Silva classifier seven level code

[SoilRotifer (Mike Robeson)]

Some background as to why the 18S rank information is inconsistent is outlined here:

Why all taxonomic levels on BarPlot have 'D' letter?

I can likely provide some insight here as I am one of the contributors that helped to format SILVA database for QIIME. The D_X__ convention was chosen to be as much of a unique and “safe” text string as possible, considering many of the bizarre taxonomy text annotations within the SILVA reference database. That is, it was meant as a quick fix to be able to search and parse these taxonomy strings. The ‘D’ was a way of annotating the “taxonomic Depth”. At the time some of the code was written, th…

-Mike