1. Introduction

As the world's population continues to grow, the demand for food is on the rise. Simultaneously, there is a growing awareness of the need to reduce the environmental impact of food production. The expansion of human habitation has also led to exploration and technological advancements in space for potential extraterrestrial living. Technological developments in material cycling within closed environments, based on human life, have the potential to address challenges both on Earth and in the emerging space habitats.

In this context, there is growing interest in innovative food technology that utilizes microalgae with high productivity per unit volume and per unit area. Among these microalgae, Euglena, known for its resilience to high concentrations of carbon dioxide and rapid proliferation, holds the potential to contribute to achieving a low-carbon society. Euglena possesses characteristics of both plants and animals, containing a rich array of 59 different nutrients. Additionally, a unique component of Euglena, called paramylon, has shown promise in maintaining gut health and immune regulation, expanding its potential applications in the healthcare sector.

On Earth, Saga Sustainable Tech Farm, operated by Euglena Co., Ltd., is actively researching resource cycling in agriculture using microalgae Euglena. After successful endeavors in liquid fertilizer and cultivation soil, they are now exploring the production of fertilizers through Euglena, cultivated using underutilized resources from sewage treatment facilities, to support sustainable agriculture.

In 2021, within the SPACE FOODSPHERE program, a collaborative effort addressing common challenges related to "food" on Earth and in space, a team is dedicated to researching and developing high-efficiency food resources using microalgae. This article explores the possibilities of space utilization, particularly focusing on the potential of microalgae-based food resources, with an eye toward the prospect of human habitation on the lunar surface by 2040.

2. Utilizing Microalgae as Resource-Cycling Food in Space

2.1 Role of Microalgae in Closed Systems

In closed environments like lunar bases, efficiently utilizing resources generated during long-term human stays is a significant challenge. The International Space Station (ISS) has incorporated technologies that convert accumulated carbon dioxide into oxygen through processes like water electrolysis and the Sabatier reaction as part of the Environmental Control and Life Support System (ECLSS).

Sabatier Reaction: 2H2O → 2H2 + O2 (Water Electrolysis) CO2 + 4H2 → CH4 + 2H2O (Sabatier Reaction)

However, excess carbon dioxide, equal to that produced by respiration, still requires disposal due to insufficient hydrogen supply for complete removal. To address this, the use of photosynthetic organisms such as microalgae is being considered to convert carbon dioxide and wastewater into oxygen and food, effectively recycling water. Microalgae, in particular, offer advantages due to their rapid growth, high edibility, and minimal waste generation (see Figure 3).

2.2 Historical Perspective of Microalgae Use in Space

Microalgae experiments related to space are not new, with numerous studies conducted over the years. The first recorded success in algal liquid cultivation in space occurred in 1960 aboard Korabl-Sputnik 2, utilizing a green algae species, Chlorella. The experiments demonstrated that basic physiological and photosynthetic functions could be maintained in orbit. Similar experiments, including Spirulina cultivation, have been carried out in low Earth orbit on the ISS.

However, the specific challenges of cultivating microalgae in the low-gravity environments of the Moon and Mars are still being explored. Different cultivation methods and considerations are needed for these environments compared to Earth's low orbit.

2.3 Investigating Euglena Cultivation for Resource Recycling

Euglena has the ability to proliferate using various carbon sources present in the culture medium and can utilize ammonia nitrogen found in urine. Furthermore, Euglena contains a high percentage of protein in its dry weight (30-50%) and a wide range of vitamins and minerals. It can also synthesize all vitamins except B1 and B12, making it a valuable source of vitamin D, which is challenging to obtain from plants alone.

To effectively cultivate microalgae in closed environments like lunar bases, artificial lighting, specifically LED panels, is commonly considered for production. Researchers have developed a flat-panel photobioreactor with built-in features to maintain stable continuous cultivation in low-gravity environments. By optimizing the design and implementing efficient mixing of cells, they have successfully demonstrated two weeks of continuous cultivation.

