I have more that 2000 OTUs detected, for which only 25% have assigned taxonomy. The quality of reads is good. I have suspicious that it is a soil sample but I am not sure about it. I know it's Miseq, V3-V4.
After blast search of unassigned OTUs the results shows : "uncultured bacteria".
My questions are: What could be the reason of detection of so many OTUs? How interpret OTUs with no taxonomy?
Many thanks for any suggestion,