Metabolic response of Chlorella vulgaris to a transient thermal environment for supporting simultaneous air revitalization and thermal control in a crewed habitat
Introduction
The multifunctionality and adaptability of bioregenerative technologies could benefit the environmental control and life support (ECLS) systems of long-duration human spaceflight. The advantage of bioregenerative technologies, specifically algal photobioreactors, includes concurrently addressing multiple ECLS functions, like fixation of cabin carbon dioxide (CO2), provision of useable oxygen (O2) and edible biomass, while progressing towards carbon loop closure. Earlier research has recognized these advantages with bioregenerative technologies. The Micro-Ecological Life Support System Alternative (MELiSSA) project models the interactions of its multiple bioreactors from lake ecosystems. This design incorporates higher plants, microalgae, and bacteria to close carbon loops and recycled solid, liquid, and gaseous wastes in long duration spaceflight [1]. An algal photobioreactor studying a portion of the MELiSSA system was flown to the ISS in 2017 and resulted in comparable oxygen production rates to the ground controls using Anthrospira [2]. The University of Stuttgart flew a membrane-based algal photobioreactor in 2019 to demonstrate integration with a life support rack on the International Space Station (ISS). The photobioreactor received a slip stream of CO2 from the life support rack and provided O2 to the cabin. The results of this experiment suggest mass savings in resupply missions when used in long duration flight [3]. Algae is typically included in studies for carbon loop closure due to its fast doubling time, high photosynthetic efficiency, and high edible biomass ratio [4]. Both referenced experiments used hydroponic algae, allowing for easy modification of biomass density (addition or removal) and nutrient supplementation through syringes and pump systems.
Spacecraft mass and volume comes at a premium for long-duration spaceflight. Therefore, water loops already allocated for cabin thermal control could house water-based algal media. Simultaneously addressing thermal control and atmosphere revitalization has the potential of closing the carbon loop for long duration spaceflight while minimizing the impact to launch mass, power, and volume, when compared to single purpose technologies. It is hypothesized that this design would allow algal media to pass through the entire cooling loop, minimizing current design modification. However, the operational temperature band of ISS cooling loops have a lower minimum temperature and cycle at a higher frequency than terrestrial, outdoor environments for algal cultures (Fig. 1) [[5], [6], [7]]. Minimizing the operational temperature of the spacecraft thermal loop minimizes the required working fluid volume [8]. Consequently, lowering the temperature of the algal medium used as the thermal loop working fluid, may reduce photosynthetic activities, thereby reducing the air revitalization capabilities of the culture [9].
Algal photosynthetic rate and media temperature are positively correlated within a species-dependent, ideal temperature range [[10], [11], [12]]. Chlorella's optimal photosynthetic temperature range has been reported as +26 °C to +36 °C, suggesting that peak photosynthetic rate may not occur if used for thermal control of the spacecraft cabin [9]. Ras mentions that outdoor reactors can experience daily fluctuations between +10 °C and +45 °C, extending most commercially-cultured species beyond their peak growth range [13]. Fortunately, algae are adaptable and are viable through diel oscillations occurring beyond optimum conditions [[14], [15], [16]]. Maxwell et al. successfully cultured Chlorella vulgaris at a constant +5 °C for 10 days, indicating that cultures are sustainable in colder temperatures [9,17]. This cooler temperature increased the doubling time from 8 to 48 h. The culture also increased its photosynthetic capacity through increasing Calvin cycle enzymes and reductions in cellular Chl content, reducing the probability of irradiance absorption, thereby reducing excitation pressure on PSII and preserving cell sustainment.
A limited number of the referenced studies investigating the effects of diurnal cycles in outdoor cultures included variance of irradiance and temperature (+3 °C to +28 °C). However, they suggest that there is no significant difference in specific growth rate between those cultures that experience temperature and irradiance cycling and constant temperature controls using time-averaged cycle temperature [[18], [19], [20]]. Davidson suggests that algae easily acclimate to temperature change in natural environments occurring over the course of weeks to 24 h, but states that little is known about the rate of acclimation, especially at shorter time scales [10]. The referenced experiments characterizing cellular response to various thermal environments occurred under steady-state temperatures or over 24 h, while spacecraft thermal loops can have a turnover rate of a few minutes. The referenced studies did not include cycle frequencies greater than once every 24 h.
