The study focused on Lake Aydat, a shallow, nutrient-rich lake that stratifies and mixes twice a year, making it a classic model system for freshwater ecology. From November 2018 to January 2020, the team collected water from the sunlit euphotic zone at the deepest point of the lake, sampling monthly and intensifying to twice weekly during late spring. Fourteen litres of water per sample were filtered to concentrate microbial biomass, which was preserved at minus 80 degrees Celsius until DNA could be extracted. This careful, time-resolved sampling design was essential: the goal was not merely to list the species present, but to track how entire communities of algae, ciliates, fungi and parasites rose and fell across a full annual cycle.

The short-read side of the comparison used Illumina sequencing of the V4 hypervariable region of the 18S ribosomal RNA gene, a fragment of roughly 380 base pairs that has become the standard marker for surveying eukaryotic diversity. The long-read side used Pacific Biosciences Sequel II technology with circular consensus sequencing, a method that reads the same DNA molecule repeatedly to correct errors, achieving accuracy comparable to Illumina. Instead of a single hypervariable region, the long-read approach amplified nearly the entire ribosomal RNA operon — approximately 4,500 base pairs spanning the 18S gene, the internal transcribed spacer regions, the 5.8S gene and part of the 28S gene. In total, the team recovered more than 153,000 filtered long reads, from which they extracted over 20,000 full-length 18S sequences averaging about 1,232 base pairs each.

Both datasets were processed with comparable pipelines, clustering sequences into operational taxonomic units at 97 percent identity and assigning taxonomy through phylogenetic placement against curated reference databases. After filtering, the short-read dataset contained 747 OTUs and the long-read dataset 546. To make a rigorous, like-for-like comparison, the researchers performed an elegant analytical trick: they extracted the V4 region computationally from the long-read sequences, creating a hybrid dataset that could be clustered together with the genuine short-read OTUs. This allowed them to ask precisely whether differences in taxonomic assignment arose from the length of the sequence itself or from the different organisms each method happened to capture.

The answer was unambiguous. At broad taxonomic levels — supergroups and divisions — the two methods agreed well, painting similar pictures of the lake’s major eukaryotic lineages. But at finer scales the approaches diverged sharply. Among clusters containing sequences from both datasets, more than a quarter showed deeper taxonomic assignment with the long-read data, in some cases resolving up to seven additional hierarchical ranks. Overall, nearly 30 percent of long-read OTUs could be classified to genus level, compared with only 11 percent of short-read OTUs. The long-read dataset also recovered 35 genera that the short-read approach missed entirely, against just 13 genera unique to the short reads — and those short-read-only sequences tended to be rare, low-abundance taxa, consistent with Illumina’s greater sequencing depth.

Certain groups benefited disproportionately from the longer sequences. Ciliates, single-celled predators that are central to aquatic food webs, were frequently assigned only to phylum by short reads, while long reads resolved 34 percent of ciliate OTUs to genus. Chytrid fungi, parasites of phytoplankton with major ecological influence, saw genus-level identification jump from 8 percent with short reads to 36 percent with long reads. Diatoms, the silica-shelled algae that underpin much of freshwater primary production, were classified to genus in 92 percent of long-read OTUs versus 56 percent of short-read OTUs. Where the short-read data suggested a lake dominated by the diatom genus Fragilaria, the long-read data revealed substantial populations of Cyclotella and Asterionella — a finding that matched earlier microscopic observations of the lake, underscoring how short-read surveys can misrepresent community composition.

The seasonal dynamics the long-read data revealed are where the study becomes genuinely vivid. Because taxa could be identified precisely, the researchers could watch individual genera succeed one another through the year. The ciliate Condylostoma peaked in June 2019, while Carchesium, a related filter-feeder with similar dietary preferences, showed a different temporal pattern — a succession that may reflect differing life-history strategies or tolerances to temperature, oxygen and nutrient fluctuations. Among the diatoms, Cyclotella flourished in spring, Fragilariforma dominated in July, and Asterionella reached its greatest abundance in the cold months of January and April. Such fine-grained temporal partitioning among close relatives is exactly the kind of ecological process that short-read data, with its blurred taxonomy, tends to obscure.

