Micro-Bioreactors in Space: Case Study of a Yeast (Saccharomyces cerevisiae) Bioreactor With a Non-Invasive Monitoring Method

Abstract

Bioreactors in space have applications from basic science to microbial factories. Monitoring bioreactors in microgravity has challenges with respect to fluidics, aeration, sensor size, sample volume and disturbance of medium and cultures. We present a case study of the development of small bioreactors and a non-invasive method to monitor dissolved oxygen, pH, and biomass of yeast cultures. Two different bioreactor configurations were tested for system volumes of 60 ml and 10.5 ml. For both configurations, the PreSens SFR vario, an optical sensor array, collected data autonomously. Oxygen and pH in the cultures were monitored using chemically doped spots, 7 mm in diameter, that were fixed to the bottom of sampling chambers. Spots emitted a fluorescent signal for DO and pH when reacted with oxygen molecules and hydrogen ions, respectively. Biomass was sensed using light reflectance at centered at 605 nm. The, optical array had three light detectors, one for each variable, that returned signals that were pre- and post-calibrated. For heterotrophic cultures requiring oxygen and respiring carbon dioxide, a hollow fiber filter, in-line with the optical array, oxygenated cells and remove carbon dioxide. This provided oxygen levels that were sufficient to maintain aerobic respiration for steady state conditions. Time series of yeast metabolism in the two bioreactors are compared and discussed. The bioreactor configurations can be easily be modified for autotrophic cultures such that carbon dioxide is enhanced and oxygen removed, which would be required for photosynthetic algal cultures.

Microbial Factories and Exploiting Synergies of Bioreactor Technologies to Produce Bioproducts

“…Growth rates were significantly different for all periods (df = 5, F = 5771, p < 0.01), indicating that growth was not steady-state. The changes in pH correlated with changes in pCO 2 , although other metabolites produced during growth could have contributed to pH [76]. It is certain that the two peaks in pH on days 4.5 and 6.5 were the result of dilution caused by the lower CO 2aq concentration of the medium.…”

“…The carbon source was 7.5 g L −1 D-glucose. For pulsed feeding of the yeast bioreactor, a 10-fold concentrated medium was used [76].…”

“…Briefly, this optical system uses chemically doped spots to measure pH and DO. Spots were glued inside a 2 mL volume optical chamber [76], which was aligned with the optical sensors of the SRF array. A second set of times series was also run for 1-2 days and data are statistically presented.…”

“…For the DO calibration, air was bubbled in the media at 4 L min −1 until the media was saturated. The deoxygenated medium was obtained by adding NaSO 3 until DO was below the detection limit, near zero [76]. For all experiments, DO was corrected for changes in temperature and atmospheric pressure.…”

“…The oxygen uptake rate, OUR (Equation ( 2)), was estimated at 8.6 g (L d) −1 based on the difference between the DO saturated medium (6.2 mg O 2 L −1 ) and the DO depleted bioreactor (1.6 mg O 2 L −1 ) and accounting for oxygen transfer through the membrane (Table 4). Using the same bioreactor set-up but for a continuous culture, Granata et al [76] showed that yeast had a maximum OUR of 14.3 g (L d) −1 , which was 40% higher than the yeast cultures in Figure 1a. The higher OUR in their experiments was probably the result of higher growth rates (4.8 d −1 ) and the operation of the bioreactor in steady-state conditions (i.e., continuous culture).…”