# Influence of Macrophyte and Gut Microbiota on Mercury Contamination in Fish: A Microcosms Study
## 4 Discussion
### Periphyto Associated with Root's Macrolytes: Main Source of MeHg for Organisms
#### 4.1.2 MeHg Produced in Aquatic Rhizsphere is Bioavailable for Organisms  
The trophic pathway is considered to be the major contamination pathway in aquatic food webs for Hg bioaccumulation and biomagnification [55; 56; 57]. The \"plant\" condition allows for the evaluation of the role played by aquatic plants in MeHg bioavailability to aquatic organisms without sediment influence. Aquatic plants are regarded as an important food source for herbivorous and scavengers of some aquatic ecosystems [22; 58; 59]. In this study, [THg] is significantly higher in the gut of periphytophageous fish for the control and contaminated \"plant\" conditions compared to the \"plant+sediment\" and \"sediment\" conditions (Figure 3), and between the control and contaminated units. This indicates that (1) the periphyton associated with aquatic plant roots could be a food source for fish--this result is generally observed with periphytophageous fish due to their diet [60], and (2) there is a trophic contamination pathway for Hg. However, a low direct contamination could be possible at Ti (just before the fish introduction) in the three contaminated conditions since [\({}^{199}\)IHg] levels in water were not negligible (Table 2) and [\({}^{199}\)MeHg] levels were also very low in terms of proportion (Figure 2). The maximum levels of [\({}^{199}\)IHg] and [\({}^{199}\)MeHg] were observed in the plant compartment compared to the sediment compartment. However, \({}^{199}\)MeHg proportion and concentrations in fish muscle were similar in both the \"sediment+plant\" and \"sediment\" conditions. Sediment could act as a limiting factor of Hg bioavailability for aquatic organisms. In [61], the ability of mangrove sediment to trap Hg, limiting its bioavailability is highlighted.  
In all conditions (control and contaminated), the MeHg % in fish is always higher than the IHg %, showing the higher bioavailability of MeHg compared to IHg (Tables 1 and 2). The significantly higher \({}^{199}\)MeHg proportion observed in fish from the \"plant\" condition supports the fact that plants (and sediment to a lesser extent) play a role in fish Hg contamination, despite a heterogeneity observed between replicates of fish muscle. The difference in plants in each unit could explain this heterogeneity. Indeed, only a single plant of _Ludwigia_ sp. was planted by aquarium (average weight \(ww\) = 40 g). Periphyton could not have the same microbial composition from one root to another, even if they are spatially close. Studies of SRP diversity (_dsrAB_ and _16S rRNA_ genes) in _Ludwigia_ sp. roots from Sanguinet Lake revealed heterogeneous microbial communities between replicates of samples [7,8]. This could be attributed to the different stages of biofilm development. In this case, Hg methylation could vary significantly from one plant to another, and hence to one microcosm to another, leading to differences in MeHg production and transfer to fish. Another explanation could be the variable amount of food eaten by the fish, some feeding more than others and therefore bioaccumulating more Hg. The duration of the experiment (15 days) may also have played a role and may have been too short to obtain clear information about it.