QC of sequencing data - increasing G content

[robertsr]

Hello,

I conducted some metagenomic sequencing as follows:

• Metagenomic sequencing from human stool samples
• PCR-free library prep using NEB kit (450bp insert)
• Illumina NovaSeq X Plus sequencing (150bp paired-end)
• 96 samples multiplexed on 1 lane

I have just got back the QC results and the Q30 scores look good (87-90% for all samples). However the base content along the reads looks strange for some samples whereby the G content begins to increase at the end of both reads whilst C content (and sometime A/T content) begins to decrease. See some photos attached of a mixture of different samples. This occurs only for some samples, whilst others remain relatively stable in GC/AT content.

Should I be worried about this? Any advice would be helpful.

Thanks!