Dear Qiime community,
I'm using Qiime2 (installed with conda) for the first time and I'm analyzing Illumina PE data obtained by 2x250 bp on V3-V4 region (341F-806R).
These are the reads :
1)Why quality of reverse reads is ( as usually seen) so bad? Are these reverse reads still usable in PE analysis or is better a single-end forward reads approach? What can I do to avoid this quality drop in reverse reads next time?
2)I've noticed that trimming and maxEE parameters in this situation have a strong effect on taxonomy composition. Bifidobacterium sp. is a core species in this microbiota and is consistently present in all the samples when I analyze only forward reads. However, this species is absent when I do PE analysis with DADA2 using the following command :
qiime dada2 denoise-paired
--i-demultiplexed-seqs demux-paired-end.qza
--p-trim-left-f 0
--p-trim-left-r 0
--p-trunc-len-f 249
--p-trunc-len-r 240
--o-table table.qza
--o-representative-sequences rep-seqs.qza
--o-denoising-stats denoising-stats.qza
Bifidobacterium is present only in few samples when I trim more the reverse reads, and increasing maxEE to 10, with the following command:
qiime dada2 denoise-paired
--i-demultiplexed-seqs ../../../demux-paired-end.qza
--p-trim-left-f 0
--p-trim-left-r 0
--p-trunc-len-f 249
--p-trunc-len-r 232
--p-n-threads 4
--p-max-ee-r 10.0
--o-table table.qza
--o-representative-sequences rep-seqs.qza
--o-denoising-stats denoising-stats.qza
Here are the results of taxonomy and the representative sequences:
rep-seqs.qzv (270.0 KB) taxonomy.same.silva_blast_v3v4_16S.qzv (1.3 MB)
How do you explain this situation? I've noticed that Bifidobacterium sequences has only 445 bp, while I expect something like 460 bp using these primers... Do you think that this length variability can be responsible of this different species composition using different denoising parameters?