1. Problem: The Journey of Humanity in Space Exploration

1.1 Humanity is Evolving into a Multi-Planetary Species

The Lascaux prehistoric cave paintings provide evidence that around 15,300 years ago, ancient humans were already fascinated by the distant starry sky. Since ancient times, the dream of voyaging through the cosmos has been deeply embedded in human aspiration.

Over 700 years ago, humanity made its initial attempt at manned rocket flight. An elderly Chinese man, with a kite in hand, fearlessly launched himself into the sky while seated in a chair strapped to rockets. Although the endeavor was undeniably unsuccessful, the quest for space exploration never ceased.

In 1961, Yuri Gagarin became the first human to journey into outer space, marking a historic milestone. Following this achievement, continuous exploration and experimentation accumulated vital experience in manned spaceflight. Ultimately, humanity took a monumental step out of Earth's cradle with the successful moon landing of Apollo 11. In 1971, the launch of Salyut 1 inaugurated the era of space stations, providing conditions for human habitation in space.

In recent years, the focus has shifted towards deep space exploration and manned missions to Mars. NASA's Perseverance rover successfully landed on Mars in 2021, carrying out scientific research and sample collection. Concurrently, China has made significant advancements in the aerospace sector, with Chang'e 4 achieving the first-ever landing of a human probe on the far side of the moon, and the construction of the Chinese space station is progressing.

1.2 Life Supporting in Deep Speace

Humanity continues to push the boundaries of space exploration, raising new questions on how to venture to more distant places, especially regarding life support systems. Currently, space stations rely on carrying supplies and periodic resupply missions from Earth to secure food resources. However, during deep space exploration, obtaining resupplies from Earth will be impractical, making the assurance of astronauts' food supply a crucial challenge.

Life support systems for deep space missions include key components such as oxygen supply, water resource management, food supply, temperature and humidity control, waste management, and pressure regulation. These systems aim to provide essential life support conditions for astronauts, allowing them to live on space stations, future spacecraft traveling farther into space, lunar bases, or Mars bases.

Initially, life support systems were simple, relying on single-use resources with almost no recycling facilities. With the demands of long-term missions, these systems have become increasingly complex and efficient. For instance, the Tiangong Space Station has achieved 100% recycling of water resources.

As we embark on deep space exploration missions, the increased distance and duration of travel will demand even more sophisticated life support systems. These systems must ensure highly efficient resource recycling within the spacecraft, providing sustainable life support for astronauts during extended missions.

2. Inspiration: Agriculture and Space

2.1 The cultivation of crops is an important part of life support system

Astronauts residing on a space station 400 kilometers above Earth depend on air, water, and food. In both the China Space Station (CSS) and the International Space Station (ISS), supplies are delivered by cargo spacecraft. While water and oxygen can be recycled through physical and chemical processes, food must be carried in sufficient quantities and is not renewable. Growing vegetables in space has been attempted on both the CSS and ISS. Plant growth relies on external light sources, primarily supplemented by adjustable spectrum LEDs, and fertilizers are brought from Earth. However, these attempts are small-scale, with food primarily supplied through transportation.

The cost of launching payloads into space is substantial. In 2021, the lowest cost was approximately $1,500 per kilogram, with the typical cost around $10,000 per kilogram (CSIS, 2022). These high costs and limited payload capacities necessitate meticulous planning for every gram. Current biological recycling systems are insufficiently efficient. We believe "biological regeneration" will be critical in addressing food supply issues and constructing life support systems in space stations. In the future, when humanity embarks on longer and more distant space missions, relying solely on carried food or regular Earth resupply will be impractical. "Biological regeneration" will become the main source of food, and ideally, the water and inorganic fertilizers needed by plants will also be included in this process.

Thus, growing crops in space is imperative. Additionally, cultivating plants in space offers multiple benefits.

Firstly, plants can provide food for astronauts. During long-term space missions, plants can be a source of fresh food, reducing dependence on Earth's supply chain and enhancing mission self-sufficiency. Different plants offer varied nutrition, improving astronauts' diet quality.

Moreover, plants absorb carbon dioxide and release oxygen through photosynthesis, providing essential breathing gases and maintaining appropriate carbon dioxide levels within the spacecraft. Growing plants helps maintain a suitable gas environment, reducing the burden on artificial life support systems. Plants also release water vapor through transpiration, helping regulate cabin humidity.

Beyond material benefits, growing plants supports astronauts' psychological well-being. Observing plant growth creates a natural, relaxing environment within the spacecraft, alleviating stress and supporting mental health.

