Full-Length 16S/18S/ITS Amplicon Sequencing by PacBio Revio & ONT PromethION — Species-Level Taxonomic Resolution Across Bacteria, Fungi, and Microbial Eukaryotes

Short-read amplicon sequencing targeting partial 16S rRNA variable regions (typically V3–V4 or V4–V5) provides genus-level classification at best — and in most microbiomes, 40–50% of sequences cannot be assigned to any known species. The fundamental limitation is not sequencing depth but read length: when the information-carrying region of a 16S rRNA gene spans approximately 1,500 bp, reading only 300–500 bp discards the phylogenetic signal contained in the remaining two-thirds of the sequence. Full-length 16S/18S/ITS amplicon sequencing overcomes this barrier by sequencing the complete gene in a single contiguous read, enabling species-level and, in many cases, strain-level taxonomic classification that is structurally impossible from short-read data alone.

CD Genomics provides full-length 16S (V1–V9), 18S (full-length SSU rRNA), and ITS (ITS1-5.8S-ITS2) amplicon sequencing on both PacBio Revio (HiFi circular consensus sequencing) and ONT PromethION (ultra-long single-molecule sequencing) platforms. Unlike sequencing providers that default to short-read approaches and offer long-read as a premium add-on, long-read sequencing is our core technology — every full-length amplicon project benefits from third-generation sequencing resolution from the start, on the platform best matched to the research question. We provide end-to-end service from primer design and long-range PCR optimization through library construction, platform-matched sequencing, and comprehensive bioinformatics analysis, delivering taxonomic profiles at a resolution that short-read methods cannot achieve.

Why Full-Length Amplicon Sequencing for Microbiome Research — Service Highlights

• Complete gene coverage for maximum taxonomic resolution — Full-length 16S (V1–V9, ~1,500 bp), full-length 18S (~1,800 bp), and complete ITS region (~400–900 bp) amplify and sequence the entire informative region, not just a fragment. Species-level classification rates increase from ~50% with V3–V4 to 75–90% with full-length reads
• Dual-platform flexibility under one service — Choose PacBio Revio HiFi for maximum single-base accuracy (Q30+, >99.9% consensus) in strain-level resolution projects, ONT PromethION for ultra-high throughput and real-time data streaming for large cohort studies, or both for comprehensive cross-platform validation
• Full-length 16S + 18S + ITS from a single service — We support simultaneous amplification and sequencing of bacterial (16S), fungal (ITS), and eukaryotic (18S) markers from the same samples, providing multi-kingdom microbial community profiling in a unified workflow
• Long-read is our core, not an add-on — We are a long-read sequencing specialist, not an NGS provider offering long-read as a secondary option. Every project benefits from third-generation sequencing resolution by default
• End-to-end bioinformatics — Full analysis pipeline including ASV-level clustering (DADA2, isONclust), taxonomic assignment against SILVA/GTDB/UNITE databases, alpha and beta diversity metrics, differential abundance analysis, and functional prediction
• Flexible sample input — Compatible with gDNA from stool, soil, water, tissue, biofilm, and environmental samples, as well as amplicon libraries prepared by the customer

Why Full-Length Amplicon Sequencing — and Why the Platform Choice Matters

The 16S rRNA gene contains nine variable regions (V1–V9) interspersed with conserved sequences that serve as universal PCR priming sites. Short-read amplicon sequencing targets 1–3 adjacent variable regions, capturing only 20–30% of the total phylogenetic information in the gene. This partial coverage imposes a hard ceiling on taxonomic resolution: at the genus level, most short-read studies achieve 80–85% classification; at the species level, classification rates drop to 40–55% regardless of sequencing depth, because the distinguishing nucleotides lie outside the sequenced fragments.

Full-length 16S sequencing reads all nine variable regions simultaneously, capturing the complete phylogenetic signal. The improvement is not marginal — it is qualitative. Species-level classification rates increase to 75–90% across diverse microbiome types, and for many genera, full-length sequences can distinguish closely related species that share >99% identity across V3–V4 alone. The same principle applies to the eukaryotic 18S SSU rRNA gene (~1,800 bp) and the fungal ITS region (~400–900 bp), where the internal transcribed spacer includes rapidly evolving regions that provide species-level discrimination within most fungal genera.

