TY - JOUR
AU - Aggelis,, George
AB - EDITORIAL LETTER Our society currently faces the twin challenges of resource depletion and waste accumulation leading to rapidly escalating raw material costs and increasingly expensive and restrictive waste disposal issue. A plethora of agricultural, aquaculture, food-processing and industrial activities generates enormous quantities of organic wastes and residues every year, the eco-friendly treatment of which constitutes a very important challenge of the society and the authorities. These types of wastes and residues include but are not limited to hydrophilic liquid or solid materials deriving from various food-processing facilities (i.e. sugar-rich wastewaters, expired sucrose-rich drinks, coffee wastes, etc.), hydrophobic residues/renewable materials (i.e. cooked/waste lipids, low-cost industrial fats, stearins, etc.), biodiesel-plant residues (mostly biodiesel-derived glycerol), lignocellulosic materials (i.e. woody biomass, grass, forestry and agricultural wastes, spent mushroom substrates, process waters rich in lignocellulosic sugars, etc.), low-purity sugars (i.e. unpurified xylose amenable to be converted into xylitol, very high polarity sucrose), aquaculture wastewaters, etc. (Koutinas et al. 2014; Coma and Chatzifragkou 2019; Dourou et al. 2020; Kothri et al. 2020). The majority of solid or semi-solid waste streams mentioned above, is currently utilized as animal feed, fertilizers and substrates for composting or vermi-composting processes. Likewise in various cases, several of these solid waste-streams previously mentioned are not at all treated since they are directly land-filled or burnt, with evident very important negative impact on the environment (Coma and Chatzifragkou 2019). As far as the food-wastes are considered, about 33% of the foodstuffs, totaling about 1.3 billion tons annually, are wasted around the world as indicated by the Food and Agriculture Organization of the United Nations 2019 report (Kothri et al. 2020). In European Union the food waste production is up to 98 million tons per year and is expected to reach around 139 million tons by 2020 (Kothri et al. 2020), while it is noted that 45% of the total food losses and wastes was found at the household level (Coma and Chatzifragkou 2019). The removal of non-toxic and non-hazardous waste materials deriving from the food industry (mostly in order to be converted into compost), costs around 0.4–0.7 US$ per kg of waste, thus, it can easily be understandable that the benefit for the food-processing plant would be very important in relation with the development of processes beneficially converting these residues in situ (Papanikolaou and Aggelis 2010). Likewise, in many instances and specifically when combinations of organic wastes are done in order to impose an organic load that is not excessively high, low-value valorization of these materials can be performed (i.e. their conversion into biogas). However, the presence of various carbon sources, proteins and minerals found in these wastes/residues, makes them suitable as growth media for various types of heterotrophic microorganisms for the purpose of higher-value microbial valorization. Indeed their utilization can result in the synthesis of various microbial metabolic compounds that present several types of applications in the food, chemical and biofuel industries (i.e. production of microbial oil not presenting particular fatty acid composition amenable to be converted into 2nd generation biodiesel, production of microbial oil containing unusual fatty acids like polyunsaturated fatty acids of high nutraceutical value, erucic acid, etc., biohydrogen, microbial polysaccharides, microbial enzymes, organic acids, polyols, etc.). The last years there has been an increasing interest concerning the treatment of several types of emerging agro-industrial residues, viz. residues the appearance and production of which has presented a significant rise the last years due to the various anthropogenic activities. Petrosyan et al. (2020) developed a process in which wild-type (wt) or genetically engineered Escherichia coli strains have been tested as regards their potential to convert spent coffee grounds (SCG) hydrolysate into hydrogen (H2). Strains tested were the wt BW25113 and the defects in hydrogen (H2)-producing/oxidizing four hydrogenases (Hyd; i.e. ΔhyaB, ΔhybC, ΔhycE, ΔhyfG). A septuple mutant (ΔhyaB, ΔhybC, ΔhycA, ΔfdoG, ΔldhA, ΔfrdC, ΔaceE) was also produced. These strains were investigated concerning their potential to produce H2 from SCG hydrolysate generated from both soluble and instant coffee wastes (Petrosyan et al. 2020). With the process proposed, the wt strain grown on SCG hydrolysate produced H2 with rate of 1.28 mL H2 per g of sugar per h, yielding in the production of 30.7 mL H2 per g of sugar, while the septuple mutant, produced H2 at 72 mL per g of sugar with a rate of 3 mL of H2 per g of sugar per h. It was concluded that genetic modifications of E. coli together with control of external parameters during growth could lead to prolonged and enhanced microbiological H2 production from organic wastes through a completely eco-friendly way based on the sustainable and zero-waste release process. Large quantities of substrate colonized by fungal