## 4 Discussion
However, research focused on the plant rhizosphere in soils reports on what is called the rhizosphere effect defined as biological, chemical, and physical changes that occur due to root exudates and rhizdeposition (sloughing of the root cap) [36]. Plant roots in this study are likely exhibiting a rhizosphere effect on the hydroponics system by attracting microbes with plant exudates released in response to high silver exposure. The diversity and abundance trends of this study are summarized in Figure 9.  
The roots in the 125 ppb and 500 ppb silver treatments of the hydroponic setup were predominated by the genera _Shingomonas_, _Spingobium_, _Ralstonia_, _Novoshingobium_, and _Cupriavidus_. Each of these genera was also found in the hydroponics irrigation water at the time of harvest but at differing relative abundances. The literature on soil-based agriculture similarly reports the observation of heavy metal exposure and the selection and subsequent proliferation of metal-tolerant species [37]. The genera selected in heavy metal soils include representatives of _Pseudomonas_ and _Acinetobacter_[38] which are distinct from the predominately water-borne genera that are enriched in our study. Specifically, we observed a selective pressure for _Cupriavidus_ from the hydroponic water tray to associate with the plant root. The _Cupriavidus_ genus is noted for its ability to grow in the presence of heavy metals [39]. This observation has interesting implications for the accumulation of resistant species if silver is used as the sole biocide in the water system. Finally, the enriched bacterial genomic pathways in the arcillite and hydroponic-grown lettuce for each silver irrigation treatment as compared to the respective 0 ppb silver control in each setup were examined. Here it was found that each of the three silver treatments in the arcillite growth setup showed enrichment in the ECASYN-PWY: enterobacterial common antigen (ECA) biosynthesis pathways in the lettuce roots as compared to the control with 0 ppb silver. The ECA is an outer membrane glycolipid shared by all members of the _Enterobacteriaceae_ family naturally found in the lettuce microbiome [40]--a family that is not represented by the top ten genera across all samples. In contrast, in the hydroponics silver-treated lettuce roots, there is complete overlap in the enriched pathways for the 125 ppb treatment with the 500 ppb enriched pathways. In both these conditions, a stunted growth phenotype was observed. The enriched pathways illustrate that fatty-acid biosynthesis and the ability to shift metabolism may help microbes under high silver treatments to associate with plant roots to overcome silver stress and access nutrients. Although the exact mechanism of action for silver as an antibacterial is not completely understood, prior work indicates that silver particles cause damage to the bacterial cell  
Figure 9: Summary of diversity and abundance trends for the microbiome of lettuce roots and shoots and irrigation water from arcillite and hydroponic setups treated with 31 ppb,125 ppb, or 500 ppb silver water. membrane and this in turn affects intracellular metabolic activity [41; 42; 43]. Furthermore, [PERSON] et al. 2019 showed that the introduction of Ag\({}^{+}\) produced pressure on antibiotic resistance genes similar to that of an antibiotic [44]. This present study did not show an enrichment in any antibiotic resistance genes, however, a microbial community of constituents with an enhanced ability to generate fatty acids may be able to counter damage to the cell membrane caused by silver. In addition, an enhancement in a diversity of metabolic strategies may help microbes to shelter on plant roots as biofilms that feed off plant exudates. Interestingly, in the 500 ppb silver treatment the ppGpp pathway--a bacterial stress response alarmone [45] and transcriptional regulator of the plastid-encoded RNA polymerase in plant chloroplasts [46]--was enriched.