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.
Highlights
Yeast is an ideal cell model for space biology since it is a eucaryotic organism with fast growth rates whose metabolism has increasing importance in genomics and systems biology and shares many molecular processes with animals (Botstein et al, 1997; Scannell et al, 2007; Baryshnikova et al, 2010; Botstein and Fink, 2011; Botstein and Fink, 2011)
Despite the differences in accuracy, pH spots autoclaved once and multiple times gave reproduceable results that could be post-calibrated to account for offsets and drift during the 15 days experiments
For the 60 ml bioreactor, optical density (OD) in the 50 ml sampling chamber corresponded to a pathlength >14 mm for maximum light penetration
Summary
Yeast is an ideal cell model for space biology since it is a eucaryotic organism with fast growth rates whose metabolism has increasing importance in genomics and systems biology and shares many molecular processes with animals (Botstein et al, 1997; Scannell et al, 2007; Baryshnikova et al, 2010; Botstein and Fink, 2011; Botstein and Fink, 2011). Yeast is usually grown in bioreactors that come in various configurations (Walther 2002). Yeast bioreactors can be operated as batch, batch-fed, and continuous cultures. Batch and batch-fed cultures give unsteady state growth and limit the number of yeast generations produced. This is because as biomass accumulates in the bioreactor nutrients are rapidly depleted and cell growth rates decrease until cells reach the stationary phase when growth ceases
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