Phytoremediation of pharmaceutical-contaminated wastewater: Insights into rhizobacterial dynamics related to pollutant degradation mechanisms during plant life cycle

Highlights

• Conventional pollutant and IBP removals increased with plant development.

• IBP reduced rhizobacterial diversity during plant development.

• Actinobacteria was dominant in the rhizosphere under IBP stress.

• Potential co-metabolism of IBP by rhizobacteria was related to root exudates.

• Potential degradation of IBP by rhizobacteria was related to bulk microbiome shift.

Abstract

Rhizobacterial dynamics, relating to pollutant degradation mechanisms, over the course of plant lifespan have rarely been reported when using phytoremediation technologies for pharmaceutical-contaminated wastewater treatment. This study investigated the rhizobacterial dynamics of Typha angustifolia in constructed wetlands to treat ibuprofen (IBP)-polluted wastewater throughout plant development from seedling, vegetative, bolting, mature, to senescent stages. It was found that conventional pollutant and IBP removals increased with plant development, reaching to the best performance at bolting or mature stage (removal efficiencies: 92% organics, 52% ammonia, 60% phosphorus and 76% IBP). In the IBP-stressed wetlands, the rhizobacterial diversity during plant development was adversely affected by IBP accompanied with a reduced evenness. The bacterial communities changed dynamically at different developmental stages and showed significant differences compared to the control wetlands (free of IBP). The dominant bacteria colonized in the rhizosphere was the phylum Actinobacteria, having a final relative abundance of 0.79 and containing a large amount of genus norank_o__PeM15. Positive interactions were evident among the rhizobacteria in IBP-stressed wetlands and the predicted functions of 16S rRNA genes revealed the potential co-metabolism and metabolism of IBP. The co-metabolism of IBP might be related to root exudates such as amino acid, lipid, fatty acid and organic acid. In addition, positive correlations between the organic compounds of interstitial water (bulk environment) and the rhizobacterial communities were observed in IBP-stressed wetlands, which suggests that the influence of IBP on bulk microbiome might be able to modulate rhizosphere microbiome to achieve the degradation of IBP via co-metabolism or metabolism.

Introduction

Pharmaceuticals, which are the typical emerging pollutants widely detected in water and soil environments, are mainly emitted to the environment through municipal wastewater (Aus der Beek et al., 2016). For example, ibuprofen (IBP) that is found at the maximum concentrations of 373.1 μg L⁻¹ and 95.0 μg L⁻¹ in the influent and effluent waters of wastewater treatment plants (WWTPs), respectively, is one of the most frequently detected pharmaceuticals in the environment (Aus der Beek et al., 2016; Gao et al., 2016). Due to the environmental risks of pharmaceuticals, it is of a great concern to remove pharmaceuticals from wastewater (Aus der Beek et al., 2016; Li et al., 2019).

Phytoremediation technologies (e.g. constructed wetlands) are regarded as one of the alternative secondary wastewater treatment or the wastewater polishing treatment processes for pharmaceutical removal from wastewater, especially showing a promising application in the decentralized wastewater treatment systems (Li et al., 2014b; Madikizela et al., 2018). To further enhance the removal efficiency of pharmaceuticals during phytoremediation of wastewater, it requires an in-depth understanding on the degradation mechanisms of pharmaceuticals. However, available research works mainly focus on the uptake of pharmaceuticals from wastewater by plant roots (Madikizela et al., 2018) and the translocation, accumulation and metabolism processes of pharmaceuticals in plant tissues (Klampfl, 2019). To date, there are few reports on the dynamics of rhizosphere microbiome related to pharmaceutical removal mechanisms during the phytoremediation of pharmaceutical-contained wastewater (Li et al., 2016b; Nguyen et al., 2019).

Rhizosphere, the thin zone surrounding plant roots, is a hotspot for numerous organisms (microorganisms and invertebrates) and is recognized as one of the most complex ecosystems on Earth (Mendes et al., 2013; Philippot et al., 2013; Sasse et al., 2018). Within the rhizosphere, there exist intricate and dynamic interactions between plant roots and microorganisms (such as bacteria, archaea, fungi, oomycetes and viruses) (Mendes et al., 2013; Philippot et al., 2013). The rhizosphere bacteria that play a positive role in acquisition for nutrients, defense against pests and disease, and tolerance to abiotic stress are referred to as the plant-growth-promoting rhizobacteria (PGPR) (Huang et al., 2014; Vacheron et al., 2013). It has been noted that the PGPR are pivotal for plant growth, health, productivity and functioning (Huang et al., 2014; Vacheron et al., 2013).

Generally, plants could induce a rhizosphere effect, so that rhizosphere microbiome is distinctly different from that of the bulk environment (Sasse et al., 2018). In detail, distinct rhizosphere microbiome is associated with plant species and genotype, plant developmental stage, root morphology, root exudation, soil properties, abiotic and biotic stress, and several other factors (Philippot et al., 2013; Sasse et al., 2018; Zhalnina et al., 2018). In the case of a specific plant in a given environment, the plant’s rhizosphere microbiome is not a static but rather a dynamic system with variations during plant growth, i.e., exerting a selection on rhizosphere beneficial microorganisms (e.g. PGPR) at different developmental stages (Philippot et al., 2013; Sasse et al., 2018; York et al., 2016).

The succession of rhizosphere microbiome during plant life cycle has been well illustrated for some plants such as oat (Shi et al., 2015), arabidopsis (Chaparro et al., 2014), and maize (Li et al., 2014a). Furthermore, a growing number of studies have reported the shifts in community composition and functional activities of rhizosphere microbiome under abiotic stresses such as salinity, heavy metal and organic pollutants (Feng et al., 2019; Mark Ibekwe et al., 2017; Rajkumar et al., 2012). Nevertheless, in the research field of phytoremediation, for the case of pharmaceuticals in wastewater which is a kind of abiotic stress to plants (Aus der Beek et al., 2016; Bartrons and Penuelas, 2017), there are knowledge gaps on the dynamic variations of rhizosphere microbiome over the course of plant life cycle.

In this study, ibuprofen (IBP) was selected as the target pharmaceutical, which belongs to the key pharmceuticals with high environmental risks in the aquatic environment (Li et al., 2019; Wang et al., 2016). The study focused on the removal of IBP from wastewater in microcosm-scale constructed wetlands using Typha angustifolia in order to reveal the potential biodegradation mechanisms of IBP associated with rhizosphere microbiome (e.g. the PCPR). The diversity, community and function variations of rhizobacteria during the developmental course of Typha angustifolia were investigated. In addition, the correlations between the environmental factors (physicochemical properties of interstitial water), OTU (operational taxonomic unit) abundances, and rhizosphere samples (bacterial communities) were evaluated.