Reliability of photobioreactors in life support systems: an investigation of methods to restore Chlamydomonas reinhardtii cultures after heat stress

Microalgal cultures in photobioreactors are essential in biological life support systems for space flight. However, photobioreactor cultures are sensitive to environmental parameters outside of their tolerance range, and crew time for repair is limited. This work aimed to increase resilience and decrease vulnerability of photobioreactor cultures by exploring methods for restarting photobioreactor cultures after heat shock, with as little use of crew time as possible. Using the microalga Chlamydomonas reinhardtii, two paths for restarting a photobioreactor culture were explored. In the first path we tested the heat resistance of C. reinhardtii biofilms, hypothesizing that microalgal biofilms would have a higher heat tolerance than planktonic cultures. That would make it possible to use a subset of microalgal culture, grown as a biofilm inside the planktonic photobioreactor culture, to restart the planktonic culture after a heat shock event, making it essentially self-restarting. As biofilm substrate we used alumina (Al2O3). Several combinations of porosities and surface functionalization were tested for biofilm growth and potential harm to cells. Alumina with pore sizes around the size of a C. reinhardtii cell had the most cells attaching to them. To test heat tolerance of C. reinhardtii biofilm, and compare it to that of planktonic cultures, we designed a heat stress test setup. We could not, however, detect any difference in heat stress tolerance between C. reinhardtii biofilm and C. reinhardtii planktonic culture. The second path explored the suitability of a silica sol-gel, manufactured with a novel method, for encapsulation of C. reinhardtii cells for medium term storage. The silica sol gel manufacturing method has been designed to be less stressful to biological cells and also consists of fewer steps than other similar methods described in literature. The method proved successful, the algal cells survived and stayed healthy for several weeks.