that inoculated plants experienced a significantly higher rate of growth than the controls. They recorded 75% more growth in the roots and shoots of these plants compared to clovers which didn’t have access to the bacteria.

Although the bacteria had a positive effect on the plants themselves, the team also reports not seeing any increase in ammonium (NH4) levels in the regolith. In other words, the soil itself did not become enriched in any meaningful way in key nitrogen compounds that other plants could tap into. Furthermore, the symbiotic relationship between bacteria and clovers planted in regolith showed some functional differences compared to those of clovers planted in potting soil.

This suggests that even with the benefit of nitrogen-fixing bacteria on their side, crops sown in alien soils might still develop at different rates to crops on Earth.

All in all, however, the research proves that there is a case to be made for growing crops on alien worlds. Although there are still many unknowns regarding this topic, and even considering a lower yield rate, it remains an attractive proposition. Shuttling materials to outer space remains extremely expensive. It’s also a very long trip to Mars. Both of these factors make it impractical to rely on food transports from Earth to feed a potential colony.

But we are making strides towards offering space explorers greater autonomy. For example, we’re exploring new ways to produce building materials from astronauts’ own bodies and waste. We’re also working on ways to obtain water from regolith.

We’re likely not ready to grow crops in space, however, and the authors note that more research is needed to understand exactly how such a process should be handled. Chief among these, they want to expand their research to other types of crops, and to address possible issues of plant toxicity in regolith.

The paper “Soil fertility interactions with Sinorhizobium-legume symbiosis in a simulated Martian regolith; effects on nitrogen content and plant health” has been published in the journal PLOS ONE.