ACTA MICROBIOLOGICA BULGARICA Volume 36 / 1 (2020) Review Capacity of fungi for biodegradation of cellulose wastes generated at manned space flight Ekaterina Krumova, Hristo Najdenski, Vladislava Dishlijska, Radoslav Abrashev, Galina Stoyancheva, Yana Gocheva, Nedelina Kostadinova, Jeni-Miteva-Staleva, Boryana Spasova, Maria Angelovа* The Stephan Angeloff Institute of Microbiology, Bulgarian Academy of Sciences, Sofia, Bulgaria Abstract The amount of waste generated on manned long-duration space missions away from Earth orbit creates the daunting challenge of how to manage the waste through reuse, rejection, or recycling. Microbial degradation, for both economic and ecological reasons, has become an increasingly popular alternative for the treatment of cellulose containing organic waste in the space stations. This approach offers several advantages: low energy, mild operation conditions, control on biological hazard of the wastes, etc. Fungi are the main cellulose-degrading microorganisms in the nature and their cellulolytic features are object of intensive studies. This review describes the ability of anaerobic, aerobic, and microaerophilic fungi to degrade organic wastes generated during manned space flight. Keywords: fungi, celullose-containing waste, biodegradation, manned space-flight Резюме По време на пилотируемите космически мисии, далеч от земната орбита в станциите се натруп- ват големи количества органични целулоза съдържащи отпадъци. Тяхното управление чрез повтор- на употреба, изхвърляне или рециклиране е изключително предизвикателство за науката. Микроб- ното разграждане на място, в космическите станции, се превръща във все по-популярна алтернатива по икономически и екологични причини. Този метод предлага няколко предимства: ниска енергия, меки условия на работа и контрол върху биологичната опасност на отпадъците. Гъбите са основните микроорганизми, които разграждат целулозата в природата и техните целулозолитични свойства са обект на интензивни проучвания. Този обзор разглежда способността на анаеробните, аеробните и микроаерофилните гъби да разграждат органичните отпадъци, натрупвани по време на пилотиру- емите космически полети. Introduction Popular depictions of space exploration as difficulties and challenges for the each manned well as government life support research programs space mission. These materials must be processed have long assumed that future planetary bases onboard the spacecraft, which requires technolo- would rely on small scale, closed ecological sys- gies based on low energy consumption, high pro- tems with crop plants producing food, water, and cessing speed, on the area, volume and mass of the oxygen and with bioreactors recycling waste. processing system. Microbial degradation, for both Among the wastes that are formed in the con- economic and ecological reasons, has become an ditions of manned space flight, a significant pro- increasingly popular alternative for the treatment of portion are cellulose-containing materials such as organic waste in the space stations. It offers several personal hygiene products, uneaten parts of plants advantages: low energy, mild operation conditions from greenhouse, etc. (Ilyin et al., 2004; 2018). and control on biological hazard of the wastes. Organic waste collection and containment present * Corresponding author: [email protected] 19 For the most part the organic wastes are dis- rates of celluloses. posable personal hygiene items used in large quan- Native cellulose (exemplified by cotton tities containing human body products, which are wool) has a greater degree of polymerization and a very dangerous from the sanitary-epidemiological smaller fraction of reducing end groups than treat- standpoint. Thus, it is very important to develop a ed cellulose (e.g. tissue) and therefore the rate and process of biotransformation of the used personal extent of hydrolysis of cotton wool is less than that hygiene products of cosmonauts, as well as vege- of tissue (Bassil et al., 2015). table and liquid organic waste, to the life support Cellulose and hemicellulose have very dif- systems of space crews. ferent physical and chemical characteristics which Structure of cellulose materials have direct relevance to their degradation in repos- Cellulose-containing wastes include agricul- itories e.g. cellulose is insoluble in alkali and hemi- ture residue, water plants, grasses, and other plant, cellulose is soluble. Hemicellulose is branched food and higiene materials and substances. In gen- polymer which contain a range of different mono- eral, agricultural waste biomass is composed of cel- saccharides. It has considerably smaller degree of lulose (C H O )n, hemicellulose (C H O )m, lignin polymerisation when compared to cellulose. Hemi- 6 10 5 5 8 