Engineering photosynthetic systems for bioregenerative life support

El projecte MELiSSA (Micro-Ecological Life Support System Alternative) de lAgencia Espacial Europea (ESA) es un ecosistema artificial concebut com una eina per estudiar i desenvolupar la tecnologia per a sistemes de suport de vida biologics requerits per a missions tripulades de llarga durada a lespai. El fet que el projecte internacional MELiSSA es desenvolupa en cooperacio amb organitzacions de diferents paisos ha permes que el treball experimental daquesta tesi es realitzes part a la Planta Pilot MELiSSA (MPP), ubicada a la Universitat Autonoma de Barcelona (Spain), i part a Controlled Environmental Systems Research Facility de la University of Guelph (Canada). Inspirant-se en un ecosistema natural aquatic, el bucle MELiSSA produeix aliments, aigua i oxigen a partir de la degradacio dels residus organics (biomassa no comestible, femta, orina i CO2) utilitzant lactivitat combinada de diferents microorganismes i plantes superiors, que colonitzen cinc compartiments interconectats. Lobjectiu daquesta tesi es avancar en el desenvolupament dels compartiments fotosintetics del bucle per tal de ser integrats a la MPP. Concretament, el treball sha estructurat en 3 unitats principals. I Compartiment dArthrospira: Shan realitzat cultius en continu a diferents velocitats de dilucio i intensitats luminiques (seleccionades segons un disseny central composat tipus Box Wilson) per determinar els limits operacionals i la maxima productivitat del fotobioreactor a escala pilot dArthrospira. La productivitat mes alta aconseguida fou de 27 mg·L-1·h-1 a una velocitat de dilucio de 0.044 h-1 i 194 W·m-2. Sha estudiat la resposta dels cultius davant de pertorbacions afectant el pH i els cabals de liquid i gas. De forma mes detallada, sha avaluat lefecte de lamoni en la produccio i composicio de lArthrospira, determinant que per tal devitar linhibicio del creixement dArthrospira cal mantenir les concentracions damoni a lestat estacionari per sota de 5.6 mM. II Compartiment de Plantes Superiors: Shan realitzat cultius de remolatxa i enciam dins de cambres de plantes estanques per obtenir dades de referencia de productivitat, composicio, consum de nutrients i fixacio de carboni. La productivitat mitjana entre els 3 cultius en discontinu i els 2 en etapes es de 15.31 g dw·m-2·d-1 per remolatxa i de 13.85 g dw·m-2·d-1 per enciam. La mesura de la fixacio neta de carboni es una bona tecnica per estimar el creixement i la produccio de les plantes dintre les cambres sense utilitzar metodes destructius. A mes, sha provat que el consum de nutrients permet estimar el contingut mineral total dins la cambra utilitzant la produccio de biomassa. Tambe sha avaluat ladequacio dun model fotosintetic per estimar la produccio de biomassa dins la cambra. Sha conclos que el model hiperbolic es adequat per descriure la resposta fotosintetica duna fulla a diferents intensitats luminiques. A mes lestimacio dels corresponents parametres ha permes determinar que ni el rendiment quantic (?), ni la velocitat fotosintetica maxima (Pmax) ni la velocitat de respiracio (Rd) depenen de ledat de la planta i unicament la Pmax depen de la concentracio de CO2. III - Integracio dels Compartiments Fotosintetics: Shan dimensionat i dissenyat les cambres de plantes que sintegraran proximament a la MPP. Les 3 cambres de plantes amb una area de produccio de 5 m2 cada una tindran una produccio de biomassa comestible (remolatxa, enciam i blat) equivalent al 20% dels requeriments diaris dun huma. La configuracio seleccionada (una cambra allargada amb dues subcamares estanques a cada banda) permetra obtenir una produccio semicontinua de biomassa i assegurar destanqueitat del sistema. Finalment, sha avaluat limpacte de la integracio dels compartiments fotosintetics a la MPP desenvolupant un model que permet calcular els balancos de nitrogen, CO2 i O2 dins del bucle i determinar en quines condicions es possible aconseguir el tancament dels mateixos. The MELiSSA project (Micro-Ecological Life Support System Alternative) of the European Space Agency is an artificial ecosystem conceived as a tool to study and develop technology for a future biological life support system required for long term manned space missions. The fact that the MELiSSA project is formed by several independent organizations of different countries made possible that part of the experimental work of this thesis was carried out in the MELiSSA Pilot Plant (MPP) located at Universitat Autonoma de Barcelona (Spain) and the Controlled Environmental Systems Research Facility located at University of Guelph (Canada). Based on the principle of an aquatic ecosystem, MELiSSA aims to produce food, fresh water and oxygen from organic wastes (inedible biomass, faeces, urine and CO2) using the combined activity of several microorganisms and higher plants, which colonize five interconnected compartments. The main contribution of this thesis is in the engineering of the photosynthetic compartments and their integration into MPP. Particularly, the work has been structured in the following three main units. I - Arthrospira Compartment: Several continuous cultures have been carried out at different dilution rates and light intensities, planned using a Box-Wilson Central Composite Design, to determine the operational limits and maximum productivity of Arthrospira pilot plant photobioreactor. The highest Arthrospira productivity attained is 27 mg·L-1·h-1 at a dilution rate of 0.044 h-1 and a light intensity of 194 W·m-2. Disturbances of normal operating conditions affecting pH, liquid and gas flow rate influence Arthrospira growth has been studied. The effect of ammonium on Arthrospira production and composition has been evaluated in detail and it is determined that to avoid inhibition of the Arthrospira growth, the steady-state ammonium concentration must be lower than 5.6 mM. II Higher Plant Compartment: Three batch and two staggered cultures in sealed environment chambers have been performed to collect baseline data of productivity, tissue composition, nutrient uptake and canopy photosynthesis from beet and lettuce trials. The mean total plant productivity among batch and staggered cultures is 15.31 g dw·m-2·d-1 for beet and 13.85 g dw·m-2·d-1 for lettuce. The net carbon exchange rate technique is a good alternative to classical growth analysis for estimating plant growth and production inside the chamber without using destructive analyses. In addition to this, the ionic uptake of the nutrient solution has been proven to be a good predictor of total canopy mineral content using the estimated biomass. Moreover, the photosynthetic study performed at leaf level has contributed to estimates of light energy related parameters for the canopy model. The rectangular hyperbola model is suitable in defining the leaf photosynthetic response to light at different CO2 levels and crop ages. No significant differences are detected for the quantum yield (?) and dark respiration rate (Rd) among CO2 levels, but in contrast, the maximum photosynthetic rate (Pmax) was found to depend on CO2 concentration. Moreover, it is observed that that ?, Pmax and Rd values remain constant through crop development. III - Photosynthetic Compartments Integration: The HPC prototype to be integrated into the MPP has been designed. It is concluded that 3 HPC prototypes with 5 m2 of growing area each, will be constructed to provide 20% of the daily crew diet with beet, lettuce and wheat. The selected configuration, an elongated chamber with two air-locks at each end, allows the semi-continuous biomass production while ensuring gas environment isolation. Finally, the impact of the integration of the photosynthetic compartments into the MPP has been evaluated using a static mass balance model for assessing the nitrogen, CO2 and O2 balances, while determining the conditions under which the closure of the mass balances can be expected.

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