mycelia, termed spent mushroom substrate (SMS), are discarded after mushroom harvesting (5 kg SMS per kg of mushrooms produced), and because its volume increases annually, the mushroom industry is currently facing challenges concerning SMS valorization and management. Spent mushroom substrate is composed of mushroom mycelia, whereas significant amounts of hemicellulose, cellulose and lignin also exist that remain unused after mushrooms cultivation (Economou, Philippoussis and Diamantopoulou 2020). Moreover, SMS contain extra-cellular lignin-modifying enzymes that had been secreted during substrate colonization (Philippoussis and Diamantopoulou 2012). Economou, Philippoussis and Diamantopoulou (2020) investigated the ability of Pleurotus ostreatus SMS (supplemented with N-rich materials) to support a second crop of the edible and medicinal P. ostreatus and P. pulmonarius mushrooms, evaluated their productivity and determined the nutritional value of their sporophores. The biosynthesis of mycelial mass, laccase enzyme and polysaccharides was estimated in the SMS before, during and after the second cycle of Pleurotus mushroom cultivation. Finally, the potential of crude laccase enzyme obtained from SMS during Pleurotus cultivation to remove phenolic compounds from olive mill and winery wastewaters, abundant residues deriving from several food-processing activities, was evaluated. The expected increase of worldwide biodiesel production from edible vegetable oils to 30 million tons by 2021 is expected to generate a quantity of c. 3 million tons of glycerol (purity ≈ 90% w/w), only from 1st generation biodiesel production (Koutinas et al. 2014). On the other hand, further significant glycerol glut is envisaged due to the 2nd generation biodiesel production and also due to other possible industrial routes of glycerol accumulation into the market volume (Papanikolaou and Aggelis 2009; Diamantopoulou et al. 2020). Therefore, alternative ways of glycerol valorization seem currently of very high importance; in one such approach, Poladyan et al. (2020) used waste glycerol that was converted into microbial biomass, consequently providing valuable biocatalysts promoting the generation of electrical current in microbial fuel cells. [NiFe]–Hydrogenases (Hyds) of Escherichia coli and Ralstonia eutropha could be applied as potential anode biocatalysts in MFCs. E. coli K12 whole cells or crude extracts and R. eutropha HF649 synthesizing Strep-tagged membrane-bound Hyds were evaluated as anode enzymes in a bio-electrochemical system. Mediators like ferrocene and its derivatives were implicated. The maximal level of bio-electrocatalytic activity of Hyds was demonstrated at 500 mV voltage. Depending on the mediator and biocatalyst, current strength varied from 5 to 42 μA. Introduction of ferrocene-carboxylic acid enhanced current strength; moreover, the current flow was directly correlated with H2 concentration. The maximal value (up to 150 μA) of current strength was achieved with a 2-fold hydrogen supply. It may be inferred that Hyds are efficiently produced by E. coli and R. eutropha grown on glycerol, while ferrocene derivatives act as agents mediating the electrochemical activity of Hyds. Gajdoš et al. (2020) investigated the possibility to cultivate mutant strains of the non-conventional yeast Yarrowia lipolytica on media composed of crude glycerol, where the conversion of the intra-cellular oleic acid (Δ9C18:1) into erucic acid (Δ13C22:1) was considered. The later fatty acid presents several interesting applications such as its use as lubricant, surfactant and composite material, whereas it can also be used as starting material for biodiesel synthesis. Initially in the strain Po1d the production of cellular Δ9C18:1 was enhanced by deleting the gene expressing in the Δ12-desaturase, and thereafter FAE1 from Thlaspi arvense was over-expressed in Y. lipolytica with the Δfad2 genotype. Indeed, the resultant strain produced very long chain fatty acids and especially Δ13C22:1. The cells of the modified strain grown on crude glycerol produced microbial oil devoid of linoleic acid and enriched with very long chain fatty acids, mainly erucic acid. Implication also of waste cooking oil as substrate induced ex novo lipid accumulation process, further enhancing the synthesis of Δ13C22:1. Diamantopoulou et al. (2020) performed a screening of eleven wt yeast strains belonging to Y. lipolytica, Metschnikowia sp., Rhodotorula sp. and Rhodosporidium toruloides grown under nitrogen-limited conditions with biodiesel-derived glycerol employed as substrate. All microorganisms presenting interesting dry cell weight (DCW) production. A total of three of these strains belonging to Metschnikowia sp. accumulated significant quantities of endopolysaccharides (one of these strains produced 11.0 g/L of endopolysaccharides corresponding to ≈ 63% of DCW). A total of six wt Y. lipolytica strains produced either citric acid or mannitol. Most of the screened yeasts presented somehow elevated lipid and polysaccharides in DCW values at the early steps of growth. This happened despite the presence of nitrogen into the fermentation medium, which seems not to coincide to the currently accepted model for storage lipid and endopolysaccharides accumulation (Papanikolaou