Study on the degradation performance and kinetics of immobilized cells in straw-alginate beads in marine environment
Introduction
Marine, covering over two-thirds of the earth’s surface, is one of the most important ecosystems. With the rapid development of economic globalization, industrial discharge and oil pollution have caused irreparable damage to the marine environment. A large number of pollutants constantly enter the ocean, reducing the resilience of marine biodiversity, destroying the environmental quality and value of tourist attractions, and jeopardizing human health through the food chain (Liu et al., 2017). As one of them, oil pollution is increasingly frequent. Oil is prone to leakage during marine transportation, which causes serious damage to the ocean and high cost of removing oil spills (Lim et al., 2016, Macaulay and Rees, 2014, Reddy et al., 2012). Traditional remediation technologies, such as thermal extraction, steam stripping, adsorption and bioremediation technology, were used to treat oil-contaminated areas (Padhi and Gokhale, 2017). Among them, bioremediation has been proved to be economical and environmentally friendly method. Thus, it has been widely utilized in remediation of oil-contaminated soil.
In the bioremediation process, microorganisms are used to degrade oil. However, in the marine environment, bioremediation is difficult to be applied because of the low survival rate and poor environmental adaptability of microorganisms. Simultaneously, the low solubility, non-polarity and hydrophobicity of oil also limit the oil degradation (Varjani and Upasani, 2017).
Immobilized cells have been proven to solve the above problems, and it supports the formation of biofilms (Chen et al., 2007). Compared with free cells, immobilized system provides a more stable living environment for microorganisms (Kureel et al., 2016, Zhang et al., 2015). Immobilization carrier is a critical factor of immobilized system, as free cells should be immobilized on a biodegradable carrier to achieve higher cell density and activity (Ivankovic et al., 2010). Alginate is a kind of biodegradable polysaccharide extracting mainly from phaeophyta, and cells immobilized with alginate were found to be more efficient in ethylbenzene degradation (Parameswarappa et al., 2008). Alginate beads have advantageous physicochemical properties such as excellent biocompatibility, presence of microenvironment and cell-limiting ability. However, they are difficult to adsorb oil due to their hydrophilic property. Therefore, hydrophobic carriers can be combined in. Straw, composed of lignin and cellulose, is considered to be a suitable immobilization carrier. It not only has hydrophobicity and oil adsorption properties but also slowly release nutrients. Simultaneously, biosurfactants can be used to improve the solubility of oil.
In this paper, straw and alginate were combined to be the immobilization carrier, and free cells, which produced biosurfactants, were embedded in straw-alginate beads. The degradation performance and kinetics of free cells and immobilized cells in straw-alginate beads were studied.
Section snippets
Chemicals
In this study, diesel was used as a target substance for microbial degradation. The diesel was provided by PetroChina (Qingdao 151st Station), Huangdao District, Qingdao, Shandong Province, P.R. China.
Enrichment and isolation of free cells
Two stains HDMP1 and HDMB2 were isolated from oil-contaminated areas as described by previous research (Wu et al., 2018, Wu et al., 2019), and HDMP1 had the highest rate (79.59%) for degradation of diesel after 7 days. HDMB2 had good biosurfactant producing properties with a positive impact on its
Isolation and characterization
Based on the 16S rDNA gene sequences and using the Gen Bank BLAST tool, the 16S rRNA gene sequence of HDMP1 and HDMB2 were aligned automatically to reference sequences of the genus obtained from the GenBank (https://blast.ncbi.nlm.nih.gov/Blast.cgi), respectively. A phylogenetic tree was constructed (Fig. 1) based on the neighbor-joining method using the software MEGA version 7.0. The complete 16S rDNA sequence of HDMP1 and HDMB2 was existed in the GenBank database, and the accession numbers
Conclusions
In this study, a new type of immobilized cells, free cells immobilized in beads combined with sodium alginate and straw that could degrade diesel and produce biosurfactant, was studied. The optimum composition of immobilized cells in straw-alginate beads was obtained, and it had good mechanical properties and diesel adsorption capacity. The best degradation rate of immobilized cells in straw-alginate beads could be 68.68%. By analyzing the degradation kinetics, immobilized cells in
Acknowledgements
This study was financially supported by the scientific research fund project of National Natural Science Foundation of China (Grant No. 51408347), the SDUST Young Teachers Teaching Talent Training Plan (BJRC20170502), and the SDUST Graduate tutor guidance ability promotion project (KDYC17023).
Conflict of Interest
The authors declare that they have no conflict of interest.
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