Perhaps the most ecologically compelling result concerns parasitism. Chytrid fungi infect and lyse phytoplankton, sometimes terminating algal blooms and reshaping entire food webs, yet host–parasite dynamics are notoriously difficult to observe in nature. The long-read data allowed the team to track the chytrid genus Zygorhizidium alongside its diatom hosts month by month. Zygorhizidium abundance rose sharply between March and May, coinciding with the spring diatom bloom — a pattern consistent with laboratory work showing that Zygorhizidium affluens infects the diatom Asterionella formosa, but now observable directly in a natural lake. A December peak in chytrids followed a November surge of the golden alga Mallomonas, echoing previous reports of chytrid-driven algal declines. The data also showed simultaneous peaks of Perkinsea parasites and colonial green algae, supporting emerging evidence that these parasites, better known from marine hosts, play a similar role in freshwaters.

The authors are careful to note that short-read sequencing retains real advantages: it is cheaper, generates far greater depth, benefits from mature analytical pipelines, and excels at detecting rare taxa. Long reads also demand higher-quality DNA, since degraded environmental samples can fail to amplify across 4,500 base pairs, and reference databases remain thinner for full-length sequences, which occasionally forced more conservative assignments. Indeed, in a small minority of cases the short reads achieved finer classification, likely because the V4 region is better represented in reference databases. These caveats matter for anyone designing a monitoring programme, and the study does not suggest that short reads should be abandoned.

But the trajectory is clear. As sequencing costs fall and reference databases grow, the study argues that long-read metabarcoding should move from specialist tool to routine instrument of environmental surveillance. The ability to resolve genera and species in time series opens the door to tracking bioindicator ciliates for water quality, detecting parasitic outbreaks before blooms collapse, and mapping the cryptic diversity that short fragments cannot distinguish. Combined with the 28S sequences the team plans to analyse next, and with emerging single-cell and genome-based reference collections, long-read surveys promise to close taxonomic gaps that have limited freshwater ecology for two decades. For the invisible majority of lake life, the picture is finally coming into focus — one long read at a time.

Subject of Research: Comparing short-read and long-read metabarcoding of eukaryotic diversity in a lake over one year

Article Title: Profiling the eukaryotic diversity over a year in a lake ecosystem through short- and long-read metabarcoding

Article References: Chauvet, M., Terzian, P., Moné, A., Debroas, D., Bronner, G., & Lepère, C. (2026). Profiling the eukaryotic diversity over a year in a lake ecosystem through short- and long-read metabarcoding. Metabarcoding and Metagenomics, 10, Article e184566. https://doi.org/10.3897/mbmg.10.184566

Image Credits: AI Generated

DOI: 10.3897/mbmg.10.184566

Keywords: metabarcoding, long-read sequencing, PacBio, eukaryotic diversity, Lake Aydat, 18S rRNA, chytrid fungi, diatoms, ciliates, freshwater ecology, seasonal dynamics, host-parasite interactions

The scale of the challenge is matched by the scale of the opportunity. Photosynthetic protists, particularly algae, are responsible for producing around half of the world’s oxygen, making them foundational to the very habitability of the planet. Other protists graze on diverse microorganisms, regulating microbial communities and supporting ecosystem health in soils, oceans, and freshwater systems alike. Their influence extends into human affairs in less welcome ways as well: certain protists spread devastating diseases such as malaria and sleeping sickness, while harmful algal blooms can damage marine ecosystems and the economies that depend on them. Understanding these organisms at the genomic level is therefore not an esoteric pursuit but a matter of environmental, agricultural, and medical consequence.

Why, then, have protists lagged so far behind bacteria, plants, and animals in genomic research? The answer lies in their extraordinary diversity. Protists deviate from nearly every standard biological assumption that makes working with model organisms straightforward. They exhibit an immense range of cell structures, chemistries, genetic makeups, and evolutionary histories. Some possess genomes of bewildering complexity, others have diverged so far from the canonical genetic code that routine analytical tools fail them entirely. This variety makes culturing, sequencing, and analysing protists inherently difficult, and it means that protocols developed for one lineage often fail spectacularly when applied to another. The result is a vast proportion of biodiversity that continues to be overlooked, simply because the technical machinery of modern genomics was not built with these organisms in mind.

Professor Neil Hall, Director of the Earlham Institute, framed the problem in stark terms. Despite representing the largest share of eukaryotic diversity, protists remain poorly classified and understudied, he noted, with their immense and largely unexplored diversity leaving a significant blind spot in our understanding of life on Earth. He acknowledged that the community has made good progress in addressing some of the technical challenges, but warned that if these bottlenecks are not continually overcome and the solutions scaled up, a vast proportion of biodiversity will continue to be overlooked. The new investment is designed precisely to move from incremental progress to systematic, scalable solutions.

