Bioconversión microbiana de residuos agroindustriales procedentes de Nicaragua con fines biotecnológicos

Résumé

Most of agro-industrial wastes produced in Nicaragua are from lignocellulosic materials which are mainly derived from agricultural activities. Among these residues, coffee pulp, rice husk, cassava, leaves and stems of banana tree and wheat straw are the most abundant. The exploitation of these residues without commercial value for biotechnological purposes may have a positive effect on the reduction of environmental impact caused by their accumulation. In this work, selected Streptomyces strains were used to transform these residues in order to know their technological feasibility. Moreover, the edible fungus Pleurotus ostreatus were grown on different mixtures of these substrates, and the production and quality of fruiting bodies were evaluated for commercial purposes. First, we analyze the chemical modifications produced by the Streptomyces strains on the coffee pulp under solid state fermentation conditions (SSF) with the following purposes: i) to obtain high-added value products (i.e. enzymes for biotechnological applications); ii) to produce a protein enrichment of the residue and iii) to reduce its phenolic compounds content. The last two aims are unavoidable requirements to be applied for animal feeding. We aware that the development of these objectives could have a social and economic importance for countries like Nicaragua, with scarce resources but plenty of agricultural residues. Pyrolysis associated to mass spectrometry and gas chromatography (Py-GC/MS) were used to show up the chemical transformations occurred on coffee pulp during the growth of Streptomyces strains, as well as their ability to reduce their polyphenols content. Moreover, nitrogen analysis demonstrates a significant increase in protein nitrogen of the coffee pulp transformed by the selected strains after 10 days of incubation. This result points out the possibility to use this biotechnological strategy for animal feeding. On the other hand, the capacity of the three Streptomyces strains used in this study to produce hydrolytic and oxidative enzymes was also established. Thus, the detection of enzymes responsible for the degradation of hemicelluloses, lignin, lipids and starch show the suitability of these residues for the production of enzymes with potential industrial applications. Another approach considered in this study was to reuse the wastes generated from agricultural practices for the production of edible mushrooms. It is known that selected fungi growing on this type of wastes are able to convert lignocellulosic biomass into a food with nutritional features suitable for human consumption. In addition, the fermented residues once harvested the mushrooms, could be used as organic amendments in degraded soils. For the production of edible mushrooms, different mixtures of agricultural residues such as coffee pulp and rice husk, very abundant in Nicaragua joined to other residues such as wheat straw, abundant in Spain, were screened. Then, growth and fruiting bodies production in the strain Pleurotus ostreatus CBS-411.71 were determined under controlled conditions. The highest biological efficiency was achieved when a mixture of coffee pulp and rice husk (mixture 2) was used. Mushrooms obtained on the different mixtures assayed, presented a high nutritional value, regarding to the content of proteins and carbohydrates. Moreover, the protein quality of the obtained mushrooms, estimated as the ratio between total amino acids and the crude protein, proved to be superior to that corresponding to the commercial mushroom used as control. It should be noted that highest values of protein quality was obtained in the mushrooms cultivated on mixture 1 (i.e. coffee pulp and wheat straw). Finally, residues obtained after fermentation with selected Streptomyces strains as well as after recovering fruiting bodies of P. ostreatus, were both applied into degraded soils as organic amendments. This study was carried out under controlled conditions in a greenhouse using Phaseolus vulgaris as reference plant. Best results were obtained when spent mushrooms substrates were used as organic amendments. These both mixtures, EII and EIII, were basically composed by rice husk, coffee pulp and pine sawdust, and EIII was also supplemented with soy flour. The addition of these organic amendments to soil improved some physical, chemical, microbiological and biochemical properties as well as the yield of Phaseolus vulgaris production. Thus, among physico-chemical characteristics, porosity, organic matter content and total humic extract, were mainly improved compared with the untreated soil. Regarding to the biological properties of amended soils, an increase in both respiratory activity and enzymatic activities involved in the cycles of the P and C were observed. It is remarkable that alkaline phosphatase and β-glucosidase were the highest enzymatic activities detected. Soil microbial biodiversity in amended soils was established through the analysis of Bacteria 16S rDNA fragments by Denaturing Gradient Gel Electrophoresis (DGGE) technique. The pattern of bands obtained in the amended soils was different to that corresponding to control soil. In addition, some differences were also observed between soils managed with different treatments. It is important to note that the number of bands detected in control soil was lesser than in the amended soils, especially in soil treated with E-II amendment. In addition, this EII-treated soil presented the highest number of bands and therefore the greatest microbial diversity as revealed by Shannon index value obtained for this amended soil.