Concrete inevitably develops cracks during service, which seriously impairs the functional performance of structures. Microbially induced calcium carbonate precipitation technology has gained attention for its eco-friendliness, cost-effectiveness, and potential for crack healing. This study used carbonated recycled fine aggregates to immobilize Bacillus cohnii and Nitratireductor indicus for preparing a microbial repair agent. Microbial self-healing recycled concrete was made by controlling the repair agent dosage and recycled coarse aggregate replacement ratio. Macroscopic performance tests were conducted to analyze the compressive, splitting, and flexural strengths of self-healing recycled concrete and assess crack healing effectiveness at various mineralization times. Combined with microscopic tests, the micro-morphology and crystal types of mineralized precipitates under different oxygen contents were analyzed. The findings show that increasing the microbial repair agent dosage reduces the concrete's strength, while significantly improving the crack healing ratio. After 28 days of healing, when the repair agent dosage was 4%, the crack healing ratio for 0.3 mm width cracks in the oxygen-rich zone, transition zone, and oxygen-deficient zone was 100.00%, 93.30%, and 96.70%, respectively. However, for 0.7 mm width cracks, the highest healing ratio of only 38.57% occurred in the oxygen-rich zone. The healing effectiveness decreases with increasing crack width. Regarding the influence of oxygen content, the healing performance is optimal in the oxygen-rich zone, followed by the oxygen-deficient zone, and least effective in the transition zone. Under different oxygen environments, the mineralized precipitates were all calcite-type CaCO3. This study offers theoretical support for prolonging the service life of recycled concrete.IMPORTANCEBased on the fact that in concrete projects under humid environments, the surface of slender cracks in concrete engineering is relatively high, while the oxygen content in the deep cracks is low, this study immobilizes a mixture of aerobic and anaerobic microorganisms in recycled fine aggregates. By analyzing the crack repair performance of microbially self-healing recycled concrete under conditions where oxygen-rich and oxygen-poor areas coexist, as well as the mineralization mechanism of mixed microorganisms, this research can provide a theoretical basis and technical support for the repair performance of recycled concrete under external loads. The research conclusions are significant for the engineering application of microorganisms and the resource utilization of construction waste. At the same time, it has important practical significance for extending the service life of concrete engineering.
The biosynthesis of calcium carbonate (CaCO3) minerals through a metabolic process known as microbially induced calcium carbonate precipitation (MICP) between diverse microorganisms, and organic/inorganic compounds within their immediate microenvironment, gives rise to a cementitious biomaterial that may emerge as a promissory alternative to conventional cement. Among photosynthetic microalgae, Chlorella vulgaris has been identified as one of the species capable of undergoing such activity in nature. In this study, response surface technique was employed to ascertain the optimum condition for the enhancement of biomass and CaCO3 precipitation of C. vulgaris when cultured in Blue-Green (BG)-11 aquaculture medium. Preliminary screening via Plackett-Burman Design showed that sodium nitrate (NaNO3), sodium acetate, and urea have a significant effect on both target responses (p < 0.05). Further refinement was conducted using Box-Behnken Design based on these three factors. The highest production of 1.517 g/L C. vulgaris biomass and 1.143 g/L of CaCO3 precipitates was achieved with a final recipe comprising of 8.74 mM of NaNO3, 61.40 mM of sodium acetate and 0.143 g/L of urea, respectively. Moreover, polymorphism analyses on the collected minerals through morphological examination via scanning electron microscopy and crystallographic elucidation by X-ray diffraction indicated to predominantly calcite crystalline structure.
Istamycins (ISMs) are 2-deoxyfortamine-containing aminoglycoside antibiotics (AGAs) produced by Streptomyces tenjimariensis ATCC 31603 with broad-spectrum bactericidal activities against most of the clinically relevant pathogens. Therefore, this study aimed to statistically optimize the environmental conditions affecting ISMs production using the central composite design (CCD). Both the effect of culture media composition and incubation time and agitation rate were studied as one factor at the time (OFAT). The results showed that both the aminoglycoside production medium and the protoplast regeneration medium gave the highest specific productivity. Results also showed that 6 days incubation time and 200 rpm agitation were optimum for their production. A CCD quadratic model of 17 runs was employed to test three key variables: initial pH, incubation temperature, and concentration of calcium carbonate. A significant statistical model was obtained including, an initial pH of 6.38, incubation temperature of 30 ˚C, and 5.3% CaCO3 concentration. This model was verified experimentally in the lab and resulted in a 31-fold increase as compared to the unoptimized conditions and a threefold increase to that generated by using the optimized culture media. To our knowledge, this is the first report about studying environmental conditions affecting ISM production as OFAT and through CCD design of the response surface methodology (RSM) employed for statistical optimization. In conclusion, the CCD design is an effective tool for optimizing ISMs at the shake flask level. However, the optimized conditions generated using the CCD model in this study should be scaled up in a fermenter for industrial production of ISMs by S. tenjimariensis ATCC 31603 considering the studied environmental conditions that significantly influence the production proces.
Production cost of enzyme is largely determined by the type of the strain and raw material used to propagate the strain. Hence, selection of the strain and raw materials is crucial in enzyme production. For Glucose oxidase (GOx), previous studies showed Aspergillus terreus UniMAP AA-1 offers a better alternative to the existing sources. Thus, a lower production cost could be logically anticipated by growing the strain in a cheaper complex media such as molasses. In this work, sugar cane molasses, supplemented with urea and carbonate salt and a locally isolated strain Aspergillus terreus UniMAP AA-1 were used to produce a crude GOx enzyme in a small scale. A statistical optimization approach namely Response Surface Methodology (RSM) was used to optimize the media components for highest GOx activity. It was found that the highest GOx activity was achieved using a combination of molasses, carbonate salt and urea at concentration 32.51, 4.58 and 0.93% (w/v), respectively. This study provides an alternative optimized media conditions for GOx production using locally available raw materials.