However, achieving full-length coverage is only half the equation. The platform used to sequence these long amplicons determines the accuracy, throughput, and cost structure of the project. CD Genomics offers two complementary long-read platforms, each optimized for different research priorities:

• PacBio Revio HiFi sequencing — Circular consensus sequencing reads each amplicon molecule multiple times, generating a highly accurate consensus sequence (Q30+, >99.9% accuracy). This platform is preferred when per-amplicon accuracy is the highest priority: strain-level discrimination, rare variant detection, and projects requiring maximum confidence in individual sequence variant calls. HiFi data integrates directly with DADA2-based ASV pipelines without error profile correction.
• ONT PromethION sequencing — Single-molecule sequencing generates ultra-long reads at substantially higher throughput per flow cell. The PromethION platform excels in large cohort studies, projects requiring real-time data quality assessment during sequencing, and applications where native DNA/RNA modification detection is of interest alongside taxonomic profiling. Recent R10.4.1 chemistry with duplex basecalling achieves Q30+ accuracy in the best-performing reads, and platform-specific denoising tools (isONclust, LACA) effectively manage systematic error profiles.

The choice between platforms — or the decision to use both in a complementary strategy — depends on the specific balance of accuracy, throughput, turnaround time, and budget that best fits each research project. Our project scientists provide platform-agnostic guidance to help you select the optimal approach.

Key Advantages of Full-Length Amplicon Sequencing

Technology Overview — PacBio Revio and ONT PromethION Workflows for Full-Length Amplicon Sequencing

Both platforms share an upstream workflow (DNA extraction, long-range PCR, library preparation, barcoding) but diverge in sequencing chemistry, data generation, and bioinformatics processing. Below we describe each platform's workflow independently, followed by a direct comparison to guide platform selection.

Phase 1 — Sample Preparation and Long-Range PCR (Shared Workflow)

High-quality genomic DNA is extracted from the sample matrix (stool, soil, tissue, water filter, biofilm) using extraction protocols optimized for the sample type and target microbial groups. The full-length target region is amplified using platform-specific long-range PCR: 16S using universal primers 27F–1492R (or 27F–1540R for maximal V1–V9 coverage); 18S using primers targeting the complete SSU rRNA gene (~1,800 bp); ITS using primers ITS1–ITS4 covering the complete ITS1-5.8S-ITS2 region. PCR conditions are optimized to minimize amplification bias while maintaining amplicon length integrity. Amplicons are purified by AMPure bead cleanup to remove primers and short fragments, quantified, and assessed for size distribution by TapeStation or Fragment Analyzer.

Phase 2A — PacBio Revio Library Construction and Sequencing

Purified full-length amplicons are prepared for PacBio SMRT sequencing (see our PacBio SMRT sequencing technology page for platform details) using the SMRTbell Prep Kit 3.0. Amplicons are end-repaired and A-tailed, SMRTbell adapters are ligated, and libraries are size-selected using AMPure PB beads to retain full-length inserts. Barcoded libraries from different samples are pooled at equimolar ratios (up to 384 samples per SMRT Cell 8M), bound to polymerase, and loaded onto the Revio system. Each SMRT Cell 8M generates approximately 80–100 Gb of HiFi data in a 24-hour run. The circular consensus sequencing (CCS) algorithm reads each amplicon molecule multiple times (typically 10–20 passes per amplicon), generating a single high-accuracy (>Q30) consensus sequence per molecule. For full-length 16S amplicons (~1,500 bp), this translates to 200,000–500,000 HiFi reads per SMRT Cell at multiplexing levels of 96–384 samples, providing 2,000–10,000 reads per sample with per-read accuracy exceeding 99.9%. On-instrument basecalling produces CCS reads without additional bioinformatics infrastructure.

Phase 2B — ONT PromethION Library Construction and Sequencing

Full-length amplicons are prepared for nanopore sequencing (see our Oxford Nanopore sequencing technology page for platform details) using the ONT Ligation Sequencing Kit (SQK-LSK114) or native barcoding kit (SQK-NBD114.96) for multiplexed projects. Amplicons are end-prepped, dA-tailed, and ligated to sequencing adapters with attached motor proteins. Libraries are loaded onto R10.4.1 flow cells on the PromethION P48 or P24 platform. Each flow cell generates 100–290 Gb of data over a 72-hour sequencing run, with read N50 typically matching the amplicon length (e.g., ~1,500 bp for full-length 16S). Real-time basecalling (super-accurate or high-accuracy mode using Dorado) enables immediate data quality assessment and coverage monitoring during the run. For amplicon libraries of uniform length, PromethION flow cells can produce 10–50 million reads per flow cell, accommodating 96–384 barcoded samples with 10,000–50,000 reads per sample. Duplex basecalling (Q30+) is available for projects requiring the highest possible single-molecule accuracy from the nanopore platform.

