Artifical Primary Succession in Lunar Regolith

With the upcoming Artemis missions, NASA, ESA, JAXA and CSA are looking to re-establish a human presence on the Moon for the first time in over 50 years. The program has several objectives which include the long-term goal of establishing a permanent lunar base camp and construction of the Lunar Gateway to support surface missions and facilitate further exploration into the depths of our solar system. If humanity is to successfully establish a permanent presence on other planetary bodies such as the Moon and Mars, the ability to cultivate plants within a settlement or base will be invaluable and likely essential, both in-terms of enabling the production of food and other useful materials in-situ in addition to providing passive atmospheric and potentially water recycling.

While soil-free growing methods such as hydroponics and aeroponics have been successfully demonstrated and offer a number of benefits in terms of reducing space required for cultivation, they also present a number of issues such as high resource requirements, potential loss of crops as a result of power interruptions, and an extensive level of monitoring and maintenance to remain viable. Conventional plant growing will likely be much more resource-efficient and sustainable, however would require a suitable growth substrate. Transporting sufficient soil or a soil-alternative from Earth would require a massive expenditure of resources thus it would be far easier and more efficient to produce a suitable substrate in-situ[1].

Previous Research Summary

Previous research has shown that it is possible to cultivate terrestrial plants within both lunar and Martian regolith simulants – these previous studies have primarily focused on attempting to cultivate conventional crop plants such as tomatoes, potatoes, and berries. However, these experiments also found that these plants are typically stunted and poorly developed and any crops produced are small and poor-quality compared to specimens grown in terrestrial soils.

Recently (2022) a paper was published by the University of Florida which demonstrated that specimens of Arabidopsis thaliana (a small and rapidly growing plant, commonly used for laboratory investigations) were able to germinate and grow in real samples of lunar regolith, retrieved between 1969 – 1972 by the Apollo 11, 12 and 17 missions. However, these specimens demonstrated poor growth and development even relative to comparison specimens grown in lunar regolith simulant samples. Tissue samples of these plants also showed high numbers of differentially expressed genes (DEG’s), with a significant number associated with stresses induced by high levels of salinity, metals, and reactive oxygen species within the regolith[2]. This study confirmed the observations that lunar regolith, in its “natural state”, is not a benign substrate and the growth of crops in such an environment would be highly problematic.

Why is Lunar regolith not an ideal substrate for growth?

Though often termed “Lunar Soil”, the lunar regolith is in fact not equivalent, or even close in composition to a terrestrial soil. Soils are mixtures of organic matter, minerals, gases, liquids, and microorganisms that are, together, able to support life [3]. New soils are created over hundreds to thousands of years from freshly created or exposed rock (the soil parent material) via a combination of biological, physical, and chemical processes, which enable soils to “evolve” throughout their lifespan. This is a nuance that seems to be elusive in many current and previous investigations into the potential uses of lunar regolith for plant growth.

There appear to be four major components of a healthy soil that are either entirely lacking or heavily depleted on the Moon that must be addressed in order to convert lunar regolith into a more suitable substrate:

Accessible Nutrients

Plants require 17-18 chemical elements in order to survive and grow “normally” [4] some of which, such as N and C are almost entirely absent from the lunar surface. Others such as K and P we are unsure of how abundant they are likely to be due to the complexity of the lunar environment while others such as Mg, Ca and Fe have been demonstrated to be present in abundance within the regolith mineralogy [5]. Unfortunately, plants require these elements not only to be present, but to be accessible. On Earth, these elements are made accessible by the decomposition and transformation of these minerals via the action of certain biological organisms and through a range of chemical reactions, a large number of which are water-based and oxidation reactions. In order for us to make use of lunar regolith as a substrate for crop growth, these elements must be made available.

Soil Organic Matter

Carbon forms the central building block for life as we know it and is essential for the construction of organic molecules – the majority of organisms obtain carbon through the consumption and digestion of other organisms however, as photosynthetic organisms, plants primarily obtain necessary carbon in the form of CO2 from the atmosphere rather than pre-existing organic molecules. So why are organic compounds so important to the functioning of a healthy soil? They are critical in order to sustain the wide array of other organisms within the soil – bacteria, protists, fungi, insects, arthropods etc. all of which play key roles in maintaining the health of the soil. Many of these organisms play key roles in the recycling of nutrients by breaking down organic matter and releasing other important nutrients, beyond carbon, such as phosphorus and sulfur into the soil in forms accessible to plants. Additionally, organic matter has a number of useful “physical” properties in soils such as promoting increased water retention, improved soil structure and buffering pH levels that benefit plant colonisation and survival [6].

