Trimming the first 5 low(er) quality bases from forward and reverse reads results in 25% more sequences removed as chimeras

Nothing to add except to reiterate that you must remove the primers. The entire primers, not just 5bps. The primers aren't sequences from the sample, they are sequences that were added into the PCR reaction, and the ambiguous nucleotides in the primers interfere with denoising and chimera detection.

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