invertebrates, and other plants, in more or less mutualistic or symbiotic way, associated with plants (Leach et al., 2017). Unique physiological and anatomical adaptations in the root system of wetland plants exist, allowing them to exploit the light, water, and nutrients. The adapted wetland plant roots are surrounded by a rhizosphere, which attracts by its water, oxygen, and nutrients a myriad of wetland organisms. Phytobiome plays a major role in phyto-technology.
The wetland macrophytes rhizosphere hosts large quantities of rhizospheric bacteria and archaea involved with nutrient transformation (Vymazal, 2011). Endophytic bacteria reside in various root tissues where they play an essential role in macrophytes, supporting them by providing critical nutrients such as nitrogen and phosphorus and other essential ones such as potassium in an easy-absorbable form (Cuellar-Bermudez et al., 2017; Shahid et al., 2020b). Rhizospheric and endophytic microorganisms are both essential in macrophytes growth promotion and phytoremediation. In tandem, they contribute to the fight against mixed contamination by biodegradation and sequestration of metals through a reciprocal support between macrophytes and their associated phytobiome (Shahid et al., 2020b). Much of the wetlands N-metabolism occur at or near the rhizophere. Wetland macrophytes roots influence bacterial processes in the outer region of their rhizosphere by modifying the availability of substrates and by providing oxygen and degradable OM (Bardgett and van der Putten, 2014). Fungi associate intimately with roots in macrophyte rhizospheres and thereby influence prominently wetland functioning and phytoremediation. The fungal and wetland plant associations perform several specialized functions, such as Fe nutrition and metal detoxification (Wenzel, 2009). Protozoa are widespread: their feeding on roots and microorganisms enhances the nutrient cycling through the microbial loop as well as it facilitates the passage of nutrients and energy up the food chain (Dart, 1990). In wetland soil, invertebrates are attracted to rhizosphere for oxygen and food. The rhizosphere invertebrates consume biota, flora, and root tissue, modifying the microbial biogeochemical processes, recycling nutrients, and influencing plant growth which is making them important for phytoremediation (Du et al., 2014). Finally, viruses profoundly influence the aquatic communities and bio-geochemical cycles of wetlands. Their role in the microbial loop and as pathogens can make viruses crucial to the function of wetland vegetation (Raven, 2006).
1.7.3 Various Aquatic Plants Used
Aquatic plants are plants whose entire life cycle is dependent on the aquatic environment (water or waterlogged soil). This category of plants includes algae, bryophytes, and some vascular plants that do not usually produce real roots because they can directly take the necessary nutrients from the water. Some of them produce rhizomes that store energy for the next season. Whether in the laboratory by hydroponic culture or in the field, various species of aquatic plants have been identified and recognized for their effectiveness in accumulating and degrading mineral and organic contaminants in water (Prasad, 2007). In order to achieve the best results in decontamination of pollutants the aquatic macrophytes should be selected upon their efficacy to accumulate dissolved nutrients, metals and other contaminants (Lu, 2009). The selected aquatic plants should be fast growing, easily to handle and to harvest (De Stefani et al., 2011). The severity of the pollution is another factor that might affect the success of a phytoremediation system. From a functional ecology point of view, a distinction is made between emergent, floating leaved, free-floating, and submerged aquatic plants. If emergent macrophytes are found in subsurface and surface-flow CWs, then the other types of macrophytes are exclusive to surface flow systems.
1.7.4 Emergent Aquatic Plants
Because of its large spread in natural habitats as well as its large use in CWs, P. australis is the emergent macrophyte, whose phenology has been probably the most extensively studied. Its yearly cycle begins with the start of growth, corresponding to start of the remobilization of resources (C, N, and P) stored in the rhizomes, when the accumulated degree days is high enough. Photosynthesis is taking place from the start of the growth, until the start of the senescence, after flowering. At that time, the resources are reallocated to the rhizomes and the plant starts decaying. The full decaying process is quite long (2 or more years), due to the silica content of the aboveground biomass [≈18 mg/g dry weight (Gao et al., 2019)]. By leaching of the decomposing litter, C, N, P, as well as minerals stored in the biomass are slowly released to the aquatic environment. Typha sp., P. arundinacea, Scirpus sp., and G. maxima are other groups of emergent plants, whose phenology is similar to Phragmites.
1.7.5 Floating Leaved Aquatic Plants
Floating-leaved macrophytes have roots and are anchored in the sediment. Water lilies or Nymphaeaceae are good examples of these rhizomatic plants, whose leaves are floating at the surface of the water. Their flowers can be emergent or floating.
1.7.6 Floating Aquatic Plants
Because of their ubiquitous or even invasive nature, their bioaccumulation capacity, and their resilience in a polluted environment, duckweeds are widely studied in phytoremediation (Ekperusi et al., 2019), especially the species L. gibba and L. minor (Mkandawire and Dudel, 2007). Duckweeds are known for their cold tolerance property (they can grow in all seasons), whereas water hyacinth (P. crassipes) can survive only in summer. Duckweeds can also propagate over wide pH range (Raskin et al., 1994). The biomass production of duckweed is rapid and it is largely used for exploiting the phytoremediation process when compared to other aquatic plants (Cunningham and Berti, 1993; Raskin et al., 1994; Salt et al., 1998).
1.7.7 Submerged Aquatic Plants
Ceratophyllaceae (coontails or hornworts) and Callitrichaceae (waterstarwort) are submerged plants typical of lentic systems, and they develop easily in ponds as well as in surface-flow CWs. They are slightly attached on the bottom and develop in the water column until the surface. Their photosynthetic rate decreases when the flow velocities increase (Madsen and Søndergaard, 1983). Their seasonal growth is the result of the balance between the energy from photosynthesis and the energy needed to maintain the plants (Best and Visser, 1987).
1.7.8 Mixture of Macrophytes and Microalgae
Oberholster et al. (2017) tested the feasibility of a biological hybrid treatment system to treat sulfur using AMD water treated by a CW and macro-algae from a pond system. The study was conducted under laboratory conditions to determine the bioaccumulation of S and other important algal growth elements such as Ca, Mg, and P from AMD water and treated CW water at different pH values. Following exposure of the microalgae to AMD and treated CW water for 192 h, Microspora tumidula showed the highest bio-accumulation of S and P which occurred at a pH of 5. Oedogonium crassum showed the highest bioaccumulation of Ca and Mg at a pH of 7. M. tumidula appears to be a good candidate for the treatment of sulfur-rich AMD using a hybrid biological system (Oberholster et al., 2017).
1.8 Phycoremediation
Phycoremediation is a biological clean-up technology involving the use of microalgae (in fact, algae and cyanobacteria) for the biological transformation of contaminants, including nutrients such as mineral and organic C, P, N, S, heavy metals, and emerging contaminants (Guleri et al., 2020; Leong and Chang, 2020). During the process of phycoremediation, algae utilize nutrients (N and P), C, and other salts from the wastewater for their growth. The clean-up process has been a research subject for many decades (Oswald, 1995). Algae are also used for their ability to oxygenate the environment due to photosynthesis (Hernandez et al., 2006). Macrophytes and algae are often associated, voluntarily or not, in SFS-CWs. In some cases, algae are considered a public health concern due to the secretion of harmful
