Multiple heavy metal removal using an entomopathogenic fungi Beauveria bassiana

Highlights

• High multimetal (84%) removal compared to individual metal (61–75%) by B. bassiana.

• Relative metal affinity changed under multimetal as compared to single metal.

• AFM revealed changes in surface roughness due to metal toxicity.

• Potential strain for multimetal removal from contaminated wastewater.

Abstract

Towards the development of a potential remediation technology for multiple heavy metals [Zn(II), Cu(II), Cd(II), Cr(VI) and Ni(II)] from contaminated water, present study examined the growth kinetics and heavy metal removal ability of Beauveria bassiana in individual and multi metals. The specific growth rate of B. bassiana varied from 0.025 h⁻¹ to 0.039 h⁻¹ in presence of individual/multi heavy metals. FTIR analysis indicated the involvement of different surface functional groups in biosorption of different metals, while cellular changes in fungus was reflected by various microscopic (SEM, AFM and TEM) analysis. TEM studies proved removal of heavy metals via sorption and accumulation processes, whereas AFM studies revealed increase in cell surface roughness in fungal cells exposed to heavy metals. Present study delivers first report on the mechanism of bioremediation of heavy metals when present individually as well as multi metal mixture by entomopathogenic fungi.

Introduction

Increased industrialisation activities have contributed largely towards the introduction of heavy metals and other pollutants into the environment (Bhattacharya et al., 2015, Gupta et al., 2014, Mishra et al., 2014, Shah, 2014). Industries directly discharge heavy metal containing waste into the river and drains which enters the food chain via vegetables irrigated with contaminated water (Nguyen et al., 2013). As a result, heavy metals accumulate in human body thereby causing potential health hazards. Exposure to these toxic metals is associated with many serious diseases, including autoimmune disorder, digestive disorder, heart disorders, liver, kidney, stomach and lung cancer (Yadav et al., 2010). Hence, heavy metals must be removed or transformed to less toxic forms in wastewater before being discharged to the environment or before using it for irrigation. Physical and chemical processes are conventionally used for removing heavy metal, but these processes are not economical when the concentration of heavy metal is very low (Li et al., 2013). Biological processes are suitable for remediating single or binary metal solutions (Bulgariu and Bulgariu, 2012, Khodabakhsh et al., 2011, Mishra and Malik, 2012). The efficiency of these processes decreases when employed for multiple heavy metal removal.

Majority of the studies have targeted bioremediation technology for removal of a single metal or at the most binary metal mixture because microbes are unable to efficiently deal with multiple heavy metals at the same time. This is due to the fact that the individual metals in mix can interact in synergistic, antagonistic and non-interactive manner to influence the resultant toxicity. In most of the studies, synergistic effect was observed in multiple metal ion mixture, whereas few studies discussed about the antagonistic effect (Gola et al., 2016). Moreover, in multi metal mixture some metals with high affinity for the biomass are effectively removed but others may still be retained in the solution. For example, Bacillus sp. L14 could uptake up to 75.7% and 80.4% of Cd (II) and Pb (II) but only 21.2% of Cu (II) was removed from initial concentration of 10 mg L⁻¹ (Guo et al., 2010). Aspergillus niger could also remove significant quantities of Cu(II) and Pb(II) from growth media, but was less resistant against Cr(VI) (Dursun et al., 2003). Mishra and Malik (2012) observed 78–100% metal removal for Cr(III), Cu(II) and Pb(II), but only 42% of Ni(II) could be removed from synthetic solution, by Aspergillus lentulus. On the other hand, 89–94% metal removal was attained for Ni(II), Cu(II), Cr(VI) and Zn(II) by Saccharomyces cerevisiae at pH 6 (Machado et al., 2010).

Beauveria bassiana is an anamorphic entomopathogenic species of fungi which is a natural enemy of insects and arachnids (Khalid et al., 2011). Large volume of literature is available on its insecticidal properties but relatively few reports exist on metal remediation using entomopathogenic fungi. Kameo et al. (2000) studied the production of metallothioneins (cysteine rich metal binding protein) in the presence of Cu(II) and Cd(II) using a Cu(II) resistance Beauveria bassiana strain. Upon heavy metal exposure, various fungal strains synthesize metallothioneins to provide resistance against heavy metal toxicity. Khalid et al. (2011) studied the bio-sorption of Pb(II) and Cd(II) (under single metal exposure) using the dead biomass of B. bassiana and observed metal uptake capacity up to 46 mg g⁻¹ and 86.3 mg g⁻¹ for Pb(II) and Cd(II), respectively. In spite of such lab scale studies, very less is known about the heavy metal bioremediation role of fungi in actual environmental conditions. Metal tolerant fungi, Beauveria bassiana and Rhodotorula mucilaginosa, isolated from constructed wetlands were found to possess high MIC for Zn(II) and Pb(II) (Scholes et al., 1999). Purchase et al. (2009) demonstrated that these strains could accomplish Zn(II) and Pb(II) accumulation at extremely low temperature (4 °C) as well thereby establishing their importance for round the year metal removal in wetlands. However, in none of the studies, simultaneous uptake of multiple metals by Beauveria species or other entomopathogenic fungi is described.

Recently, AFM (Atomic Force Microscopy) has been used to detect nano-mechanical properties induced to the micro-organism, mammalian cells and nanoparticles in different conditions. However, limited studies have been conducted to determine the changes in the nano-mechanical properties of fungal cell walls (Das et al., 2008). In view of above, the objective of the present study was to investigate the ability of environmentally benign fungal strain to deal with the individual metal as well as multiple metals stress and to understand the impact of metal (single versus multiple) exposure on morphological characteristics and surface properties of the fungal strain. Briefly, a fungal isolate with great potential for simultaneous uptake of multiple metals has been described.