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Simultaneous pH, CO2 and O2 Measurements in Algae Photobioreactor
pH Flow-Through Cells successfully evaluated for later use in the International Space Station ISS
Gisela Detrell and Jochen Keppler
Institute of Space Systems, University of Stuttgart, Germany
At the Institute of Space Systems several experiments with algae photobioreactors were conducted, in order to design a system to be tested in the International Space Station in collaboration with DLR and Airbus Defence and Space. As different parameters can be varied, such as algae circulation velocity, type and intensity of illumination, etc., the performance of the algae needs to be evaluated. It can be analysed from the oxygen (O2) production rate and the carbon dioxide (CO2) consumption. For monitoring these parameters PreSens O2 and CO2 sensor spots integrated in flow-through cells are used, which are read out with the OXY-4 mini and the pCO2 mini meters. Moreover, PreSens pH sensor spots integrated in flow-through cells and operated with the pH-1 mini are applied to ensure that the optimum pH range for algae growth is kept.
The use of biological technologies, for example algae, will play an important role in closing the carbon cycle in life support systems for future human space flight missions. Algae are able to produce O2 from CO2 and generate edible biomass through photosynthesis. However, before algae can be part of the life support system, several tests need to be carried out, as different design parameters such as the algae type, the photobioreactor design or light, will have a high influence on algae performance in space. To evaluate the growth performance of the algae culture, different parameters can be monitored: O2 production rate, CO2 consumption rate and cell density. At the Institute of Space Systems, several tests are currently carried out in breadboard models, in order to design an experiment to be launched to the International Space Station, in 2018. For this tests, several PreSens sensors are used: the sensor spots O2, allowing non-invasive measurements of both dissolved O2 concentration in the algae solution and gaseous O2 concentration in the atmosphere inside the experiment box; the sensor spots CO2, to measure dissolved CO2 in the algae solution; and the sensor spots pH, to measure the pH of the algae solution.
Materials & Methods
Several experiments are being carried out on Earth, in order to define the proper design of the space experiment, for example, test of different reactor geometries or different LED combinations. Therefore, a breadboard set-up (Fig. 1) has been built at the Institute of Space Systems (IRS). The breadboard includes all subsystems / components that will be required in the flight design:
- algae cultivation loop (a reactor, a pump and tubing)
- illumination by LED panels
- gas management (CO2 source, gas exchange membrane, O2 removal)
- temperature control (cold plate and ventilation)
- sensoric monitoring
The experiment compartment (EC) contains an atmosphere with 7 - 9 % CO2, a maximum of 25 % O2 and N2. The pressure inside the EC is kept to atmospheric level by venting excess air. This atmosphere is in contact with the algae inside the reactor chamber through a gas exchange membrane: CO2 diffuses in the reactor, the algae 'consume' it and release O2, which diffuses out into the EC. When the maximum O2 level is reached, the experiment compartment is flushed with N2. As CO2 is being consumed by the algae, new CO2 is introduced to the EC to keep the level within the required limits. The algae are constantly kept in motion inside the reactor with pumps, circulating through the algae loop. Connected to this loop, several sensors provide information about the algae performance: CO2, O2, pH and algae density. Illumination is provided by LED panels, the intensity of which can be changed during the experiments. Several temperature sensors allow for a thermal control, keeping a temperature range between 26.8 - 27.2 °C. Finally, a nutrients supply and harvesting device allows to provide the algae with the required nutrients while extracting part of the cultivation for further analysis.
The PreSens sensors (CO2, O2 and pH) are located in T-connectors integrated in the algae loop (Fig. 2). Moreover, an extra O2 sensor has been used for some experiments to measure the oxygen concentration in the experiment compartment.
Several experiments are carried out to test and improve the flight model design and to better understand the behaviour of the algae under different conditions. An example of such an experiment is the analysis of how the absence of light affects the algae. In this experiment light was switched off for a defined time period (about 2.5 hours). Special focus was kept on the dissolved gas concentrations (O2 and CO2) and how they developed during this time period. Additionally, the pH value was monitored.
Figure 3 shows the experiment data. As soon as the light is switched off, a significant drop in the dissolved O2 concentration can be observed. Simultaneously a significant increase in dissolved CO2 concentration occcurs and along with it a drop in pH values can be monitored. When the light is switched on after the dark phase, the dissolved O2 concentration and pH rise again while CO2 drops. This results show an expected reaction of the algae: during absence of light, the algae metabolize ('breathe') O2, resulting in a rapid decrease of the dissolved O2. Simultaneously, the algae produce CO2, which leads to an increase in dissolved CO2 concentration. As a result pH drops, since CO2 is acidic. When the light is switched on again, the algae go back into their normal metabolism of CO2. Their photosynthetic activity then leads to the formation of O2. Since the dissolved CO2 concentration is lowered, pH values increase again.
PreSens optical sensors (for CO2, O2 and pH) were successfully used for monitoring in an algae suspension and helped gaining a better understanding of our algae's photosynthetic activity. In the flight model, however, PreSens O2 and CO2 sensors will not be applicable, as an oxygen sensor with analog signal will be required, as well as a CO2 sensor for measurements in the gas phase. The pH flow-through cell, on the other hand, will be part of the monitoring system in the photobioreactor experiment, which will be brought to the ISS in 2018.
One of the experiments carried out in pretests has analysed the effects of absence of light on the algae. From the experiment it can be clearly observed that in dark conditions the algae breathe O2 and produce CO2, as we humans do, but as soon as the light is switched back on the algae recover their expected funciton for the space application: O2 production from the CO2 released by humans.
 S. Belz, J. Bretschneider, H. Helisch, G. Detrell, J. Keppler, W. Burger, A. Yesil, M. Binnig, S. Fasoulas, N. Henn, P. Kern, H. Hartstein, C. Matthias. Preparatory Activities for a Photobioreactor Spaceflight Experiment enabling Microalgae Cultivation for supporting Humans in Space. 66th International Astronautical Congress, Jerusalem, Israel. 2015.