**1. Introduction**

Constructed wetlands (CWs) are purposely designed and constructed systems, based on the physical, chemical, and biological principles and processes of natural wetlands [1]. The vegetation, soil, and microorganisms are the main components of a CW that contribute to pollutant removal processes from wastewater. The associated environmental and economic benefits have established CWs as a viable option for wastewater treatment [2]. These have been widely applied in the treatment of various types of wastewater, such as municipal, agricultural runoff, storm runoff, and industrial [3–8]. Floating treatment wetland (FTW) is a novel technology, based on a floating vegetated system, that has unique

abilities to remediate wastewater [9,10]. In FTWs, plants are supported by a buoyant mat or raft that floats on the surface of the water [11]. The roots of the plants develop below the floating mat, extending down the water column, and develop an extensive root system beneath the water level [10,12,13]. The development of a widespread and dense root system is necessary for the e ffective performance of FTWs [14]. FTWs move freely and thus cover a wider area of water than the emergen<sup>t</sup> root system. In a FTW system, the rhizomes and dense root structure develop a special hydraulic flow in the water zone between the mat and the bottom of the water body, and the floating roots act as a filter [15]. This leads to an e ffective removal of pollutants from the water due to the availability of the increased surface area of roots for adsorption and absorption [16]. The roots and rhizomes provide a habitat for microbial growth and development. The roots and attached biofilms perform di fferent physical and biochemical processes for the removal of pollutants from the contaminated water [17,18]. In FTWs, pollutants are removed by three main processes, namely adsorption, sedimentation, and biodegradation [19].

The benefits associated with FTWs have made it a promising ecological remediation technology in the field of wastewater treatment. These benefits include economic and convenient construction, no digging/earth moving or extra land acquisition, easy operation and maintenance, floating mats that are adjustable with a change in the water level, and excellent treatment performance [10,20,21]. Furthermore, the planted vegetation provides economic and ecological benefits such as the use of vegetation as fodder, providing a habitat for wildlife/aquatic animals, and enhancing the aesthetic value of the pond [10,22]. Globally, FTWs are being applied to remediate various types of wastewater, such as eutrophic water, sewage and domestic, storm water runo ff, and industrial [23–29].

Microbes have a fundamental role in the remediation of polluted water by FTWs. The bacteria attached to the roots form biofilms through a repeated proliferation process [30]. The oxygen and exudates released by the plants create a substrate for microbial growth and colonization on the root beneath the water level [31]. Thus, along the vegetation, the performance of FTWs also depends upon the metabolism of the microbial community in water, attached to the roots and floating mats [32–34]. The application of plants in combination with microorganisms in FTWs is an e ffective and sustainable approach for the treatment of wastewater [35]. The plant–microbe interaction enhances the e fficacy of FTWs [36]. Although the plant–bacteria interaction plays an essential role in the removal of contaminants from aquatic ecosystem, the interaction of the plant with bacteria in the FTWs is not well explored [37].

This paper discusses this important component of FTWs and provides a detailed overview of the specific role of microorganisms in FTWs. We have summarized the important species of bacteria that colonize the roots of plants. Furthermore, the specific role of rhizospheric bacteria, endophytes, and algae in the pollutant removal process in FTWs has been elaborated.

#### **2. Mechanism of FTWs**

In FTWs, pollutants are removed from the wastewater by di fferent mechanisms induced by plants, microbes, and their mutualistic relationships. The presence of a vegetated floating mat in a water body boosts the pollutant removal e fficiency of the system by modifying the physicochemical properties of the water [38,39]. The physical characteristics of the plant's roots and the nutrient uptake are interdependent/interlinked. The type of medium in which the roots exit and the nutrients present in the medium specify the root's physical characteristics [9,40]. In general, the roots of plants filter the particulates present in the water. Nutrients are taken up by the plant's roots and accumulated in them, as well as in the parts of the plant above the mat [14]. Most organic pollutants are degraded by microorganisms present on the roots. However, some of the organic pollutants are taken by the plants. The organic pollutants can either be accumulated in the biomass of vegetation or degraded by endophytic bacteria present inside the plants [41,42].

The plants in FTWs contribute to the pollutant removal process by entrapping pollutant particles in the roots [11,43,44]. The roots of plants act as physical filters, and remove suspended particulate matter from the water. For an e ffective removal, there should be dense roots, so that they can act as a physical filter and a bio-sorbent [15].

The bioactive substances released by the roots have a unique role in the removal of nutrients. These substances balance pH, and increase the humic content in the water, which results in the adsorption and/or precipitation of pollutants in the form of insoluble material [15,21]. The neutral pH induced by the vegetation helps in the settlement of dissolved particulate pollutants [24]. Moreover, these substances alter the physicochemical condition of water, and increase metal and nutrient removal and the sorption characteristics of biofilms [45,46]. For example, plants may remove phosphorus by direct uptake, but the key mechanisms of phosphorus removal are sorption, settlement at the bottom, and physical entrapment in the roots [47]. The FTWs also inhibit the growth of algal communities by removing nutrients from the water, thus reducing their population [48].

Roots act as a suitable surface for the formation of biofilms, which enhance the degradation of organic pollutants and removal of nutrients from wastewater [11]. Root exudates aid in the retention of microbes on the roots by providing them with nutrients [49]. The roots also provide oxygen to rhizospheric bacteria for aerobic degradation of organic matter. The biodegradation of organic matter into simple nutrients occurs when it comes in contact with the biofilm [50,51]. Plants remove these nutrients through direct uptake [52]. Trapping in the biofilm of the roots of macrophytes is an essential mechanism for particulate matter removal. Furthermore, roots let microbial colonies assimilate the carbon compounds and help in the reduction in biological oxygen demand and chemical oxygen demand [26]. Floating wetlands can work under both aerobic and anaerobic conditions. However, the nutrient removal under aerobic conditions is higher than under anaerobic conditions [53]. Other organic compounds are degraded by heterotrophic microorganisms either aerobically or anaerobically, depending upon the oxygen level in water [54].

#### **3. Important Components of FTWs**

FTW is composed of plants that are vegetated in a floating mat. Di fferent types of material are used as floating mats. The detail of these important components is described below (Figure 1).

**Figure 1.** Schematic representation of floating treatment wetland and pollutant removal process.

#### *3.1. Growth Media*

Di fferent types of growth media have been used to provide support to the plants growing on the floating mat. This growth media can be coconut fiber, peat, soil, bamboo crush, sand, peat rice straw, and compost [55]. The selection of growth media also influences the pollutant removal process. For instance, the use of rice straw as growth media improved the total nitrogen removal process by the formation of thick biofilms, boosting the nitrification/denitrification process [56].
