Biodegradation of diclofenac by two green microalgae: Picocystis sp. and Graesiella sp.
Graphical abstract
Introduction
The presence of emerging contaminants as pharmaceutical and personal care products (PPCPs) in the aquatic environment continues to give rise to concern due to their environmental risk and toxicological properties. Indeed, these chemical compounds can cause multiple changes in the physiological state of organisms, and their occurrence in the environment may affect non-target species (Valavanidis et al., 2014).
Among the most used PPCPs, Diclofenac (DCF) is a non-steroidal anti-inflammatory drug (NSAID) widely prescribed as an antipyretic analgesic. It is often found as a persistent toxic waste and one of the most widely available drugs in the world. Recent studies based on the Intercontinental Marketing Services (IMS) health data (which serves 82% of the global population) from 86 countries estimated that about 1443 ± 58 tons of DCF are consumed globally on an annual basis (Acuña et al., 2015). However, this is only an indication of the DCF consumption for human health-related applications and does not include DCF's veterinary uses.
A large part of the consumed DCF is excreted in urine and feces in original form so entering municipal wastewater. DCF is ineffectively removed by conventional wastewater treatment plants (WWTPs) (Langenhoff et al., 2013; Sophia and Lima, 2018). Thus, it can be discharged into the environment with treated wastewater effluent, recycled water, and wastewater plant sludge. Actually, DCF is detected in several aquatic environments all over the world at concentration ranges from few hundreds to thousands of ng/L (Lonappan et al., 2016). It may be transported through food chains (Cuellar-Bermudez et al., 2017) causing toxic adverse effects on many aquatics organisms, even at environmentally low concentrations (Xu et al., 2019).
Therefore, it is mandatory to investigate alternative treatments for DCF removal from wastewaters. Among them, those based on the use of microalgae are emerging as a sustainable and economical solution (Escapa et al., 2017). Recent studies reported that many microalgae species could remove several pharmaceutical contaminants, including DCF (Xiong et al., 2018). Therefore, using microalgae to remove DCF from wastewater could be prospective.
To date, only very few studies have been carried out on the assessment of DCF removal using microalgae (Escapa et al., 2017; Villar-Navarro et al., 2018). Furthermore, the ability of microalgae to accumulate DCF as well as its biodegradation and/or biotransformation products were not yet investigated. Thus, information regarding DCF effects, removal and biotransformation products in the presence of microalgae are required.
Among microalgae, a particular interest is given to the use of extremophilic species in bioremediation systems (Varshney et al., 2014). Such organisms are assumed to have specific qualities allowing them to tolerate high concentration of several pollutants and further could be more efficient in the fast contaminant removal from wastewaters (Peeples, 2014).
The aim of the present work is to evaluate and compare, under laboratory culture conditions, the effects and the removal efficiency of DCF by two extremophilic microalgae strains Picocystis sp. and Graesiella sp. isolated from two polluted ponds in Tunisia for the purpose of selecting resistant species for their bioremediation use. To fulfill those purposes, the effect of DCF on growth and photosynthesis of both species was assessed together with their abilities to remove and accumulate DCF. Then, the resulting biotransformation and biodegradation products were addressed.
Section snippets
Chemicals and reagents
Diclofenac sodium salt (≥98%) was purchased from Sigma Aldrich. Ultra-pure water (UPW) was delivered by Elga Pure Lab System (resistivity 18.2 MΩ cm, COT <50 μg C/L). Acetonitrile (ACN) with 0.1% formic acid (FA) was purchased from JT Baker (LC-MS grade) and used in association with UPW with 0.1% formic acid.
Salts and reagents for medium preparation suppliers are specified in Supplementary material 1. All chemicals used were of analytical grade.
Algal strains and culture conditions
Two extremophilic chlorophyta species were
Effect of DCF on microalgae growth
The effect of DCF concentrations on Picocystis sp. and Graesiella sp. growth during 5 days of exposure was evaluated by the growth inhibition percentage (PI%) (Table 1). Results showed that the DFC has slight effect on both species. The PI of Picocystis sp. and Graesiella sp. did not reach 40% for both species during the whole period of exposure and even at the highest DCF tested concentration (200 mg L−1). The maximum PI recorded was 21% for Picocystis and 36% for Graesiella implying that
Discussion
Microbial removal of pollutants relies on a trade-off between tolerance of the organisms to the toxicity of the targeted compounds, and their efficiency to effectively sequester or degrade them. To date only few studies on DCF toxicity to microalgae were reported, mostly on freshwater cyanobacteria and chlorophyta species (Supplementary material 3). The measured sensitivities to diclofenac spread from 84.5% growth inhibition in the freshwater cyanobacterium Cylindrospermopsis raciborskii
Conclusion
Based on IC50 values, Picocystis and Graesiella were, relatively, tolerant to DCF compared to other chlorophyta species as reported in the literature. The growth of both species was inhibited by less than 40% even at 200 mg L−1 DCF. Interestingly, Picocystis and Graesiella exhibited high DCF removal efficiencies reaching 73% and 52% of 25 mg L−1 as initial concentration, respectively.
The main proportion of DCF removal (69% and 44% of 25 mg L−1 initial DCF concentration) was insured by
Declaration of competing interest
Authors declares that the research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.
Acknowledgements
Authors are thankful to the French Research Institute for Development (IRD), France, for financing the Ph.D stipend of Sabrine Ben Ouada under the Laboratory LMI Cosys-med project.
The support of this work under “Contrat Programme of Laboratory of Environmental Bioprocesses” by The Ministry of Higher Education and Scientific Research of Tunisia is also gratefully acknowledged.
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