Genetic Characterization of Selected Cyanobacteria and their Role in Remediating Environmental Pollutants

Abstract

Bioremediation using microorganisms such as bacteria, cyanobacteria, microalgae, yeasts, and fungi is efficient, cost-effective, and eco-friendly. Among these, cyanobacteria can carry out oxygenic photosynthesis, fix atmospheric nitrogen and pose flexibility to sustain a range of harsh conditions. These also offer the promise for treating environmental pollutants found in soil and water. Despite huge prospects little or no work is available on indigenous/local cyanobacterial species of Pakistan. Therefore, this research was initiated to isolate and assess the potentials of local cyanobacterial strains capable of remediating heavy metals especially Cadmium (Cd) and Lead (Pb). Three different sites were surveyed for the collection of algal samples. Initially 45 different taxa were isolated belonged to the group: cyanobacteria, green algae and diatoms. Among these, ten axenic strains of cyanobacteria were obtained. Six taxa were identified up to species level while four were identified to the genus level. Six taxa have been reported for the first time from Pakistan, viz. Desertifilum tharense MK-2, Nodosilinea nodulosa MK-4, Fischerella muscicola MK-8, Westiellopsis prolifica MK-9, Desikacharya MK-7, Synechocystis fuscopigmentosa MK-13. The purified strains were screened for Cd and Pb tolerance. Since obtained from sewage sites, most of the strains showed tolerance to Cd and Pb concentrations. Screening results further showed that Nostoc sp. MK-11 was the most tolerant followed by F. muscicola MK-8. Therefore, these two strains were selected for further analysis. Cells of Nostoc sp. and F. muscicola were cultured and their biomass was harvested, dried and ground to fine powder. The dried biomass of Nostoc sp. revealed highest biosrption of Pb and Cd at 60-minute contact time, with maximum biosorption capacities of 75.757 mg/g for Cd and 83.963 mg/g for Pb. Biosrption of Pb and Cd on dried biomass of Nostoc sp. followed the Pseudo second order Kinetics and Langmuir isotherm models indicating chemisorption mechanism in homogeneous manner. Biosorption-desorption tests showed above 90% metal recovery besides the fact that the dried biomass of Nostoc sp. holds the potential of reusability, thus, are cost-effective and eco-friendly in nature. Dried biomass of F. muscicola displayed rapid kinetic biosorption at 60 and 90 minutes for Cd and Pb, respectively, with maximum biosorption at pH 7 and 5. Langmuir isotherm modeling revealed biosorption capacities of 63.5 mg/g for Cd and 70.2 mg/g for Pb, signifying Summary 2 effective biosorption in homogeneous manner. Biosrption of metals onto the dried biomass of this strain also followed the pseudo-second-order kinetics showing the chemisorption of metals. Functional groups responsible for heavy metal binding, highlighting hydroxyl, amine, carbonyl, carboxyl, and sulfoxide groups crucial for metal ion complexation, showed biosorbent potential for Cd and Pb, while the Scanning electron microscopy highlighted the altered morphology after the biosorption process. Furthermore, desorption of Cd and Pb using 0.1 M Hydrochloric acid showed efficient metal desorption with a recovery rate of 90%. Iron oxide nanoparticles (IONPs) synthesized using extracts of two cyanobacteria (F. muscicola and Nostoc sp.) and parallel analyses were carried out. The Langmuir isotherm calculated maximum adsorption capacities for F. muscicola based IONPs as 94.161 and 93.370 mg/g for Pb and Cd, respectively. Similarly, Langmuir isotherm calculated maximal adsorption capacities of Nostoc sp. mediated IONPs as 105.932 and 118.764 mg/g for Cd and Pb, respectively. The recyclability of IONPs revealed retention of their adsorption efficiency. Hence, this study indicated that Nostoc sp. has higher potentials of adsorbing Cd and Pb ions compared to F. muscicola, both these strains are derived from the indigenous algal flora thriving in polluted water. The study recommends exploring diverse ecological niches and hybrid water treatment systems, advancing genetic engineering, and improving biosorption methods, with field trials needed to validate the feasibility and ecological safety of cyanobacteria-based solutions.