Fungal carbonatogenesis process mediates zinc and chromium removal via statistically optimized carbonic anhydrase enzyme.

Introduction: With rapid elevation in population, urbanization and industrialization, the environment is exposed to uncontrolled discharge of effluents filled with broad-spectrum toxicity, persistence and long-distance transmission anthropogenic compounds, among them heavy metals. That put our ecosystem on the verge or at a stake of drastic ecological deterioration, which eventually adversely influence on public health. Therefore, this study employed marine fungal strain Rhodotorula sp. MZ312369 for Zn²⁺ and Cr⁶⁺ remediation using the promising calcium carbonate (CaCO₃) bioprecipitation technique, for the first time.

Results: Initially, Plackett-Burman design followed by central composite design were applied to optimize carbonic anhydrase enzyme (CA), which succeeded in enhancing its activity to 154 U/mL with 1.8-fold increase comparing to the basal conditions. The potentiality of our biofactory in remediating Zn²⁺ (50 ppm) and Cr⁶⁺ (400 ppm) was monitored through dynamic study of several parameters including microbial count, CA activity, CaCO₃ weight, pH fluctuation, changing the soluble concentrations of Ca²⁺ along with Zn²⁺ and Cr⁶⁺. The results revealed that 9.23 × 10⁷ ± 2.1 × 10⁶ CFU/mL and 10.88 × 10⁷ ± 2.5 × 10⁶ CFU/mL of cells exhibited their maximum CA activity by 124.84 ± 1.24 and 140 ± 2.5 U/mL at 132 h for Zn²⁺ and Cr⁶⁺, respectively. Simultaneously, with pH increase to 9.5 ± 0.2, a complete removal for both metals was observed at 168 h; Ca²⁺ removal percentages recorded 78.99% and 85.06% for Zn²⁺ and Cr⁶⁺ remediating experiments, respectively. Further, the identity, elemental composition, functional structure and morphology of bioremediated precipitates were also examined via mineralogical analysis. EDX pattern showed the typical signals of C, O and Ca accompanying with Zn²⁺ and Cr⁶⁺ peaks. SEM micrographs depicted spindle, spherical and cubic shape bioliths with size range of 1.3 ± 0.5-23.7 ± 3.1 µm. Meanwhile, XRD difractigrams unveiled the prevalence of vaterite phase in remediated samples. Besides, FTIR profiles emphasized the presence of vaterite spectral peaks along with metals wavenumbers.

Conclusion: CA enzyme mediated Zn²⁺ and Cr⁶⁺ immobilization and encapsulation inside potent vaterite trap through microbial biomineralization process, which deemed as surrogate ecofriendly solution to mitigate heavy metals toxicity and restrict their mobility in soil and wastewater.