Certain abiotic factors limit photosynthetic processes in aquatic macrophytes, among them, light, inorganic carbon, and temperature are the most relevant, but also oxygen concentration and current velocity (Madsen and Maberly 1991). The depth of the water body attenuates light exponentially (Pedersen et al. 2013), thus its penetration can be reduced by suspended particles, pigments in planktonic algae and dissolved organic matter (Staehr et al. 2012b). Considering that macrophytes are capable of transforming radiation energy into photosynthesis (Dale and Gillespie 1977), and that suspended particles reduce the penetration of light into the water body (Staehret al. 2012b), the presence of glitter may have resulted in this decrease, since it behaves like a suspended sediment, and that the photosynthetic rates were lower in the incubations of E. densa with the microplastic. Despite resulting in effects similar to turbidity, the addition of microplastic glitter would possibly not result in the addition of nutrients, a situation similar to turbidity caused by soil particles (Machado et al. 2020).
In addition, it should be considered that on the surface of the water there is a possibility of reflectivity, transmission and absorption of incident energy, and the quantity and quality of suspended particles determine whether this will be absorbed, dispersed and transmitted. The reflectance of water varies according to its surface and its interaction with direct solar radiation, its diffusion and transmission. Naturally, the water surface reflects some of the direct solar radiation (Novo et al. 1989), but the reflectance increases because of the elevation of suspended sediments (Bhatti and Nasu 2007). Furthermore, the smaller the particle size, the higher the reflectance (Holyer 1978). Thus, the microplastic glitter, acting as a suspended sediment, increased the reflectance of the water, reflecting radiation energy at a higher intensity and decreasing the absorption process, thus the underwater radiation may have been affected.
Considering that the reflectance of metals is highly intense (Stenzel 2016), and that glitter contains metal particles to achieve high reflectivity (Locher et al. 2018; Yurtsever 2019b), these microplastics also behaved as a reflective surface. According to Stenzel (2016), light can be absorbed or scattered in the volume or on the surface of the body, or else transmitted or reflected (partially or fully). The larger amount of microplastics may result in continued reflection of radiation energy between the glitter particles themselves, increasing the possibility of light remaining inside the bottle, which could explain the higher PN and PG rate in T3 (0.0235 g) than in T1 (0.0058 g). This effect can occur in aquatic ecosystems, at specific points with particle resuspension as a function of hydrodynamic flow or wind-induced resuspension (Bertrin et al. 2017), where E. densa (a macrophyte of submerged, rooted habit and wide global distribution) colonizes the backwaters of lotic environments (Pezzato and Camargo 2004) or the littoral zones of lentic environments (Vári 2013).
However, it should be considered that the wave energy (formed by the fetch) is one of the limitations for the distribution of submerged macrophytes, with some species showing areas of preference for colonization where sediment suspension and water mixing is lower (Chambers 1987), such as E. densa, which has reduced distribution where water movement, and consequently sediment resuspension, is higher (Bertrin et al. 2017). Therefore, it is possible to assume that in natural environments, the photosynthetic response of the E. densa would be like T1, considering the reduced resuspension of microplastics and the larger dimensions of the water body compared to the bioassays used in this study, resulting in a wider space and distance between the resuspended glitter particles.
Thus, the glitter interfered in the radiation that could be absorbed in the photosynthetic processes. According to Morini and Muleo (2003), light regulates plant growth and development, relating directly with photosynthesis rates of submerged macrophytes (Menendez and Peñuelas 1993; Menendez and Sanches 1998), being extremely essential for photosynthetic processes (Freedman and Lacoul 2006). Therefore, the presence of microplastic may decrease photosynthetic rates in a monospecific bank of E. densa, however this process will not be determinant in the dissolved oxygen balance at these spots. However, it should be noted that reducing the photosynthesis rate can prejudice the macrophyte, since, as stated by Simpson and Eaton (1986) the efficiency of this process can determine the success of a specie.
The variation between photosynthetic rates in the same species may be due to the phenological stage of the plant, as well as the specific environmental conditions in which the plant develops (Rodrigues and Thomaz 2010), time of year, time when the plant is found in the water column (Pezzato and Camargo 2004), and temperature (Haramoto and Ikusima 1988). Therefore, considering that the specimens of E. densa used in the research had different phenological stages, to reproduce a real aquatic environment, and that the study was developed during different seasons of the year, such aspects may have influenced the large outliers of the treatments, like those presented by T2 and T5.
In general, although the application of glitter is not often on a daily occurrence, some single-use situations can result in a substantial amount in wastewater treatment plants and consequently in aquatic ecosystems, such as carnival festivals and other celebrations. However, there are few investigations considering glitter as a microplastic pollutant, possibly due to little understanding about its composition, and this concern is currently expressed by society (Tagg and Ivar South, 2019). About eight different studies have documented glitter particles found in samples taken from the environment, with variation in the sizes, shapes, and colors of the microplastics. The samples taken at the water surface in much of the assessments of the microplastic presence in aquatic compartments may be one reason for the small amount of glitter observed, taking into consideration that the microplastic would likely be in the sediments of the aquatic compartments and treatment plant sludge.
The glitter could also not be detected during density separation or flotation of the sediment, sludge, water, and soil samples aimed at extracting microplastics, or from dissolving the color coating in acidic solution (Yurtsever 2019b). It is interesting to note that the change in particle coating was only possible in an infrequent situation under natural conditions, and therefore, the PET would remain intact when deposited in the aquatic compartment, with a higher probability of interfering with the macrophyte's photosynthetic rates due to its metallic surface.
Although there are multiple restrictions and subsidies for decreasing the waste of disposable plastics, there are no targeted measures for glitter, which is introduced into the ecosystem already in small fragments that can accumulate in the environment as a pollutant. Therefore, it is important to formulate regulations and restrictions regarding the production, and use of this individual microplastic or in other products on the market (Yurtsever 2019a), since the continued introduction of microplastics in aquatic ecosystems will constitute a major problem in the future (Sarijan et al. 2020). It is necessary to consider glitter as a contaminating microplastic, capable of interfering in essential activities for the ecosystem, such as the photosynthesis of aquatic macrophytes, due to its microplastic structure and metallic surface, which can increase water reflectance and light reflection. The possibility of interference in the stomata is also considered, as reported by Dong et al. (2020).