Illumina library construct design. The Illumina library was created with a two-step
protocol to add cluster binding and sequencing primer sites to the construct in the
second round of PCR amplification. First-step PCR amplification primers
(PE16S_V4_U515_F, 5′-ACACG ACGCT CTTCC GATCT YRYRG TGCCA
GCMGC CGCGG TAA-3′ and PE16S_V4_E786_R, 5′-CGGCA TTCCT GCTGA
ACCGC TCTTC CGATC TGGAC TACHV GGGTW TCTAAT-3′) contain primers
U515F and E786R targeting the V4 region of the 16S rRNA gene, as described
previously20,42. Additionally, a complexity region in the forward primer (5′-YRYR-3′)
was added to aid image-processing software used during Illumina next-generation
sequencing. The second-step primers incorporate the Illumina adapter sequences
and a 9-bp barcode for library recognition (PE-III-PCR-F, 5′-AATGA TACGG
CGACC ACCGA GATCT ACACT CTTTC CCTAC ACGAC GCTCT TCCGA
TCT-3′; PEIII-PCR-001-096, 5′-CAAGC AGAAG ACGGC ATACG AGATN
NNNNN NNNCG GTCTC GGCAT TCCTG CTGAA CCGCT CTTCC GATCT-3′,
where N indicates sample barcode position). Libraries from 2008 were created with
the primer skipping protocol, as previously described41