and Aggelis 2010; Philippoussis and Diamantopoulou 2012; Dourou et al. 2020). Rhodosporidiumtoruloides DSM 4444 produced interesting lipid quantities during growth on glycerol under carbon-excess conditions. This strain, cultivated in high glycerol content media, presented important lipid-accumulating capacities (maximum lipid produced = 12.5 g/L, maximum lipid in DCW = 43.0–46.0% w/w, conversion yield of lipid produced on glycerol consumed = 0.16 g/g). Replacement of crude glycerol by commercial-type xylose (another low-cost material deriving from lignocellulose biomass and used as starting material amenable to be chemically converted into xylitol), had as result a somehow lower lipid production by the aforementioned strain. In xylose/glycerol blends, xylose was preferentially assimilated. Rhodosporidium toruloides total lipids were mainly composed of triacylglycerols. Cellular lipids of all yeasts cultivated on glycerol, contained mainly oleic and palmitic acid, constituting interesting starting materials for the synthesis of 2nd generation biodiesel. Aquaculture plays an important role in the human nutrition and the economic development but is often expanded to the detriment of the natural environment. Several research projects, aiming at cultivating micro-algae in aquaculture wastewaters (AWWs) and simultaneously to reduce organic loads and minerals, along with the production of micro-algal cell mass and metabolic products of potentially very high added-value (i.e. rarely found poly-unsaturated fatty acids, important functional polysaccharides, proteins), are underway. Dourou et al. (2020) reviewed all of the above-cited concepts; micro-algal cell mass is of high nutritional value and is regarded as a candidate to replace, partially at least, the fish meal in the fish feed. Moreover, micro-algal cell mass is considered as a feedstock in the biofuel manufacture, as well as a source of high-added value metabolic products (polysaccharides, lipids, proteins, fibers, etc.). The production of these valuable products can be combined with the reuse of AWWs in the light of environmental concerns related with the aquaculture sector. Many research papers reviewed in the currently presented paper demonstrate that plenty of micro-algal species/genera are able to efficiently grow in AWWs, mainly derived from fish and shrimp farms and produce valuable metabolites reducing the AWW pollutant load. It can be concluded therefore, that that the bio-remediation of AWWs combining with the production of micro-algae cell mass and specific metabolites is probably the most convenient and economical solution for AWWs management and can contribute to the sustainable growth of the aquaculture. Several poly-unsaturated fatty acids, specifically the ones belonging to the ω-6 and ω-3 families, present various important functionalities for the human health (Papanikolaou and Aggelis 2010; Kothri et al. 2020). Dihomo-γ-linolenic acid (DGLA), arachidonic acid (AA),eicosapentaenoic acid (EPA) etc., are precursors of the biosynthesis of several types of eicosanoids, including prostaglandins and leukotrienes. Moreover, several poly-unsaturated fatty acids are of high significance for the normal function of many systems possessing also anti-thrombotic, anti-irritant and especially anti-tumor properties whereas they are also effective in the treatment of various diseases such as inflammatory disorders, rheumatoid arthritis and atopic eczema (Bellou et al. 2016; Kothri et al. 2020). The main commercial source of (mostly ω-3) poly-unsaturated fatty acids is that from various types of fishes. However, there are several disadvantages concerning the production of fish oil, including the danger of depletion of fish population due to overfishing and the possible presence of contaminants in them, such us heavy metals (Kothri et al. 2020). The most reliable alternative way for the production of poly-unsaturated fatty acids is probably through the use of oleaginous microorganisms. Micro-algae and fungi are the main producers of these fatty acids, while oleaginous yeasts produce poly-unsaturated fatty acids after appropriate genetic modification (Bellou et al. 2016). An important difficulty of lipid production through the use of microorganisms is related to the high cost of the fermentation (Koutinas et al. 2014). To overcome this difficulty combination of microbial lipid production with other biotechnological applications is proposed, including bio-treatment of various wastes and/or byproducts. Kothri et al. (2020) successfully reviewed the state-of-the-art of knowledge concerning the major poly-unsaturated fatty acid-producing microorganisms (i.e. fungi and micro-algae), as well as the recent attempts to produce microbial oils rich in the above-mentioned high-value fatty acids from microorganisms cultivated on several types of organic residues. Conflict of interest None declared. REFERENCES Bellou S , Triantaphyllidou IE, Aggeli D et al. 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TI - Microbial products from wastes and residues
JF - FEMS Microbiology Letters
DO - 10.1093/femsle/fnaa156
DA - 2020-10-20
UR - https://www.deepdyve.com/lp/oxford-university-press/microbial-products-from-wastes-and-residues-dVeelypbrg
VL - 367
IS - 19
DP - DeepDyve
ER -