Liquid Water

The presence of liquid water is an essential requirement for life as we know it, and despite evidence for the presence of water being confirmed, both as ice at the lunar poles and within the shadows of lunar craters, the surface of the Moon is extremely dry, hundreds of times more arid than any terrestrial desert [7]. Thus any regolith would require through hydration to prepare it for the hosting of even the hardiest desert-adapted plant. Water is also a key solvent and chemical reactant, on Earth the majority of chemical and biological, and a number of physical, weathering processes that convert parent materials into soils require or are facilitated by the presence of water, without which the development of regolith into a functional soil will be impossible.

The Lack of a Soil Ecosystem/Biome and Associated Activity

The Moon, as far as we are aware, is entirely sterile, thus the lunar regolith is completely devoid of biological activity and severely depleted in organic compounds. On Earth, soils host incredibly complex ecosystems which are still not fully understood however, there appear to be some major groups of microorganisms whose presence in a soil seems to be essential or at-least highly beneficial for increasing soil productivity and fertility. These major groups include, but are not limited to:

• Soil bacteria – bacteria are the smallest and most abundant organisms within the soil biome, typically numbering between 5 – 50 million cells per g of soil in arable areas (and this is still likely to be an underestimate). Soil bacteria play several roles including decomposition of organic material, increasing nutrient availability, pathogen repression and promoting increased plant growth and activity [8][9][10][11].
• Fungi – soil fungi have, primarily, evolved to fill the role of highly versatile and specialised decomposers, able to degrade a number of resistant organic materials such as cellulose, lignin and chitin, that would otherwise remain intact, removing key elements (primarily carbon) from the natural biogeochemical cycles and limiting the availability of certain nutrients for other members of the soil ecosystem [8]. However, there is a specialist sub-group of fungi that fulfil an alternative function that while, not currently thought to be essential to soil fertility, has been found to be of great benefit to establishing and maintaining a plant population in an area.
  • Mycorrhizae – mycorrhizae are able to form symbiotic associations with approximately 80-90% of all known plant species and reside within the roots of their hosts. The hyphae that these fungi extend out into the soil are able to serve a number of purposes including increasing the range of the root network providing increased access to nutrients for the plant, improving soil structure and secreting exudates into the soil that promote the breakdown of compounds and minerals. Numerous pieces of research have been published which highlight the improvements to the growth, resilience and overall health of plant populations observed as a result of the introduction of mycorrhizal networks [12].
• Blue-Green Algae - Despite commonly being associated with aquatic environments, blue-green algae are found in virtually all environments on Earth where there is light, they can exploit for photosynthesis. Cyanobacteria are major contributors to the cycling of both C and N through ecosystems and are able to function both symbiotically with plants and fungi or as free-living organisms. Blue-green algae along with lichen are some of the earlies pioneer species to colonize new environments and select species have been found to display extreme resilience to a wide range of environmental conditions while contributing to early soil development processes [13].

Why are multiple plant species better than one?

In 2021 a paper was published by Furey & Tilman [16] concerning the correlation between plant biodiversity and the ability to regenerate soil fertility with time. It is well known that different plant species have different modes of life which result in differing levels of liberation, retention and use of nutrients. For example, leguminous plants are able to host the N-fixing bacteria Rhizobia within nodules on their roots, thus the presence of said plants in an area increases the availability of N and helps counteract N depletion. With all previous experiments attempting to grow plants in lunar or Martian regolith, specimens are grown as “monocultures” and as such, if the experiments had been allowed to continue it is likely the regolith would become increasingly abundant in a select group of nutrients and deficient in others until the plants died. In the paper, Furey and Tilman found that, across the board, higher plant diversity resulted in greater improvements to soil fertility than monocultures. They also found that a combination of: (1) Plants capable of facilitating N-fixation (e.g. legumes), (2) Plants capable of facilitating increased availability of other key plant nutrients such as K (e.g. forbs), and (3) Plants capable of rapidly generating high levels of biomass and thus increasing the organic material content of soils (e.g. grasses), was extremely beneficial to improving the fertility of a soil.

As such, in this investigation, plant species have been selected that can occupy multiple ecological niches and it is our intention to compare the growth of “consortia” of multiple species with monocultures to see if these findings can be replicated in the lunar regolith simulant.

Project Summary

While it would be possible to address these issues artificially for example through the addition of fertiliser and other growth promoting compounds, in a space-setting these resources would have to be conveyed to the habitat from Earth or produced in-situ, requiring significant material demands. Thus this investigation will focus primarily on attempting to use plant and microorganism species to affect and moderate these modifications to the regolith with little to no human assistance. In light of this, we will be focusing on attempting to grow “pioneer species”. Pioneer species is a collective term for the first organisms able to colonise a newly exposed area of land. Said species typically possess some combination of unusual characteristics that enable them to survive in these new hostile environments and as a product of their growth and existence, they contribute to and accelerate the conversion of the bare rock to a soil. As they fundamentally alter the chemical and physical properties of the environment, by a range of mechanisms, the environment is slowly made more habitable and as time progresses, more complex and less hardy species are able to colonise and survive in the area and eventually outcompete the pioneer species. This is known as ecological succession[14]. Primary succession is defined as a type of ecological succession in which the pioneer organisms colonise a barren, lifeless habitat. Pioneer species have evolved to take advantage of this hostile environment and possess a range of specialised adaptations [15].

