MELiSSA (Micro Ecological Life Support System Alternative) is developing bioregenerative Life Support technologies for long-term Space missions. The MELiSSA concept is conceived as a loop with several compartments, each one performing a specific function, providing all together edible material production, atmosphere regeneration and water recovery with a concomitant use of wastes, i.e., CO2 and organic wastes. Each one of the compartments is colonized with specific bacteria or higher plants depending on its specific function. The MELiSSA Pilot Plant is a facility designed for the terrestrial demonstration of this concept, hosting laboratory rats as a crew mock-up mimicking the respiration of humans. Currently, the MELiSSA Pilot Plant focus on the integration of three compartments: Compartment 3 (nitrifying packed-bed bioreactor based on the co-culture of immobilized Nitrosomonas europaea and Nitrobacter winogradsky), compartment 4a (an air-lift photobioreactor for the culture of the edible cyanobacteria Limnospira indica with concomitant oxygen production) and Compartment 5 (an animal isolator with rats as mock-up crew). The output from these tests shows a high robustness and reliability and the performance of oxygen producing and oxygen consuming compartments is successfully demonstrated under transitory and steady-state conditions. This contribution reports on the current state of development of the MELiSSA Pilot Plant Facility and the most recent results of the integration work.

A new method to determine the hemispherical incident light (photon) flux density onto cylindrical photoreactors or photobioreactors is presented. It applies to situations where the photo(bio)reactor is radially illuminated by surrounding artificial sources or solar light. It relies on the direct measurement of a fluence rate with a spherical sensor put at the center of the reactor. Theoretical relations leading to the calculation of the hemispherical incident light flux density from the fluence rate value are established, and a web application performing those calculations is made available. It relies on a view factor, the expression of which, established for the first time with any assumption of the angular distribution of light at the boundary, is given. This requires a proper definition of the degree of collimation for the incident radiation field. Two different and complementary experimental devices are used to validate the method. As a result, the proposed method appears to be simple and reliable; it even looks faster and more accurate than actinometry for this particular geometry.

MELiSSA (Microecological Life Support SystemAlternative) is a developing technology for regenerative life support to enable long-term human missions in Space and has developed a demonstration Pilot Plant. One of the components of the MELiSSA Pilot Plant system is an 83L external loop air-lift photobioreactor (PBR) where Limnospira indica (previously named Arthrospira sp. PC8005) is axenically cultivated in a continuous operation mode for longperiods. Its mission is to provide O 2 and consume CO 2 while producing edible material. Biological and process characterization of this PBR is performed by analysing the effect of two main variables, dilution rate (D) and PFD (Photon Flux Density) illumination. A maximum oxygen productivity (r O 2 ) of 1.35 mmol l −1 h −1 is obtained at a D of 0.025 h −1 and PFD of 930 µmol m −2 s −1 . Photoinhibition can occur when a 1 g l −1 cell density culture is exposed to PFD higher than 1700 µmol m −2 s −1 . This process is reversible if the illumination is returned to dim light (150 µmol m −2 s −1 ), proving the cell adaptability and capacity to respond at different illumination conditions. Influence of light intensity in cell composition is also described. Specific photon flux density (qPFD) has a direct effect on phycobiliproteins and chlorophyll content causing a decrease of 62.5% and 47.8%, respectively, when qPFD increases from 6.1 to 19.2 µmol g −1 s −1 . The same trend is observed for proteins and the opposite for carbohydrate content. Morphological and spiral structural features of L. indica are studied by confocal microscopy, and size distribution parameters are quantified. A direct effect between trichome width and CDW/OD ratio is observed. Changes in size distribution are not correlated with environmental factors, further confirms the adaptation capacity of the cells. The systematic analysis performed provides valuable insights to understand the key performance criteria of continuous culture in air-lift PBRs.