Moreover, experiments have been conducted using human urine diluted with water and supplemented with vitamins and trace metals to adjust the medium for Euglena cultivation. The results indicate that Euglena can grow using components from urine, laying the groundwork for efficient nitrogen cycling in closed systems.

2.4 Considering Euglena Strain Improvement Through Genome Editing

In recent years, the concept of the bioeconomy has gained attention as a means to achieve the United Nations Sustainable Development Goals (SDGs). Euglena Co., Ltd. has been conducting research and development in various applications, ranging from food and healthcare to fiber, feed, fertilizer, and biofuels, based on the 5F biomass strategy.

Notably, Euglena has achieved a breakthrough by successfully editing its genome, a crucial step toward molecular breeding of industrially valuable strains. In particular, a strain with a disrupted EgGSL2 gene, responsible for paramylon synthesis, has been created. This strain shows increased protein content, making it suitable for protein production in space and on Earth.

The efficient production of protein using urine as a resource is not limited to lunar habitation and holds potential for sustainable terrestrial applications. By cultivating microalgae using underutilized resources such as livestock waste and wastewater, a closed-loop system for resource recycling can be established.

In conclusion, microalgae, especially Euglena, offer promising solutions for sustainable food production on Earth and in space, addressing the challenges of resource recycling and providing essential nutrients for future lunar bases and beyond. Through advancements in cultivation methods, genome editing, and resource utilization, microalgae are positioned as valuable assets in our pursuit of a sustainable and thriving future.

3. Potential Utilization of Microalga Euglena as a Fermentation Aid

In the context of utilizing materials in space, fermentation, a process involving the transformation of substances by microorganisms, holds promise as a valuable tool. Waste materials rich in organic matter, such as human waste, can be converted into compost through microbial fermentation, contributing to material recycling. This compost can then be used for food production in space-based facilities like plant factories. Furthermore, with regard to food, fermentation can transform non-edible parts of vegetables and items with low shelf-life into fermented foods. In the limited dietary variety of space, employing limited processing techniques to diversify nutrients and flavors is feasible and warrants further investigation. Additionally, aside from commonly known fermented foods, it's possible to produce seasonings like sodium glutamate and alcoholic beverages (i.e., alcohol) as well.Figure 6: Examples of Applications for the Utilization of Fermentation ProcessesEuglena not only boasts rich nutritional content but also functions as a prebiotic, promoting the growth of beneficial bacteria like lactobacilli in the human gut, helping to balance the gut microbiota (Nakashima, Sasaki, Sasaki, Yasuda, Suzuki, and Kondo, 2021). Euglena has been shown not only to contribute to the proliferation of other microorganisms but also to serve as a fermentation aid. In 2016, it was confirmed that adding Euglena powder to rice and koji during the sake brewing process enhanced the fermentation process. The resulting "Midori-koji" exhibited increased enzyme production and contained higher levels of sulfur compounds, such as ergothioneine and glutathione, which are known for their strong antioxidant properties. These findings suggest potential applications in the health and cosmetic industries, among others.Currently, there is ongoing research to explore Euglena's potential for addressing challenges in developing countries. In Indonesia, traditional fermented soybean food known as "tempe" is produced by fermenting soybeans with tempe fungi. The impact of Euglena addition to the tempe fermentation process was evaluated. In addition to making tempe in the conventional way, Euglena was added at a rate of 1% of the soybean weight during fermentation. Sensory evaluations were conducted on aspects like soybean odor, pleasant aroma, favorable taste, and overall assessment, using a 7-point rating scale. The results showed that Euglena-added tempe scored favorably in all evaluation criteria compared to conventional tempe. This study demonstrated that adding Euglena during the tempe fermentation process could enhance its flavor. Future research will quantitatively evaluate parameters such as antioxidant content and enzyme activity in Euglena-added tempe.

