Hi there,
I am also experiencing a similar problem while using qiime2-2021.11 as I work through my first qiime dataset. My amplicon is the V4-V5 region of the 16S rRNA gene (used 515F/926R) with 300bp pe reads.
They appear to be in Casava 1.8 pe demuxed format, so I have imported them as such.
When I examine the dada2 results, the % of input passed filter is extremely low (>5%) and the % merged and of non-chimeras is correspondingly less than 1% for all samples (see attached) This is despite the quality of the demux file looking quite acceptable and typical (also attached).
I have tried denoising both a trimmed and untrimmed version of the reads ( I was not involved with the sequencing of this data so wasn't sure if adapters and primers had already been trimmed), but this step doesn't make a difference to my loss of reads at the denoising stage.
demux-paired-end.qzv (322.9 KB)
16s_denoising_stats.qzv (1.2 MB)
My pipeline looks like this:
qiime tools import \
--type 'SampleData[PairedEndSequencesWithQuality]' \
--input-path raw_data \
--input-format CasavaOneEightSingleLanePerSampleDirFmt \
--output-path analysis/seqs/demux-paired-end.qza
qiime demux summarize \
--i-data analysis/seqs_trimmed/trimmed_sequences.qza \
--o-visualization analysis/visualisations/demux-paired-end.qzv
# View read quality
qiime tools view analysis/visualisations/demux-paired-end.qzv
qiime dada2 denoise-paired \
--i-demultiplexed-seqs analysis/seqs_trimmed/demux-paired-end.qza \
--p-trunc-len-f 300 \
--p-trunc-len-r 220 \
--p-n-threads 0 \
--output-dir analysis/dada2out \
--verbose
# Inspect denoising results
qiime metadata tabulate \
--m-input-file analysis/dada2out/denoising_stats.qza \
--o-visualization analysis/visualisations/16s_denoising_stats.qzv \
--verbose
qiime tools view analysis/visualisations/16s_denoising_stats.qzv
Hope that provides the key bits of info. Any troubleshooting tips/thoughts are appreciated.
Thanks!
Bonnie