Finally, plant cultivation is significant for scientific research. Space plant growth experiments provide data on how microgravity, cosmic radiation, and other factors affect plant growth, physiology, and gene expression, advancing agricultural science. These experiments also provide crucial data for improving life support systems for future long-term deep space missions, enabling humanity to venture further into the universe.

2.2 Growing Crops in Space is Difficult

However, plant growth and development require various resources. In space, water, energy, and inorganic fertilizers are limited resources essential for plant growth. Therefore, it is necessary to establish a completely new cyclic space agriculture system.

2.3 Inspiration from GIAHS: The Rice Fish Culture

The rice-fish symbiosis system has been practiced in China for 2,000 years (You Xiuling, 2006). In 2005, it was recognized by the Food and Agriculture Organization of the United Nations as one of the first Globally Important Agricultural Heritage Systems (GIAHS). These systems represent ongoing human-managed communities that have developed sustainable practices in intricate relationships with their surrounding geographical, cultural, and agricultural landscapes, as well as broader biophysical and social environments. Nowadays, the traditional rice-fish symbiosis agricultural practice continues in Qingtian County, Zhejiang Province, China. This practice involves raising a unique local fish species, known as "field fish," in the rice paddies, creating a distinctive landscape where water and field fish coexist (Figure 1). Qingtian County, noted for its mountainous terrain and limited arable land, is described as "nine mountains, half water, and half farmland." The rice-fish symbiosis system is the most efficient agricultural production solution derived from generations of local farmers' experiences and practical innovations.

Within this system, rice and weeds are the primary producers, fish, insects, and various aquatic animals are the primary consumers, and bacteria and fungi are the decomposers (Figure 2). The rice plants offer shade and organic matter for the fish, while the fish contribute by controlling weeds, aerating the soil, supplying oxygen, and preying on pests. Additionally, fish waste is converted into fertilizer, enriching the soil with organic material and nutrients. This "fish eat insects and weeds – fish waste fertilizes the field" process sustains the system's cycle, reducing reliance on external chemical inputs and enhancing biodiversity.

2.4 Conclusion

The rice-fish symbiosis system provides us with profound insights. Both manned spacecraft on long-term missions and the agricultural communities in Qingtian County, operate in environments with highly limited resources. To achieve efficient resource utilization in such ecosystems, it is essential to extend the pathways of material cycling and maximize the recovery of waste energy.

For instance, in the rice-fish symbiosis system, fish waste is used to fertilize the rice fields, reutilizing the metabolic by-products of fish for plant growth. This practice extends the cycling of elements such as nitrogen and phosphorus within the ecosystem. Moreover, fish help in weed and pest control, thereby repurposing the otherwise wasted energy in weeds and pests for fish production, ultimately achieving energy recycling.

3. Overview: Developing a Self-Sustaining Cyclical Ecosystem for Space Exploration

3.1 Our Assumption: Cyclical Ecosystem

A typical ecosystem includes producers, consumers, and decomposers. Producers synthesize organic matter from inorganic substances and capture environmental energy in biochemical form, making it available for the first time to biological organisms. The organic matter produced by these producers serves as the food source for all other heterotrophic organisms, including humans, making producers the fundamental components of the ecosystem. Decomposers continually break down complex organic materials into simpler inorganic substances, ultimately returning them to the environment as nutrients for autotrophic organisms.

In the rice-fish symbiosis system, humans are consumers, rice is the producer, and decomposers are present in the soil. Introducing fish as an additional consumer improves the material cycling and energy flow. In space, with only humans as consumers and crops as producers, the absence of decomposers prevents the completion of material cycling and energy flow processes within the ecosystem. To establish a cyclical ecosystem, an additional component must be included to decompose the metabolic by-products of consumers and provide nutrients for producers (Function 1). Due to the limited energy resources in space, adding another consumer would increase energy dissipation. Therefore, we need this additional component to also fulfill the role of "fish" in the rice-fish symbiosis system by recycling waste energy (Function 2).

3.2 Our Choice: Shewanella

Shewanella oneidensis (S. oneidensis) is known for its unique metabolism and ability to perform extracellular electron transfer (EET), positioning it as a significant electrogenic microorganism. The energy-producing capabilities of S. oneidensis offer a valuable energy source for space missions. Moreover, our preliminary research has shown that its electricity-generating ability is intricately linked to its phosphorus metabolism. This linkage could allow the coupling of energy production with phosphorus metabolism, making S. oneidensis highly useful in fields such as wastewater treatment and energy generation.

