Contamination with heavy metals is a dangerous environmental issue as the mining processing, and use of these elements has enhanced in different world regions (Dhal et al. 2013). Heavy metals are one of the primary pollutants, which exist in the soil for many years. These pollutants are toxic and pose a severe threat to food safety, human and environmental health due to their accumulation in the food chain (Song et al. 2017, Tao et al. 2017). Several physicals, chemical, and biological methods including surface capping, encapsulation, electrokinetic extraction, soil flushing, chemical immobilization, landfilling, soil washing, phytoremediation, bioremediation, solidification, and vitrification have been developed for cleanup and remediation of metal contaminated soils (Liu et al. 2018a).
The biological or bioremediation methods displays numerous advantages, e.g. cost-effective (Liu et al. 2018a, Shah &Daverey 2020), environment friendly (Shah &Daverey 2020), high public acceptance (Liu et al. 2018a), simple to implement(Liu et al. 2018a), sustainable(Megharaj &Naidu 2017, Pandey et al. 2016), without disturbing the soil fertility, and biodiversity (Ahmad et al. 2016, Xiao et al. 2019), compared to others. Bioremediation can be classified into phytoremediation, microbial remediation, and animal remediation, based on the organism utilized (Luo et al. 2017). Although the phytoremediation technique is an appropriate option for remediation of heavy metal contaminated soil, it faced some challenges, including the lengthy period for cleanup, the slow growth rate of the plant, hard to mobilize some metals, an application for the site with low pollution, transfer accumulated metals into the food chain (Mahar et al. 2016). Recently, many researchers focused on improving its process efficiency by using amendment materials and microorganisms (Girolkar et al. 2021, Gong et al. 2021, Sharma et al. 2021). For example, several studies proved that microbe-assisted phytoremediation (e.g., pseudomonas, burkholderia, bacillus, rhizobium, enterobacter, bradyrhizobium, paenibacillus, thiobacillus, Arbuscular mycorrhizal fungi, etc.) showed a positive effect on the phytoremediation by enhancement of the growth, health, and establishment of the plant, and sequestration of heavy metal in soil (Harindintwali et al. 2020, Khalid et al. 2021, Lebeau et al. 2008). On the contrary, few research applied soil algae (e.g., cyanobacteria) for soil remediation (Biglari Quchan Atigh et al. 2020b). However, cyanobacteria inoculation experience demonstrated that their species such as Nostoc, Anabaena, etc have outstanding capabilities for the restoration of degraded soil in the arid area (Chamizo et al. 2018b, Li et al. 2019).
Cyanobacteria are known as blue-green bacteria, belong to the kingdom prokaryotes, which are found in various habitats, even in the desert, salty soil (Li et al. 2019, Rossi et al. 2017). They are considered as ecosystem engineers (Jones et al., 1996), due to their ability to sequestration of carbon, fixation of nitrogen, secretion nutritious, therefore improvement of soil conditions (fertility, microbial community, and productivity) and as results provide an appropriate environment for the existence of micro and macro-organisms (Kheirfam &Roohi 2020, Kheirfam et al. 2017, Mugnai et al. 2018). The previous studies proved that cyanobacteria have the potential to be developed as a bioremediating agent for plant growth promotion and salt-affected soil remediation through most effective mechanisms such as nitrogen fixation, production of extracellular polysaccharides, and growing the organic carbon contents (Li et al. 2019, Muñoz-Rojas et al. 2018). Therefore, cyanobacteria inoculation can be considered a novel technique for remediation of contaminated soil (Biglari Quchan Atigh et al. 2020a).
The combined utilization of microorganisms, plant, and amendment materials was suggested as an innovative, effective method for remediation of heavy metal contaminated soil. Among various amendments, biochar “a carbon-rich material” was considered as an exciting remediation option for contaminated soil. According to literature (Chen et al., 2018; Lahori et al., 2017; Siedt et al., 2020), it is most effective in increasing the soil water holding capacity, alterations chemical soil properties (e.g., pH, cation exchange capacity, and buffering capability), retaining plant available water, increment nutrient concentration, decreasing the bioavailability of heavy metal, reduce plant uptake, and microbial communities. Furthermore, some studies indicated that the combination of biochar with animal-remediation ( such as earthworms, Eisenia fetida ) or microbial-remediation (such as bacteria, Pseudomonas sp., Enterobacter sp. ) enhanced the soil fertility and metal uptake by the plant due to its synergistic effects (Chen et al. 2019, Sanchez-Hernandez et al. 2019, Tu et al. 2020, Wu et al. 2019, Xiao et al. 2020). However, the combination mechanism between microorganisms and biochar for the transformation or stabilization of heavy metal in the soil is not well understood. There is also a research gap in the effect of biochar on microbial assisted phytoremediation, especially from the aspect of plant growth and heavy metal bioavailability.
In this study, Oscillatoria sp, a filamentous cyanobacteria species, purtolaca oleracea, and biochar were employed as microbial, plant, and material agents to remediate soil contaminated with heavy metals (Cr (III), Cr (VI), Fe, Al, and Zn), respectively. However, to the best of our knowledge, interactions between biochar and cyanobacteria on heavy metal uptake by purtolaca oleracea grown on contaminated soil have not been reported. Hence, the specific aim of this study is to: (1) evaluate the effect of cyanobacteria inoculation accompany with biochar amendment on the properties of soil and heavy metal bioremediation efficiency, (2) explore the influence of Oscillatoria sp - purtolaca oleracea - biochar partnerships on soil properties and heavy metal bioremediation efficiency, (3) assess the heavy metal accumulation in parts of purtolaca oleracea under metal stress and assisted remediated materials.