Hi, all! I download fastq.gz files of GSE162708 from ENA which only have 2 files of each sample(usually scRNA-seq has 3 files I1 , R1 & R2 ). Then I run fastp as following

for i in `seq 1 4`
do
    echo SRR1320051${i}
    fastp -i ./fastq/SRR1320051${i}_1.fastq.gz -o SRR1320051${i}_R1.fastq.gz \
        -I ./fastq/SRR1320051${i}_2.fastq.gz -O SRR1320051${i}_R2.fastq.gz  -w 6
done

Then I get QC report , but I can't understand why Per base sequence content of Read1 looks so strange . The original R1 sequence like this

@SRR13200511.1 1/1 CNGGGCTAGCGTCAAGTTTCGGCTCCGCCTCGGCGCAAATTTTTTTTTTTTTTTTTTTTTTTTTTTTGGGTGGGGGGGAAAGGGGGGGATTGTAATGG + F#FFFFFFFFFFFFF:FFFFFFFFFFFF:::,FF::F,,,FFFF:,:,F::,:F:FFF:::F,,,,,F,:,:,FFFFF,,,:,FFF,:,,,:,,,,FF

@SRR13200511.2 2/1 TNGCGTGCAAGTGCAGTGACCGCCACTATTCATCGCAAATTTTTTTTTTTTTTTTTTAAAAAAATATGTTTTTATATTATAATTTACCTATATTATAT + F#FFFFFFFFFFFFFFFFFFFFFF:FFF,F:,,:F,:,:FFFFFFFFFFFFFFFFFFF,FFFF,,:,:F,FFF,,,,:,,FF,FF,,,,::,,,,,,,

@SRR13200511.3 3/1 GNTCTGGGTGCAGTGATAAAACGTTCATGTAAGATGATTTTTTTTTTTTTTTTTTTTTAAACCCCCACAAAAAAATTTTTAAAAAAACAAGAAGGGTC + F#FFFFFFFFFFFFFFFF:FFFFFFFFF,F,,FFF,FFFFFFFFFFFFFFFFFFFFFFF,,F:FFF,FF,,,::FFF,,F:,FF:F,:,,,,:,:F,,

zcat SRR13200511_2.fastq.gz | awk -F "" '{print NF}'|head

18
150
1
150
18
150
1
150
18
150

It would be greatly appreciated if anyone can tells what cause the strange Per base sequence content !