194 Chapter 6 Sialic acids (nonulosonic acids) are present in the “Ca. Methanoperedens” enrichment Our study furthermore aimed to expand the knowledge on sialic acids (nonulosonic acids) as a potential mechanism of adaptation to salinity stress for “Ca. Methanoperedens”. There was a marked difference in the abundance of pseudaminic acid/legionaminic acid type nonulosonic acids -measured as (Pse/ Leg)AcAm and (Pse/Leg)Ac2)- but not for N-acetylneuraminate (Neu(5)Ac)-type nonulosonic acid (Fig. 6A). Moreover, we observed changes in sialidase activity for a Neu(5)Ac probe (Fig. 6A). We then investigated the metabolic potential for sialic acid production by metagenomics. “Ca. Methanoperedens” harbored the potential for sugar conversions of the measured sialic acids (Fig. 6B). Geobacter seemed to be the most versatile community member, as it appeared to be able to independently produce the three different types of sialic acids measured (Fig. 6B). “Ca. Methanoperedens” still contributed the most to the overall gene coverage, considering that it dominated the enrichment both at 0% and 1.5% salinities. Furthermore, we retrieved low transcription levels for some genes encoding sialic acid production enzymes of “Ca. Methanoperedens”, for both freshwater and brackish salinities (Fig. 6B). For those transcripts, a low general increase in expression could be observed (Fig. 6B).
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