From urine to food and oxygen: effects of high and low NH4+:NO3- ratio on lettuce cultivated in a gas-tight hydroponic facility

In situ production of food, water and oxygen is essential for long-duration human space missions. Higher plants represent a key element in Bioregenerative Life Support Systems (BLSS), where crop cultivation can be based on water and nutrients recovered from waste and wastewater. Human urine exemplifies an important waste stream with potential to provide crops with nitrogen (N) and other nutrients. Dynamic waste composition and treatment processes may result in mineralized fractions with varying ammonium (NH4+) to nitrate (NO3-) ratios. In this study, lettuce was cultivated in the unique ESA MELiSSA Plant Characterization Unit, an advanced, gas-tight hydroponic research facility offering controlled environment and continuous monitoring of atmospheric gas composition. To evaluate biological and system effects of nutrient solution NH4+:NO3- ratio, two crop tests were run with different NH4+ to total N ratio (NH4+:N) and elevated concentrations of Na+ and Cl- in line with a urine recycling scenario. Plants cultivated at 0.5 mol·mol-1 NH4+:N (HiNH4+) achieved 50% lower shoot biomass compared to those cultivated at 0.1 mol·mol-1 NH4+:N (LoNH4+), accompanied by higher shoot dry weight content and lower harvest index. Analyses of projected leaf area over time indicated that the reduced biomass observed at harvest could be attributed to a lower specific growth rate during the close-to-exponential growth phase. The HiNH4+ crop produced 40% less O2 over the full cultivation period. However, normalization of the results indicated a marginal increase in O2 production per time and per projected leaf area for the HiNH4+ crop during the exponential growth phase, in line with a higher shoot chlorophyll content. Mineral analysis demonstrated that the biomass content of NH4+ and NO3- varied in line with the nutrient solution composition. The ratio of consumed NH4+ to consumed N was higher than the NH4+:N ratio of the nutrient solution for both crop tests, resulting in decreasing NH4+:N ratios in the nutrient solution over time. The results provide enhanced insight for design of waste processes and crop cultivation to optimize overall BLSS efficiency and hold valuable potential for improved resource utilization also in terrestrial food production systems

Perspectives for plant biology in Space and analogue environments

Advancements in plant Space biology are required for the realization of long-duration exploratoryclass manned missions, where the re-supply of resources from Earth is not feasible for technical and economic constraints. Plants are key organisms in Bioregenerative Life Support Systems (BLSS) for the regeneration of resources (i.e. oxygen production through photosynthesis, water recovery by transpiration, and wastes recycling) and the production of fresh healthy food. Moreover, plants play a role in psychological support for astronauts. The definition of cultivation requirements for the design, realization and successful operation of BLSS must take into account the effects of Space factors on plant growth, development and reproduction. Altered gravitational fields and radiation exposure are the main Space factors inducing changes in gene expression, cell proliferation and differentiation, signaling and physiological processes, with consequences on tissue organization and organogenesis, thus on the whole organisms functioning. In this paper, the main findings of gravityand radiation-related research of the last years are summarized, highlighting the knowledge gaps that is still necessary to fill. A focus on existing facilities as well as requirements for future facilities to achieve fundamental biology goals is reported. Possible future experiments in the short-mediumlong term are proposed to achieve the targets of crewed Space exploration

Perspectives for plant biology in space and analogue environments

Abstract Advancements in plant space biology are required for the realization of human space exploration missions, where the re-supply of resources from Earth is not feasible. Until a few decades ago, space life science was focused on the impact of the space environment on the human body. More recently, the interest in plant space biology has increased because plants are key organisms in Bioregenerative Life Support Systems (BLSS) for the regeneration of resources and fresh food production. Moreover, plants play an important role in psychological support for astronauts. The definition of cultivation requirements for the design, realization, and successful operation of BLSS must consider the effects of space factors on plants. Altered gravitational fields and radiation exposure are the main space factors inducing changes in gene expression, cell proliferation and differentiation, signalling and physiological processes with possible consequences on tissue organization and organogenesis, thus on the whole plant functioning. Interestingly, the changes at the cellular and molecular levels do not always result in organismic or developmental changes. This apparent paradox is a current research challenge. In this paper, the main findings of gravity- and radiation-related research on higher plants are summarized, highlighting the knowledge gaps that are still necessary to fill. Existing experimental facilities to simulate the effect of space factors, as well as requirements for future facilities for possible experiments to achieve fundamental biology goals are considered. Finally, the need for making synergies among disciplines and for establishing global standard operating procedures for analyses and data collection in space experiments is highlighted