How to use assignTaxonomy function in dada2 for nifH sequences using the Gaby and Buckley database (nifH_database_2012.arb)

I am trying to use the assignTaxonomy function in the dada2 pipeline to assign taxonomy to my nifH sequences. I have used this function for 16S taxonomy assignment and it worked. However, I used the same 16S taxonomy database as the dada2 example (benjjneb.github.io/dada2/tutorial.html). I think there may be a problem with the format of the nifH database that I am trying to use but I'm not sure how I should modify it. Thanks!

I have completed the dada2 steps from filtering my samples until removal of chimeras which is my object seqtab.ex.chi. I have then tried to assign the taxonomy by creating a path to the nifH database I downloaded from here http://www.css.cornell.edu/faculty/buckley/nifh.htm

My code:

Taxonomic assignment (Gaby&Buckley)

gb.taxa = assignTaxonomy(seqtab.ex.chi, "~/Desktop/Bryce/nifH/nifH_database_2012.arb", multithread = T)

Error:

Error in .Call2("fasta_index", filexp_list, nrec, skip, seek.first.rec, : reading FASTA file /Users/geneious/Desktop/Bryce/nifH/nifH_database_2012.arb: ">" expected at beginning of line 2