New Diatom Species Hidden in Polish Mountain Springs Rewrites Diatom Family Tree

Deep in the Bieszczady Mountains of southeastern Poland, cold water seeps from springs that have been flowing for millennia, largely undisturbed by human activity. Scientists sampling these pristine habitats have now uncovered something remarkable: a diatom species entirely new to science, hidden in plain sight among specimens that researchers had long mistaken for its better-known relatives. The discovery, published in the journal PhytoKeys, not only adds a new name to the catalogue of life but also delivers the first molecular data ever obtained for the diatom genus Odontidium, reshaping how scientists understand the evolutionary history of this poorly studied group.

Diatoms are microscopic, single-celled algae encased in intricately patterned glass-like silica shells. They are among the most important primary producers in freshwater ecosystems, and their species diversity is enormous, with many taxa still awaiting formal description. The genus Odontidium, characterised by prominent transapical ribs running across its valve face, is frequently reported from cold, nutrient-poor streams and spring habitats, particularly in alpine and high-elevation regions. Yet despite its regular appearance in these environments, the taxonomy of Odontidium has remained contentious, with boundaries between it and the closely related genus Diatoma shifting repeatedly over nearly two centuries of study.

The new species, formally named Odontidium fontinale, was discovered during field investigations of spring habitats within Bieszczady National Park, the largest national park in the Polish Carpathians and part of a UNESCO transboundary biosphere reserve. Researchers collected algal material from four springs between 604 and 1,174 metres above sea level, sampling both sandy sediments and submerged mosses at each site. The specific epithet fontinale, derived from the Latin word for spring, reflects the habitat from which the species was collected and where it appears most abundant.

What makes this discovery particularly significant is the integrative approach the research team employed. Rather than relying solely on traditional morphological comparison, the scientists combined detailed light and scanning electron microscopy with molecular phylogenetic analysis based on the chloroplast rbcL gene. This dual strategy proved essential, because Odontidium fontinale closely resembles Odontidium mesodon, a species frequently reported from spring ecosystems across Europe. The two taxa share a similar valve outline and prominent transapical ribs, making them nearly indistinguishable under routine light microscopy.

However, careful morphometric analysis of 295 valves revealed consistent differences. Odontidium fontinale possesses smaller valves, measuring 9 to 16 micrometres in length and 5 to 7 micrometres in width, compared with the broader valves of O. mesodon, which reach 8 to 11 micrometres in width. The new species also displays a lower density of transapical ribs, typically one to three per 10 micrometres, whereas O. mesodon carries three to six ribs in the same interval. Scanning electron microscopy exposed a further distinguishing feature: in O. fontinale, the narrow sternum does not extend into the apical pore field, while in O. mesodon it reaches almost to the end of the valve.

The molecular evidence proved equally decisive. Using monoclonal cultures established from single cells isolated in the laboratory, the team obtained rbcL sequences from two strains of the new species and three strains of O. mesodon. Maximum likelihood phylogenetic analysis placed the two O. fontinale strains in a distinct, maximally supported clade, separate from but sister to the O. mesodon clade. Together, these lineages formed a strongly supported monophyletic Odontidium group, confirming both the distinctiveness of the new species and its placement within the genus. These sequences represent the first molecular data ever generated for Odontidium, a genus that had previously been almost entirely absent from diatom phylogenetic databases.

Perhaps most surprisingly, the phylogenetic results challenge long-held assumptions about Odontidium’s closest relatives. The genus has traditionally been regarded as most closely related to Diatoma, based on morphological characters such as colony formation and girdle band structure. However, the molecular phylogeny indicates a closer relationship between Odontidium and the genera Meridion and Asterionella than with Diatoma. This finding, enabled by the inclusion of newly sequenced Meridion circulare strains in the analysis, suggests that morphological similarity may not always reflect true evolutionary proximity in freshwater araphid diatoms.

The discovery also carries broader implications for how scientists assess diatom diversity in mountain spring ecosystems. Springs are increasingly recognised as biodiversity hotspots and refugia for specialised taxa adapted to stable, cold conditions. The fact that O. fontinale was recorded at two geographically isolated springs within the study area suggests it may have a wider distribution than currently documented. The researchers note that morphologically similar valves have been illustrated in previous studies from other sites in the Bieszczady Mountains and beyond, raising the possibility that small specimens of O. fontinale may have previously been misidentified as O. mesodon in diatomological surveys.

An additional methodological insight emerged from the study: cultured specimens of both O. fontinale and O. mesodon were generally smaller than those observed in environmental samples, indicating that descriptions based solely on cultured material may underestimate the true morphological size range of a taxon. The researchers emphasise that both environmental and cultured material should be considered when assessing the variability of newly described diatom species. As molecular tools become more widely applied to freshwater algae, scientists expect that many more cryptic species remain hidden within morphologically conservative genera, particularly in the understudied spring habitats that dot mountain landscapes worldwide.

Subject of Research: Description of Odontidium fontinale sp. nov., a new spring-dwelling diatom species from Poland, based on integrative morphological and molecular evidence.

Article Title: Integrative taxonomy of a new Odontidium ( Tabellariaceae ) species from Poland: Combined morphological and molecular evidence

Article References: Sulkowska, J., Olszyński, R. M., Wetzel, C. E., & Żelazna-Wieczorek, J. (2026). Integrative taxonomy of a new Odontidium (Tabellariaceae) species from Poland: Combined morphological and molecular evidence. PhytoKeys, 280, 51-72. https://doi.org/10.3897/phytokeys.280.211531

Image Credits: AI Generated

DOI: 10.3897/phytokeys.280.211531

Keywords: Odontidium fontinale, diatoms, Bieszczady National Park, integrative taxonomy, rbcL, phylogeny, Tabellariaceae, mountain springs, freshwater biodiversity, Carpathians, new species, SEM