Thank you a lot Wocka.
Actually I'm learning to use Usearch too because one of my goals is assess which is the best current pipeline on 16S rRNA gene amplicons data analysis and which different advantages and disadvantages each one offers.
So I'll try to use the three ones that you mentioned above: QIIME, MOTHUR and Usearch.
But there is many problems to reach my goal. One of them is the different database used and another one is the different OTU picking methods, e.g. de novo, closed-reference and open-reference.
Anyway I'll try.
Again, thank you, António