OTU sequences must have OTU identifiers
An OTU table is generated by the otutab command or closed_ref command.. OTU identifiers are extracted from the sequence labels in the search database which contains OTU or ZOTU sequences.
OTU identifier syntax
First, usearch looks for an annotation otu=xxx; in the label. If this is found, then xxx is the OTU identifier. Otherwise, the OTU identifier is the sequence label with all annotations removed, i.e. all characters up to the first semi-colon (;) or the end of the label, which ever comes first.
The following labels all contain an OTU identifier with 123.