In 2019, NASA launched the Micro-12 project to explore the potential applications of S. oneidensis in space environments. Micro-12 took S. oneidensis to the ISS to measure the effects of the space environment on its growth, physiological responses, and biofilm formation. Upon the samples' return to Earth, further analysis was conducted to evaluate the impact of the space environment on S. oneidensis. The findings revealed that microgravity and other space conditions do not significantly affect the bacterium's abilities.

Under these circumstances, leveraging synthetic biology to modify S. oneidensis to fulfill these complex roles becomes our most viable option.

3.3 Our Plan

We have noticed that human feces (after dehydration) contain about 1.7% phosphorus, which led us to consider the unique coupling of S. oneidensis's electricity generation and phosphorus metabolism. We plan to utilize S. oneidensis to process the phosphorus in human feces, enriching it for use as fertilizer through bacterial action. At the same time, the current produced by S. oneidensis during phosphorus metabolism can power LED lights needed for plant growth or be fed into the space station's power system, thus boosting the station's energy supply (Figure 3).

Our project, "Shivacosmic Greens," creates a symbiotic system integrating humans, plants, and microorganisms. In this system, we achieve water purification, phosphorus recycling, and efficient energy usage. We use microorganisms to process human waste, enrich the phosphorus content, and break down lactic acid while generating a certain amount of electricity. The phosphorus-enriched microbial biomass serves as fertilizer, and the dephosphorized wastewater undergoes further treatment. The electricity generated during phosphorus enrichment is used to power plant cultivation. We have also developed hardware facilities for microbial cultivation and electricity generation in space. This provides innovative insights and tools for establishing a sustainable life support system for long-term deep space missions.

4. Our Solution

4.1 Phosphorus Recovery: Optimizing Phosphorus Metabolism and Electricity Generation

Energy metabolism is important for electron transport in the electric production efficiency of Shewanella, At present, the existing literature mainly focuses on several aspects below to try to improve its electricity generation efficiency: a) To study the expression of cytochrome associated with electron transport on the surface of Shewanella. b) Elevates proteins associated with intracellular electron transport (such as those involved in the synthesis of Nicotinamide adenine dinucleotide (NAD)), but proteins related to triphosphate (ATP) metabolism have been less studied.

Our previous work focused on phosphorus metabolism, as we observed that the levels of some phosphorus-related substances in cells are potentially correlated with electrification in Shewanella: a) ATP plays a major role in phosphorus metabolism and can be used for energy supply (e.g., electricity generation). b) polyphosphate (PolyP) is a high-energy compound that regulates intracellular phosphorus metabolism, but it has not been demonstrated that it is involved in electron transfer in Shewanella. For this reason, our research focuses on discovering the relationship between them.

4.2 Utilization of Human Metabolic Waste: Optimizing Lactic Acid Metabolism

In the vast expanse of space, resources are precious, and waste is an inevitable byproduct of human activity. Our innovative approach transforms this waste into valuable assets, utilizing the inherent properties of Shewanella. To enhance the metabolic breakdown of lactic acid substrates and boost its electrochemical prowess, we’ve introduced a global transcription regulator, CRP (cAMP Receptor Protein), via plasmid. This strategic modification elevates the bacterium’s ability to convert organic waste into electrical energy, a critical function in the closed-loop life support systems of space habitats.

Moreover, to fortify the resilience of Shewanella in the non-adverse conditions of wastewater, we've integrated a global regulatory protein, IrrE. This addition significantly improves the bacterium's tolerance to salinity, acidity, and radiation—hazards that are all too common in the harsh environment of space. By bolstering its defenses, we ensure that our microbial ally thrives, contributing to the sustainability and self-sufficiency of space exploration endeavors.

4.3 Adaptation to Space Environment: Improving Radiation Resistance

During deep space exploration, manned spacecraft may leave the protective shield of Earth's magnetic field, exposing astronauts to galactic cosmic rays (GCR) and solar energetic particles (SEP). Prolonged exposure to cosmic radiation can cause irreversible damage to living organisms. Shewanella will experience a decline in their electricity production ability after long-term exposure to radiation. Thus, there is a need to search for effective means of protection against radiation damage. We introduce manganese ions (Mn2+) and cationic tripeptides into the culture medium to form polyP- Mn2+-tripeptide droplets, protecting the cell's DNA and proteins, thereby enhancing the radiation resistance of Shewanella.

References

[1] To Be Done