The planet we live on is called the Green Planet for a pretty good reason. Our lush rainforests and sprawling grasslands make it no question that we live in a vegetation-heavy planet. One could even say we’re the only planet in this neighborhood with vegetation! What if I told you that scientists are studying bacteria to see if this vegetation can be achieved elsewhere as well?
Oh yeah. That’s green. Rainforest in Mexico, source.
Plants need water, sunlight, and carbon dioxide to grow into strong and healthy lifeforms that give our planet its vibrancy. The soil that they grow in play a huge role as well. Some plants have limited access to nitrogen, which is necessary to make the chlorophyll they need for photosynthesis, and is what makes the plants green in the first place. Without chlorophyll, plants can’t photosynthesize and get energy from sunlight, resulting in withering and decay. A particular type of plant called legumes decided to lean into the power of friendship to help fix this problem.
Legumes are plants in the pea family, and are responsible for providing us with soybeans, peas, and lentils. They have a special type of relationship with rhizobacteria, called mutualistic relationship, when two different organisms benefit from living together. In the legume-rhizobacteria relationship, the bacteria make atmospheric nitrogen into plant-accessible nitrogen-based ammonia. Legume roots provide special bumps called nodules for the rhizobacteria to latch onto, and share nutrients so the rhizobacteria can grow. This way, legumes can get accessible nitrogen to produce chlorophyll, and the rhizobacteria have a place to live in.
Sinorhizobium meliloti, also known as Ensifer meliloti and Rhizobium meliloti, is a species of rhizobacteria found in legumes. As such, their jobs are to turn atmospheric nitrogen into ammonia for the plants to use to develop chlorophyll. They’re so good at their job though, that scientists at the University of Central Arkansas used it to see if they could do similar jobs in Mars as well!
Legumes are known for their ability to grow mostly everywhere and produce foodstuffs that are nutritiously dense, and thus are a good candidate for being a main crop for our first forays into space. However, as previously discussed, if we want legumes to survive and thrive, rhizobacteria need to be present as well.
Dr. Arijit Mukherjee and graduate student Randall Rainwater wanted to see if the mutualistic symbiosis can also be sustained on Martian soil, and created an experiment to see if this was the case. Prior research has shown that the Martian atmosphere contains nitrogen, and that Martian soil is rich with nutrients but not necessarily plant-accessible. Thus, it would be theoretically possible for the legume-rhizobacteria symbiotic relationship to thrive on Mars.
Mars: the Red Planet, and our next real estate goal. Source.
To test this, Mukherjee and Rainwater obtained five different Martian soil samples that were sourced from the Mojave Desert. These simulations were similar to Martian soil in aspects of water retention, weathering, and mineral availability. Sand was used as a control substrate. In all five Martian simulations and sand, Medicago truncatula was grown. M. truncatula, also known as barrel clover, is an often-used legume for scientific research. The plants were split into two groups equally across all substrates: Group A had S. meliloti in the soil, while Group B had Sinorhizobium medicae, another rhizobacteria.
Barrel clover, our main character for this study! Source.
After two weeks, the plants were dug up from their substrate, and their mass was measured. Biomass of M. truncatula was used as a measurement to see how well legumes might grow in Martian soil. Compared to plants grown in sand, all of the plants grown in Martian soil simulants had significantly less biomass, except for plants grown in MMS-2 (Martian Mojave soil subtype 2). This was true for both Groups A and B, which were inoculated with different rhizobacteria. The authors proposed that this difference was due to the fact that MMS-2 had more plant-accessible compounds, such as iron oxide and sulfates.
Figure adapted from Rainwater & Mukherjee, 2021. Figure A shows the average biomass per plant of M. truncatula after 14 days of inoculation with S. meliloti, while Figure B shows the same for plants inoculated with S. medicae.
In addition to measuring biomass, the scientists also analyzed the plant roots, lookin. The genes that code for root nodules and lateral roots are genetically similar, and although this doesn’t necessarily mean the growth of one can predict the growth of the other, the scientists still thought it was important to take note. For both Groups A and B, all five types of Martian soil simulants had a statistically significant decrease in the number of lateral roots. The authors state that further research is needed to determine why this may be the case.
Figure adapted from Rainwater & Mukherjee, 2021. Figure A shows the average number of lateral roots per plant of M. truncatula after 14 days of inoculation with S. meliloti across control substrate and five Martian soil simulants. Figure B shows the same for plants inoculated with S. medicae.
In addition, the number of lateral roots did not determine the number of root nodules. Despite having a statistically significant decrease of lateral roots, most plants, regardless of substrate type or rhizobacteria type, had comparable numbers of root nodules when compared to control. MMS-1 unsorted soil inoculated with S. meliloti and MMS-1 fine soil inoculated with S. medicae did have a significant decrease in nodule numbers, however. Unfortunately, no reason was suggested to explain this difference.
Figure adapted from Rainwater & Mukherjee, 2021. Figure A shows the average number of root nodules per plant of M. truncatula after 14 days of inoculation with S. meliloti across control substrate and five Martian soil simulants. while Figure B shows the same for plants inoculated with S. medicae.
Finally, the authors stained the root nodules with a specific gene fusion to determine the presence of the gene nifH. nifH is a gene that codes for an enzyme that is necessary for nitrogen fixation. Prior literature has used nifH as a valid marker of nitrogen fixation in soil studies. Plants from all substrate types inoculated with S. meliloti indicated presence of nifH, which further indicates nitrogen fixation.
Figure adapted from Rainwater & Mukherjee, 2021. Figure A depicts an unstained root nodule. Figure B represents root nodule from plant grown in sand, Figure C represents root nodule from MMS-1 Coarse simulant, Figure D represents root nodule from MMS-1 Fine simulant, Figure E represents root nodule from MMS-1 Unsorted simulant, Figure F represents root nodule from MMS-1 Superfine simulant, and Figure G represents root nodule from MMS-2 superfine. Figures B-G are stained root nodules, with blue representing nifH expression. The black bar is 0.5mm.
The results from this study indicate that rhizobacteria and legume symbiosis can indeed occur in Mars, which may point to legumes being a viable crop in Martian endeavours. Two different rhizobacteria and five different types of Martian soil simulants were used as well. While biomass and number of lateral roots may not thrive in Martian simulants, it is crucial to note that the number of root nodules remain comparable. This, in addition to the fact that all root nodules displayed nifH expression, indicate that legumes may truly have a fighting chance in Mars.
It would be remiss to say that this study is definitive proof that legumes can grow on Mars. More research is needed, especially those that replicate Martian atmosphere and weather in addition to Martian soil. Seeing actual yield in different types of legumes would also help in the search for deciding on Martian crops. But isn’t it so interesting to think that even on Mars, we would still need tiny little bacteria on our plant roots to ensure we can eat during our interstellar conquest?
About the Author:
Grace Jaeeun Lee ‘25 is a Biology and Psychology double major at Mount Holyoke College. In her free time, she enjoys designing stickers and watching medical dramas. She hopes to survive her final undergraduate finals season.
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