The partnership brings together complementary strengths from across Europe. Alongside the Earlham Institute, the collaboration includes the Scottish Association for Marine Science (SAMS), the Culture Collection of Algae and Protozoa (CCAP), and Aalborg University in Denmark, uniting international expertise in protist culturing, single-cell genomics, and long-read sequencing. The project will address technical challenges and bottlenecks for sequencing protists from two sources: bulk cultures maintained in laboratories, and single cells isolated directly from the environment. This dual approach reflects a recognition that many of the most interesting protists have never been brought into culture at all, and that environmental sampling is the only way to reach them.

The Culture Collection of Algae and Protozoa, housed at the Scottish Association for Marine Science, manages one of the world’s largest and most taxonomically diverse protist culture collections. Within the project, CCAP will provide researchers with stable cultures of key protist lineages, enabling them to scale up biomass and enrichment, and will contribute expertise in developing ecologically informed cultivation methods. Drawing on existing knowledge and success stories from across the protist community, the researchers will begin with ten major classes of protists before scaling up their efforts, testing existing protocols across species to compare, optimise, and evaluate successes in extraction, cell lysis, and DNA amplification methods. This comparative, systematic approach is intended to replace the ad hoc trial-and-error that has characterised much protist genomics to date.

The SAMS contribution extends well beyond the established collection. Dr David Green, Principal Investigator in Molecular Microbiology at the Scottish Association for Marine Science, explained that SAMS researcher Dr Frederik De Boever has developed a new cultivation approach based on how these organisms live in nature. That ecologically grounded strategy has led to the creation of a unique research collection including more than 50 genera from some of the least understood groups of protists, such as Provora, centrohelids, telonemids, rhodelphids, kathablepharids, and heterotrophic Paulinella. Together, Green noted, the CCAP and SAMS collections cover an exceptionally wide range of evolutionary diversity, giving scientists a powerful resource for exploring how these organisms evolved and function, and one that is likely to lead to many important and exciting discoveries.

The Earlham Institute arrives at the collaboration with a proven track record of extracting genomic surprises from non-model organisms. Its researchers have previously revealed fascinating diversity and evolution among protists, including species harbouring previously unknown Legionella bacteria, and unique divergence in the DNA code itself, findings that underscore just how radically these organisms can depart from biological convention. Through its high-performance sequencing platforms and technical skill in scalable short-read and long-read workflows, together with single-cell sequencing and analysis, Earlham scientists will work with SAMS to optimise cell lysis and DNA extraction methods, identified as one of the major technical bottlenecks in the field. Breaking open protist cells without destroying their genetic material is far harder than it sounds when cell walls, scales, and chemistries vary wildly across lineages.

Meanwhile, the collaboration with Aalborg University targets the frontier of cultivation-independent genomics. Ramunas Stepanauskas, Professor of Single Cell Genomics at Aalborg University, described single-cell genomics as a key technology for recovering the genomic blueprints of these dominant yet elusive eukaryotes, most of which remain uncultured. According to Stepanauskas, the project will advance cultivation-independent genome recovery from unicellular eukaryotes, unlocking the fundamental biology, evolutionary history, and biotechnological potential of this vast and underexplored component of the planet’s biota. The ability to sequence the genome of a single cell plucked directly from seawater or soil, without ever growing it in a laboratory, promises to open windows onto lineages that have never been seen under controlled conditions.

Open science and a collaborative culture sit at the heart of the undertaking. Knowledge and skills exchange across all partners, at all stages of the workflow, is designed to ensure that learnings are shared and that methods and protocols are cross-validated across disciplines. The ambition reaches beyond the four institutions involved: by equipping the wider research and innovation community with improved pipelines, culturing resources, and technical protocols, the project aims to deliver increased reproducibility and to enable successful international protist sequencing projects for years to come. As Professor Hall put it, the partners are excited to be bringing this community together to drive forward improvements in the technical challenges facing the field. If the collaboration succeeds, the invisible majority of eukaryotic life may finally begin to receive the genomic attention its diversity demands, with consequences ranging from new industrial enzymes and sustainable agricultural tools to a fundamentally richer map of the tree of life.

Subject of Research: Protist genomics and single-cell sequencing technology development

Article Title: Major investment to unlock the potential of protist research

Article References: Major investment to unlock the potential of protist research. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: protists, genomics, single-cell sequencing, Earlham Institute, SAMS, CCAP, Aalborg University, Gordon and Betty Moore Foundation, long-read sequencing, algae, eukaryotic diversity, biomanufacturing