**4. Conclusions**

In this study, synthesis of MgO-NPs was pursued by using the metabolites on the biomass filtrate of *Rhizopus oryaze* with Mg (NO3)2.6H2O as a precursor. The production technique was optimized by studying the precursor concentration, the contact time between the fungal biomass filtrate and precursor, incubation temperatures, and pH values. The color change to turbid white and the maximum SPR at 282 nm confirmed the successful synthesis of MgO-NPs, which is characterized by TEM, SEM-EDX, XRD, DLS, FT-IR spectroscopy, and XPS analyses. These analyses revealed that the biosynthesized MgO-NPs are crystalline, spherical, and well-dispersed, with sizes ranging between 8.0 to 47.5 nm. The fungal-induced MgO-NPs offer potential anti-microbial activity against *S. aureus*, *B. subtilis, P. aeruginosa, E. coli,* and *C. albicans,* with varied inhibition zones based on NP concentrations. Moreover, these NPs provide a modest, eco-friendly, and cost-effective way of controlling *C*. *Pipiens* populations with enhanced repellent activity as compared with commercial compounds. Additionally, they could be used efficiently to decolorize tanning effluents through the reduction of physicochemical parameters such as TSS, TDS, BOD, COD, and conductivity. Moreover, the biosynthesized MgO-NPs exhibit high efficacy to bio-adsorb chromium ions from tanning effluents. This study provides a simple, ecofriendly, cost-effective, and rapid approach to inhibit the growth of the microbial pathogen, prevent the spread of adverse insects, treat some of the worst environmental contaminants, and adsorb the most hazardous heavy metal.

**Author Contributions:** Conceptualization, S.E.-D.H. and A.F.; methodology, S.E.-D.H., A.F., E.S., M.M.S.F., A.M.E., M.G.B., and M.A.A.; software, S.E.-D.H., A.F., E.S., M.M.S.F., A.M.E., M.G.B., and M.A.A.; validation, S.E.-D.H., A.F., E.S., M.M.S.F., A.M.E., M.G.B., and M.A.A.; formal analysis, S.E.-D.H., A.F., E.S., A.M.E., M.G.B., M.A.A., M.F.H., and M.F.A.; investigation, S.E.-D.H., A.F., E.S., M.M.S.F., A.M.E., M.G.B., M.F.A., and M.A.A.; resources, S.E.-D.H., A.F., E.S., M.M.S.F., A.M.E., M.G.B., M.A.A., M.F.H., and M.F.A.; data curation, S.E.-D.H., A.F., E.S., A.M.E., M.G.B., and M.A.A.; writing—original draft preparation, S.E.-D.H., A.F., E.S., M.M.S.F., A.M.E., M.G.B., M.A.A., M.F.H., and M.F.A.; writing—review and editing, A.F., E.S., M.M.S.F., A.M.E., M.G.B., M.A.A., M.F.H., and M.F.A.; visualization, S.E.-D.H., A.F., E.S., M.M.S.F., A.M.E., M.G.B., M.A.A., M.F.H., and M.F.A.; supervision, S.E.-D.H. and A.F.; project administration, S.E.-D.H. and A.F.; funding acquisition, M.F.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** Authors extend their appreciation to Mamdouh Salem El-Gamal (Head of Microbial Physiology Lab), Botany and Microbiology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt, for the great help in the current study.

**Conflicts of Interest:** The authors declare no conflict of interest.
