192 Chapter 6 encoding a retron-type reverse transcriptase (ltrA) was upregulated, suggesting a prophage-related shift (Fig. 4C). We paired the metatranscriptomic observations to the metaproteome. Some proteins of interest with yet uncharacterized function included a putative spore-production linked protein (YtfJ), which was significantly downregulated (Supplementary Fig. 11). Additionally, a cell-division related protein belonging to the ‘sepF_superfamily” was significantly upregulated from 0% to 1.5% salinity (Supplementary Fig. 11). PHA in “Ca. Methanoperedens” decreased at increasing salinities “Ca. Methanoperedens” stained positive for PHA accumulation in our enrichment culture at salinities 0% and 1.5% (Fig. 5A, Supplementary Fig. 13). This observation aligned with the high expression of genes encoding PHA synthase in the metatranscriptome data (phaC and phaE; top 3% of genes of the total “Ca. Methanoperedens” expression at 1.5%; Fig. 5B, Supplementary Table 8 and 10). We also quantified PHA derivatives at 0%, 1.5%, and 3% salinities and observed a clear decrease per biomass weight (Fig. 5C). Albeit the metatranscriptome indicated a significant upregulation of the phaE and phaC subunits (Fig. 4A), we observed a decrease in abundance of the PhaE enzyme at 1.5% salinity in the proteome dataset (Supplementary Fig. 11).
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