Coupling urine treatment and water recycling with Limnospira Indica cultivation for air revitalisation and food production under closed loop conditions

Long-duration human space missions require considerable amounts of water, oxygen (O2) and nutritious biomass to support the crew. Carrying sufficient supplies for 5-6 persons, for a return trip to Martian or Lunar bases is not feasible with present launch technology. Additionally, the spaceflights must be well-equipped to deal with metabolic waste streams. It is therefore important to look for practical alternatives to support life on such missions. One solution is to aim at self-sufficiency in terms of food, water and O2 production and waste management by employing regenerative life support systems (RLSS) like ESA's MELiSSA (Micro-Ecological Life Support System Alternative). The cyanobacterium Limnospira indica (formerly Arthrospira sp. PCC 8005) has been reported to assimilate both organic and inorganic nitrogen (N) sources. The possibility to use nitrified urine, as recycled source of nitrogen, water and other elements, and gaseous carbon dioxide, as recycled carbon source, for Limnospira indica cultivation could facilitate oxygen and food production. However, the high concentrations of various salts in urine have been reported to be inhibitory for Limnospira indica growth. Furthermore, source separated urine is highly unstable and thus needs to be stabilized to avoid the loss of nutrients, downstream usage problems and unsafe emissions. Partial nitrification of urine generates different N forms, mainly nitrate but in case of process instability also nitrite. It is therefore important to investigate the tolerance of Limnospira indica to high concentrations of different salts and N-sources (as mixture), in order to valorize urine as Limnospira indica medium. The present study evaluated the salt tolerance and the effect of mixed N-sources on the growth, the biochemical composition and the general metabolism of Limnospira indica. The preliminary experiments indicated that Limnospira indica can grow at high concentrations of salt which are normally present in human urine. In fact Limnospira indica cells exhibited comparable growth rates in Zarrouk Medium and Zarrouk medium supplemented with a 'urine-like' salt concentration. The effect of high salt concentrations on the metabolic and physiological characteristics of Limnospira indica has been analyzed. The study results clearly indicated that it is possible to valorize nitrified urine and N-source mixture for Limnospira indica cultivation. These results have opened new avenues towards solving the issues of wastewater remediation using photosynthetic cells. Such a possibility would further bring us step closer towards the (1) creation of photosynthetic biorefineries for production of high-value co-products using cheaper nutrients and (2) creation of waste-to-value based circular economy by embedding cultivation of photosynthetic microorganisms with wastewater remediation.