This study investigated the impacts of a rapid, dynamic temperature environment to the oxygen production capabilities and viability of algae. The species Chlorella vulgaris was selected due to its heritage in spaceflight experiments, widely published terrestrial data, fitness for human consumption, and ability to grow in a wide range of pH, temperature, and light regimes [21,22]. This study aims to characterize any modifications to Chlorella's oxygen production capabilities when used to support spacecraft thermal control and provide additional information for ECLS systems designs that include bioregenerative technologies.
Section snippets
Culture maintenance
A parent Chlorella culture (Chlorella vulgaris, Carolina Biological) was kept in a clean, but not sterile lab environment, similar to ISS cabin conditions. Bold's (Bristol's) Modified Media (50x Bold Modified Basal Freshwater Nutrient Solution, Sigma Aldrich) [2.94 mM NaNO3, 0.17 mM CaCl2–H2O, 0.3 mM mgSO2-7H2O, 0.43 mM K2HPO4, 1.29 mM KH2PO4, 0.43 mM NaCl after final dilution] was used as the culturing media. The parent culture was placed under a bank of cool white fluorescent bulbs
Growth profiles
Both cultures experienced an expected three-day lag (or acclimation) phase, followed by exponential growth, as identified by the linear portions of the semi-log cell density plot (Fig. 4, A). Optical density showed a similar three-day lag phase before transitioning into exponential growth. Minimal cell division was measured through cell density of the cycled temperature plate (day 1 = 4.97 × 105 cell mL−1; day 7 = 6.10 × 105 cell mL−1, 22% increase), but more than doubled in optical density
Conclusions
The oxygen production rate and culture health through growth rate and chlorophyll quantum yield of C. vulgaris were characterized under two different temperature conditions: constant 19 °C and sinusoidal profile ranging from 9 °C to 27 °C. The temperature profile, range, and period reflect the conditions experienced inside of a water-based thermal control loop of the ISS. Previous studies examining culture response to cyclic outdoor environmental changes typically focused on simultaneous diel
Funding
This work was funded by a NASA Space Technology Research Fellowship, United States [NNX15AP52H].
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Special acknowledgement to BioServe Space Technologies for allowing this research within their environmental chambers, the use of their spectrophotometer and other various lab equipment, and feedback of their personnel on the execution of these experiments. Thank you to Dr. Daniel Barta for his patience and guidance throughout the NASA Space Technology Research Fellowship tenure. Thank you to the reviewers and their comments that greatly improved this work.
Dr. Emily E. Matula completed her Ph.D. at the University of Colorado Boulder in the Smead Aerospace Engineering Sciences department with a Bioastronautics focus. Her dissertation titled, “Characterization of photobioregenerative technology for simultaneous thermal control and air revitalization of spacecraft and surface habitats” focused on the potential of a multifunctional, bioregenerative environmental control and life support system. Specifically, using microalgae for carbon loop closure
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Dr. Emily E. Matula completed her Ph.D. at the University of Colorado Boulder in the Smead Aerospace Engineering Sciences department with a Bioastronautics focus. Her dissertation titled, “Characterization of photobioregenerative technology for simultaneous thermal control and air revitalization of spacecraft and surface habitats” focused on the potential of a multifunctional, bioregenerative environmental control and life support system. Specifically, using microalgae for carbon loop closure in air revitalization while using the water-based media for habitat or cabin thermal control.
Dr. James A. Nabity is an Associate Professor in the Smead Aerospace Engineering Sciences department at the University of Colorado Boulder with research focus in the field of Bioastronautics – the study and support of life in space. My research group develops ionic liquid membranes for atmosphere revitalization and CO2 capture, use ionic liquid solvents to extract minerals and oxygen from regolith, explore the effects of space radiation on habitat layout and crew performance, develop bioregenerative systems, and investigate heat transport and fluid flow in microgravity.
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