# Mercury Uptake and Transport by Plants in Aquatic Environments: A Meta-Analysis
## 1 Introduction  
Heavy metals are some of the deadliest pollutants and their concentrations in the environment have considerably increased owing to human activities [1]. [PERSON] [2] and [PERSON] et al. [3] have shown that five potentially toxic heavy metals, namely arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), and lead (Pb), are the most dangerous and persistent pollutants. Hg, which has been listed as a global pollutant by the United Nations Environment Program, can be transported through air in a gaseous form at room temperature. When Hg enters water, it undergoes methylation, produces highly toxic methylmercury, and accumulates in food chains; this results in an increase in its concentration, with increasing trophic levels [4]. Studies have shown that Hg can reduce the photosynthetic rate of algae and inhibit cell division [5]. It has also been observed that water bodies with Hg concentrations of 0.003 g/L exert a toxic effect on goldfish [6]. Additionally, owing to the escalating nature of toxins in food chains, Hg is accumulating at an exponential rate in the internal organs of mammals [7]. When this bioaccumulated Hg reaches humans, it can cause brain-, heart-, kidney-, and lung-related diseases, and can even contribute to chromosome breaks [8, 9].  
Conventional methods for remediating heavy metal pollution in water, such as ion exchange, precipitation, and reverse osmosis, are not cost-effective and can adversely affect aquatic ecosystems [10]. Fortunately, phytoremediation technology, which focuses on the ability of plant roots, stems, and leaves to absorb and accumulate heavy metal elements, has been extensively investigated as a practical alternative to conventional methods [11]. Certain floating, submerged, and emergent plants have been identified as potential heavy metal accumulators [12, 13, 14, 15]. These aquatic plants generally absorb heavy metals via chelation and compartmentalization and, thus, demonstrate tolerance to highheavy metal concentrations [16]. Specifically, glutathione and chelating peptides derived from it play an important role in plant-assisted detoxification by binding to heavy metals and reducing free heavy metal ions in plant cells [17].  
Several researchers have conducted experiments on the remediating effect of aquatic plants on heavy metal pollution; some review articles have also qualitatively described the heavy metal absorption mechanism of these plants [10; 11; 12; 13; 14; 15; 16]. However, systematic and statistical methods for the comprehensive evaluation of the Hg absorption capacity of aquatic plants are still lacking. Furthermore, the absorption capacities of aquatic plants for different heavy metals in polluted water are different, even when the influencing factors are the same [18]. Therefore, it is necessary to comprehensively and quantitatively analyze the Hg absorption and transport capacities of different types of aquatic plants that are exposed to different Hg concentrations and various environmental factors.  
Meta-analysis is a quantitative statistical analysis method that includes point estimation and bias-corrected 95% confidence interval (CI), other central trends, as well as accuracy [19]. It can be used to synthesize and compare research results on a specific research topic and also summarize the overall rules of the topic [19; 20]. Meta-analytical methods have been increasingly used in the study of ecological problems [20]. Therefore, in this study, meta-analysis was used to quantitatively analyze (1) the Hg absorption capacity of different plants, (2) the absorption of Hg ions in plant roots and their transportation throughout the plants, and (3) the effects of different influencing factors on the heavy metal absorption capacities of the plants.