One quick recommendation, the column titles end up being:
rank_names(PHYLOSEQ.OBJECT)
[1] "ta1" "ta2" "ta3" "ta4" "ta5" "ta6" "ta7"
You can use this code to give your column titles:
colnames(tax_table(OBJECT.GOES.HERE))=c("Domain", "Phylum", "Class", "Order", "Family", "Genus", "OTU")