Figure 1. Integrated full-length 16S/18S/ITS amplicon sequencing workflow. PCR amplicons are prepared in parallel for PacBio Revio SMRTbell libraries (circular consensus sequencing) and ONT PromethION libraries (single-molecule sequencing), followed by platform-matched sequencing and bioinformatics analysis.

Phase 3 — Bioinformatics Processing (Platform-Specific)

PacBio HiFi data: CCS reads are demultiplexed by barcode, primer sequences are removed, and reads are quality-filtered. HiFi reads can be processed directly in DADA2 (via the "learnErrors" function adapted for HiFi error profiles) or QIIME2 with the q2-dada2 plugin to generate amplicon sequence variants (ASVs). Alternatively, de novo ASV clustering using isONclust provides reference-free ASV inference. Taxonomic assignment is performed against the SILVA 16S, GTDB, or UNITE ITS reference databases.

ONT PromethION data: Basecalled reads are demultiplexed, adapter-trimmed using Porechop, and quality-filtered (Q-score filtering). Because ONT reads have a distinct error profile (predominantly indels in homopolymer regions), standard DADA2 processing requires platform-specific adaptation. We deploy isONclust for reference-free consensus ASV inference, LACA (Long Amplicon Consensus Analysis) for denoising and error correction, or a modified DADA2 pipeline with ONT-specific error models. Taxonomic assignment follows the same reference databases used for HiFi data, with confidence scores adjusted for the platform-specific error profile.

Choosing the Right Platform for Full-Length Amplicon Sequencing

Both PacBio Revio and ONT PromethION deliver full-length amplicon sequencing with species-level resolution that short-read platforms cannot match. However, each platform has distinct performance characteristics that make it better suited for specific research contexts. The table below provides a side-by-side comparison to guide platform selection, drawing on published head-to-head evaluations including Biada et al. (2025, Frontiers in Microbiomes) and Hui et al. (2025, Gut Microbes).

In practice, the most important consideration is how the data will be analyzed. PacBio HiFi data is directly compatible with established short-read ASV pipelines (DADA2, QIIME2) because its error profile matches the assumptions of these tools. ONT data requires platform-aware processing (isONclust, LACA, or ONT-modified DADA2) but delivers comparable biological conclusions when properly analyzed. For projects where both accuracy and read depth are critical, our dual-platform strategy provides the most comprehensive solution, integrating PacBio HiFi accuracy with ONT ultra-deep coverage in a unified analysis framework.

Why Choose CD Genomics

Long-read specialist — not a short-read provider with a long-read side service.

CD Genomics is a long-read sequencing-focused CRO. Our full-length amplicon sequencing service is built around PacBio Revio and ONT PromethION platforms, not around NGS with long-read as a premium add-on. This means every project receives platform-optimized experimental design, library preparation, and bioinformatics from a team that thinks in long reads, not short reads.

True dual-platform flexibility with platform-agnostic guidance.

We operate both PacBio Revio and ONT PromethION platforms in-house, with dedicated teams for each platform's library chemistry and sequencing workflow. We help you select the platform — or the platform combination — that best matches your research priorities, without pushing you toward one platform over another based on availability or margin.

Platform-appropriate bioinformatics — we know the difference matters.

PacBio HiFi and ONT data require fundamentally different bioinformatics approaches. We do not apply a one-size-fits-all pipeline. Our analysis workflows are platform-matched: DADA2/QIIME2 for HiFi data, isONclust/LACA for ONT data, with validated cross-platform normalization when both data types are integrated in a single study. Every deliverable includes platform-specific quality metrics so you know exactly what each dataset represents.

Proven track record across diverse microbiome applications.

We have delivered full-length amplicon sequencing projects spanning human gut, soil, marine, plant-associated, industrial fermentation, and clinical microbiomes, with sample processing volumes from pilot-scale (dozens of samples) to population-scale (thousands of samples). Our published case study collaborations demonstrate the species-level resolution and biological insights that full-length amplicon sequencing — on the right platform — can deliver.

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