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

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.

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.

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.