198 Chapter 6 Vercelli Strain 1” (Fig. 3C and Supplementary Fig. 10). In addition, our phylogenetic analysis suggested HGT events from bacteria to ANME archaea. In fact, both kamA and ablB genes from ANME root back to those from Firmicutes species. Although it has been documented that “Ca. Methanoperedens” is particularly receptive to the acquisition of genes from a variety of bacteria and archaea (Leu et al., 2020b), the role of the Borg elements in such a transfer is unclear. An exploratory visualization of upregulated stress-related transcripts pointed out some related to the production of arginine and glutamine (argG and glmS), which have been well established as compatible solutes (Burg & Ferraris, 2008; Goude et al., 2004) (Fig. 4A). Another clear transcriptional shift was the high expression of genes encoding for cation transporters (Fig. 4C). These transporters could be involved in regulating cellular osmotic pressure, as reported for a brackish acclimated N-DAMO culture when exposed to marine salinities (Frank et al., 2023). We also observed a different response for two different types of ferritin, classified as “ferritin like” family (upregulated) and “ferritin like AB” family gene (downregulated) (Fig. 4C). Although the latter still holds an uncharacterized function, the ferritin-like upregulated transcript, similarly to the bacterioferritin, has been linked to a range of functions such as iron regulation, and reactive radical production (Claret, 2022). We also observed the upregulation of a gene encoding for a large mechanosensitive channel, mscL, which helps cells cope with hypoosmotic conditions (Levina et al., 1999). This is in agreement with the observed downregulation of mscL upon hyperosmotic stress (Fig. 4C). Our study indicates a putative salt-stress trigger of a morphology shift as we detected an upregulation of several morphology-associated transcripts (Fig. 4B). Recently, this pleomorphic shift was reported for “Ca. Methanoperedens nitroreducens” (McIlroy et al., 2023). Here, granular/PHA-accumulating “Ca. Methanoperedens” cells were differentiated from their planktonic free-living rodshaped morphotypes. Such transcript upregulation was also previously observed in an oxygen-stress “Ca. Methanoperedens” study (Guerrero Cruz et al., 2018). Similarly, some Methanosarcina species have been reported to disaggregate and grow as single cell upon salt-exposure (Sowers et al., 1993). Taken together, this indicates a potential link between stress and phenotypic shifts. However, considering that
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