4. Challenges and Opportunities of Plankton-Based Food

When attempting to consume plankton directly suspended in water, as seen in some aquatic organisms, it is limited to methods like filter feeding, which involves using specialized appendages or gills to strain the plankton. From an engineering perspective, increasing the concentration of plankton is necessary, either through the use of centrifuges or membrane filtration methods like hollow fiber membranes. Moreover, from a culinary standpoint, merely compacting plankton is insufficient, as it lacks the desired texture and the mosaic-like distribution of major food components, such as chewiness and juiciness, which are considered important for overall satisfaction in food consumption. To address these challenges, there is potential in using extracts from microalgae, also known as phytoplankton or plant-like plankton.In terms of nutrition, concerns arise when consuming a single species of microalgae continuously, as it may lead to imbalanced nutrient intake. To tackle this, Algae Nutrition Formula™, an algorithm developed by Euglena Co., Ltd., aims to achieve a complete nutrition profile by combining various microalgae species. This research aims to realize a production module for continuous nutrient supplementation in the space environment using this algorithm and vital data. If successful, it could offer a new food system in space that efficiently recycles materials, sustains nutrient requirements, and enhances consumer satisfaction with limited space.Figure 7: Nutritional Analysis of Microalgae Euglena, Chlorella, Spirulina, and Nutritional Balance Image of Microalgae Mix

5. Expected Roles of Food in Space

Thus far, Euglena has demonstrated its potential as a nutrient-rich food source, a fermentation aid, and a contributor to the production of cultured meat. Moreover, Euglena exhibits various functional properties that support human health. For instance, a unique component of Euglena, paramylon, cannot be absorbed as a nutrient by humans but functions as dietary fiber. It has shown potential effects in reducing cholesterol absorption, alleviating atopic dermatitis, and modulating immune responses.Spaceflight is known to have adverse effects on human health due to factors like microgravity and high levels of cosmic radiation. In addition to conditions such as muscle atrophy, it has been observed that spaceflight can cause liver function impairments. In experiments involving rats, oral administration of paramylon induced the recovery of the enzyme superoxide dismutase (SOD), responsible for removing reactive oxygen species, when liver damage was induced. This suggests that Euglena consumption might be effective in mitigating oxidative stress-related issues that arise in the space environment.Using the "Sulfur Index Analysis" technique developed by the University of Tsukuba and Euglena Co., Ltd., a study was conducted in collaboration with the Japan Aerospace Exploration Agency (JAXA) on the livers of mice that underwent spaceflight. This investigation comprehensively examined the status of sulfur compounds central to redox reactions in living organisms. The results revealed a reduction in the levels of reducing sulfur compounds such as cysteine, ergothioneine, and glutathione in the livers of mice that went to space. These sulfur compounds are known contributors to antioxidant processes within the body. Their decreased levels are presumed to be due to their consumption in mitigating oxidative damage caused by reactive oxygen species during spaceflight. This finding highlights the potential effectiveness of sulfur-based antioxidants in maintaining health in space. Particularly noteworthy is the revelation that ergothioneine levels decrease by approximately half compared to ground levels during normal space activities. Ergothioneine cannot be synthesized within mammalian bodies and is only obtained in trace amounts from certain sources like mushrooms. Therefore, it may be challenging to replenish reduced ergothioneine levels through conventional dietary sources during space activities. This research outcome is expected to provide new guidance for the development of space food.

6. Conclusion and Future Prospects

In the realm of space exploration, considerations such as lunar base development and Mars colonization are underway. In anticipation of long-term stays on celestial bodies like the moon and Mars, the development of a stable and sustainable food supply system in low-gravity closed environments is imperative. When contemplating the production of higher plants like vegetables, the generation of inedible parts and residues poses a challenge. By utilizing microalgae like Euglena, efficient food production can be achieved, along with the assurance of nutrients that may be lacking in plant-based diets. Additionally, microalgae contribute efficiently to air and water regeneration.Ongoing efforts are directed toward the development of highly efficient food production methods using microalgae in closed-loop systems. These methods involve various microalgae species, artificial lighting, urine, and organic waste recycling solutions. Through these developments, the aim is to contribute to the establishment of methods that efficiently address resource production and recycling challenges, both on Earth and in space.

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acknowledgments

This work was supported by MAFF strategy project “Development of a highly resource-recycling food system that supports long-term stays on the moon, etc.” Grant Number JPJ01857.