4 celluloses from different plant species have dif- [(C9H10O3(OCH3)0.9–1.7]x, pectin, extractives, glycosylated proteins and several inorganic mate- ferent compositions but the major constituents are rials (Akhtar et al., 2015). The main occurrence of xylans, galactoglucomannans, glucomannans and cellulose is the accessible lignocellulosic substance arabinogalactans. in forests, with wood as the most essential source Lignin, the third largest available biopolymer (Sundarraj and Ranganathan, 2018). Cellulose oc- in nature, consists of a phenyl propane (p-coumaryl curs in almost pure form in cotton fiber (Kaliaet al., alcohol, coniferyl alcohol and sinapyl alcohol) unit 2011). However, in wood, plant leaves and stalks, it linked with ester bonds that forms a complex with is found in combination with other materials, such hemicellulose to encapsulate cellulose, making it as lignin and hemicelluloses. In addition to cellu- resistant towards chemical and enzymatic hydrol- lose and hemicellulose, wood also contains lignin. ysis. Lignin-enriched biomass is more resistant for Cellulose is the most abundant form of or- the depolymerisation of holocellulose (cellulose ganic waste, including these of space stations. It is and hemicellulose) to produce fermentable sugars. a fibrous, tough, and water-insoluble polymer and In addition, during the degradation process, it may plays an essential role in maintaining the structure form furan (furfural and hydroxymethyl-furfural) of plant cell walls. It has been shown to be a long- compounds that could inhibit fermentation (Akhtar chain polymer with repeating units of D-glucose et al. 2015). In generally, the saccharification of linked by b-1,4-glycosidic bonds to form linear lignocellulosic biomass is still technically prob- polymeric chains of over 10 000 glucose residues. lematic because of digestibility of cellulose, which These residues are highly organized into micro and is hindered by structural and compositional factors macrofibrils. (Akhtar et al., 2915). Cellulose contains both highly crystalline Cellulose-containing wastes generated during regions where individual chains are linked to each the manned space flights other and less-ordered amorphous regions; material Recovery of various organic wastes accumu- in amorphous regions may not be able to partici- lated in the manned space flight is an actual prob- pate in reactions if blocked by a preceding crystal- lem of modern astronautics and future interplane- line region. The stabilities of crystalline forms are tary missions. Optimal and rapid biodegradation similar and may depend on the temperature. A va- of lignin and other cellulosic materials by fungi is riety of different crystalline forms of cellulose are paramount in the use of these organisms to achieve known. Cellulose Ia, Ib are meta-stable and after effective biomass recycling in ALS. It is thus im- recrystallisation or treatment with strong base yield perative for a spacecraft’s environmental control cellulose II. In each crystalline state the patterns of and life support system (ECLSS) to be able to han- hydrogen bonding differ; the orientation of chains dle all types of waste, be they solid, liquid, or gase- differs between phase I, where the chains are paral- ous. Such waste accumulation also presents consid- lel, and phase II where the chains are anti-parallel. erable vehicle storage and crewmember handling The different phases have different chemical and challenges. Each space mission needs logistics that physical properties although it is not known to what includes, but are not limited to, water, oxygen, ni- extent these differences influence the degradation trogen, clothing, waste collection, hygiene, health- 20 care and consumables (Lopez et al., 2015; Wang et ferent specificities, working together. Cellulases are al., 2018). A necessary element of the closed LSS enzymes that hydrolyze the β-1,4-glycosidic link- of manned spacecraft is considered greenhouses ages of cellulose, such as endo-1,4-β-glucanases for the reproduction of the vegetative part of the (Egs, EC 3.2.1.4), cellobiohydrolases (or exo-1,4- diet of cosmonauts. Among the crops suggested for β-glucanases) (CBHs, EC 3.2.1.91) and β-glucosi- growth in space are wheat, rice, carrots and mush- dases (EC 3.2.1.21) (Pérez et al., 2002). EGs break- rooms (Nyochembeng et al., 2019). Uneaten parts down cellulose by attacking the amorphous regions of plants are waste from greenhouses. Cellulosic to produce more accessible new free chain ends wastes include feces, food refuse, paper, cotton, for the action of CBHs. An effective hydrolysis of clothing, wipes, grey tapes etc. In addition, wastes cellulose also requires b-glucosidases, which break may be in mixed form such as “semi-solid” (Pul- down