Under the assumption that currently viable plant growth would only be possible within the interior of a lunar habitat, in this project, there are several objectives we are intending to address:

(1) Assessing whether pioneer or specially adapted plant species are better suited to colonizing lunar regolith and display improved growth and less negative impacts than previously utilized plant species when grown under the same conditions.

(2) Investigating whether, as has been suggested by previous research, a consortium of multiple plant species, able to carry out different functions regarding the improvement of soil fertility is able to support itself in regolith and display improved growth and development over a monoculture under the same conditions

(3) Investigate whether the early addition of microorganisms, both as individual species and as a collective has a positive impact on the early growth and development of plant species in lunar regolith simulants or if their impact and effectiveness is limited during the early colonization and they would be better utilized later in the soil development process.

References

[1] Barbosa, G. et al. (2015) “Comparison of land, water, and energy requirements of lettuce grown using hydroponic vs. conventional agricultural methods,” International Journal of Environmental Research and Public Health, 12(6), pp. 6879–6891. Available at: https://doi.org/10.3390/ijerph120606879.

[2] Paul, A.-L., Elardo, S.M. and Ferl, R. (2022) “Plants grown in Apollo lunar regolith present stress-associated transcriptomes that inform prospects for Lunar exploration,” Communications Biology, 5(1). Available at: https://doi.org/10.1038/s42003-022-03334-8.

[3] Needleman, B.A. (2013) What Are Soils?, Nature Education Knowledge Projecy. Nature Publishing Group. Available at: https://www.nature.com/scitable/knowledge/library/what-are-soils-67647639/ (Accessed: February 7, 2023).

[4] Nutrient Management Competency Area 1: Basic Concepts of Plant Nutrition (2010) NorthEast Region Certified Crop Advisor. Cornell University. Available at: https://nrcca.cals.cornell.edu/soilFertilityCA/CA1/CA1_print.html (Accessed: February 7, 2023).

[5] Taylor, S.R. (1975) Lunar Science - a post-apollo view: Scientific results and insights from the lunar samples. Oxford: Pergamon Press.

[6] Körschens, M. (2002) “Importance of soil organic matter (SOM) for biomass production and environment (a review),” Archives of Agronomy and Soil Science, 48(2), pp. 89–94. Available at: https://doi.org/10.1080/03650340214162.

[7] Taylor, G.J. et al. (2019) “Modal analyses of lunar soils by quantitative X-ray diffraction analysis,” Geochimica et Cosmochimica Acta, 266, pp. 17–28. Available at: https://doi.org/10.1016/j.gca.2019.07.046.

[8] Burgers, A. (2012) Soil Biology. Saint Louis: Elsevier Science.

[9] Cain, Michael L.; Bowman, William D.; Hacker, Sally D. (2011). "Chapter 16: Change in Communities". Ecology. Sinauer Associates. pp. 359–362. ISBN 978-0-87893-445-4.

[10] Willey, Joanne M.; Sherwood, Linda M.; Woolverton, Christopher J. (2011). "Chapter 29: Microorganisms in Terrestrial Ecosystems". Prescott's Microbiology. McGraw-Hill. pp. 703–706. ISBN 978-0-07-131367-4

[11] Bhatti, A.A., Haq, S. and Bhat, R.A. (2017) “Actinomycetes benefaction role in soil and Plant Health,” Microbial Pathogenesis, 111, pp. 458–467. Available at: https://doi.org/10.1016/j.micpath.2017.09.036.

[12] Jeffries, P. et al. (2003) “The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and Soil fertility,” Biology and Fertility of Soils, 37(1), pp. 1–16. Available at: https://doi.org/10.1007/s00374-002-0546-5.

[13] Yang, Y. and Lei, J. (2003) “The prospect function of terrestrial nitrogen-fixing blue-green algae on the fixation of Desert,” Ecosystems Dynamics, Ecosystem-Society Interactions, and Remote Sensing Applications for Semi-Arid and Arid Land [Preprint]. Available at: https://doi.org/10.1117/12.466462.

[14] Sturla Fridriksson (1987) Plant Colonization of A Volcanic Island, Surtsey, Iceland, Arctic and Alpine Research, 19:4, 425-431, DOI: 10.1080/00040851.1987.12002623

[15] Pioneer species - definition and examples - biology online dictionary (2022) Biology Articles, Tutorials & Dictionary Online. Available at: https://www.biologyonline.com/dictionary/pioneer-species (Accessed: February 7, 2023).

[16] Furey, G.N. and Tilman, D. (2021) “Plant Biodiversity and the regeneration of Soil fertility,” Proceedings of the National Academy of Sciences, 118(49). Available at: https://doi.org/10.1073/pnas.2111321118.