Asian diatom quietly colonises the Danube as warming waters open Europe’s rivers to new arrivals

Deep in the phytoplankton of the Danube River, a microscopic stowaway has been quietly establishing itself. Researchers analysing samples from Croatia and Hungary have documented the first occurrences of the diatom species Discostella asterocostata in Europe’s second largest river, and the earliest known record of the species anywhere in European freshwater ecosystems. The findings, published in the journal Metabarcoding and Metagenomics, trace a remarkable biogeographical journey from Asia through North America to Europe, and raise pressing questions about how climate change and human activity are reshaping the microbial communities of large rivers.

Discostella asterocostata was originally described from China in 1985 under the name Cyclotella asterocostata, based on scanning electron microscopic examination of samples collected in the 1970s. The species was later transferred to the genus Discostella in 2004, after researchers recognised that the position of its marginal fultoportulae, the strutted processes sitting between the valve costae, distinguished it from related centric diatoms. Over the following decades, the species turned up in Japan, South Korea and the Russian Far East. Then, in a publication in 2021, it was reported from North America, where re-examination of archived plankton samples revealed it had already been present in Beaver Lake, Arkansas and the Wabash River, Indiana since 2012. Sediment core analyses from several American lakes pushed its known presence on that continent back to at least the mid-1980s.

Europe, by contrast, remained a conspicuous gap. Freshwater centric diatoms have been intensively documented on the continent for decades through national floras, monitoring programmes and decades of electron microscopic surveys of the Danube phytoplankton. The species’ very distinctive morphology, a ring-shaped pattern in the central area of the valve, makes it difficult to miss under the microscope. The researchers therefore argue that its detection in the Mura River in 2016 roughly coincides with its first appearance in Europe, rather than representing an earlier overlooked record.

The new study draws on routine biomonitoring samples collected between 2016 and 2022 at eight sites along the Mura, Drava and Danube Rivers in Croatia, and monthly phytoplankton samples from thirteen sites along the Hungarian Danube and two of its side arms between May 2021 and April 2022. In the Croatian samples, the species was identified through light and scanning electron microscopy, appearing in plankton from April to September at all eight sites. Its contribution to total phytoplankton biomass was always modest, ranging from 0.2 to 3.2 percent within the class Mediophyceae, but its consistent presence across years and sites suggested it had already become a permanent member of the community.

The Hungarian story is more revealing. In the first scanning electron microscopic survey of 2021 samples, only a single valve of Discostella asterocostata was found, in a sample from the Ráckevei-Soroksári Danube side arm in July. The species was absent from the Diat.barcode database version used in the initial metabarcoding analysis, so the researchers could not identify it molecularly at first. When updated barcode sequences from China became available, the team re-analysed their previously unclassified Discostella sequences and found a near-perfect match, with uncorrected p-distances below 0.01 and at most one or two base pair differences across the 263-base-pair rbcL region used for metabarcoding.

The re-analysis transformed the picture. Molecular detection revealed the species throughout the entire Hungarian section of the Danube by 2021 to 2022, with higher relative read numbers during the warm water period and greatest abundance in the Ráckevei-Soroksári Danube. Even more strikingly, re-analysis of unclassified sequences from earlier phytobenthos surveys of 328 Hungarian surface water samples collected in 2019 and 2020 identified the species in five oxbow lakes along the southern Hungarian Danube, with the earliest molecular detections in September 2019. Two unassigned amplicon sequence variants from the 4th Joint Danube Survey, also collected in 2019, proved identical to the researchers’ sequences, providing independent confirmation that the species was already present in the region six years before its first microscopic record in Hungarian plankton.

Morphometric comparison added another layer to the evidence. The researchers measured eight valve features, including five diameter measurements, the count of marginal strutted processes and the number of outer and central striae, on 35 Croatian and 10 Hungarian valves, then compared the results with published morphometric data from Chinese and American populations. Principal component analysis showed that the first principal component, accounting for 91.8 percent of the variance in the dataset, was strongly correlated with cell diameter. More importantly, the populations from Croatia, Hungary, the United States and Asia completely overlapped on the second and third principal component axes, indicating that the four populations are morphologically indistinguishable. Phylogenetic analysis of the rbcL sequences placed all Discostella asterocostata variants in a monophyletic group well separated from other Discostella species, reinforcing the conclusion that the European populations have not diverged from their Asian ancestors.

The ecological preferences of the species offer a clue to its future in Europe. Spearman correlation analysis of Croatian samples showed a significant positive correlation between the species’ biomass and water temperature, and significant negative correlations with total organic carbon, nitrogen forms and chloride. The Hungarian molecular data confirmed these patterns, with relative read numbers positively correlated with temperature and negatively correlated with total organic carbon, total nitrogen and nitrate. The species also showed a negative correlation with discharge, suggesting a preference for slow-flowing or stagnant waters. In short, Discostella asterocostata favours warmer waters with lower nutrient loads, a profile that differs from the dominant centric diatoms of very large European rivers and that aligns uncomfortably well with the trajectory of climate warming.

How the species reached Europe remains uncertain, but the researchers consider several plausible vectors. In North America, invasive Asian carp, particularly silver carp and bighead carp, have been suggested as carriers, since their filtering apparatus can collect algae larger than 10 micrometres from the water and some algae may survive digestion and be excreted alive in faeces. Asian carps were introduced to Hungary from East Asia as early as 1963, but the researchers consider it unlikely that they explain the species’ appearance there, given the timing. The pattern of spread, with the species appearing in floodplain and protected oxbow lakes and in the Ráckevei-Soroksári Danube before it was detected in the main channel of the Hungarian Danube, suggests that waterbirds may have played a role as dispersal vectors, carrying viable cells between waterbodies. Ship ballast water and fish transport water remain additional possibilities that cannot be excluded.

For now, the species has not shown invasive behaviour in European rivers. It has never been the dominant centric diatom in any phytoplankton sample, and its rapid spread, though indisputable, has not been accompanied by the high abundances that would trigger ecological alarm. But its clear temperature-related seasonal dynamics, its preference for warm, nutrient-poor and slow-flowing waters, and its expansion pattern across the Danube catchment all support the hypothesis that ongoing warming of freshwater systems could promote its further dispersal and increased abundance. The study’s authors emphasise that integrating traditional microscopy with high-throughput molecular techniques such as metabarcoding is essential for the early detection of non-indigenous diatom species, particularly when they occur at low densities that routine counts of 400 to 500 valves can easily miss. As rising water temperatures and enhanced dispersal pathways continue to reshape aquatic ecosystems worldwide, the quiet colonisation of the Danube by a tiny Asian diatom may prove an early warning of changes still to come.

Subject of Research: First European record of the non-indigenous diatom Discostella asterocostata in the Danube River

Article Title: From Asia through America to Europe: Molecular and morphological evidence for a non-indigenous diatom species, Discostella asterocostata (B.Q.Lin, S.Q.Xie & S.X.Cai) Houk & Klee in Hungary and Croatia

Article References: Stanković, I., Shemesh, S., Hanžek, N., Šušnjara, M., Gligora Udovič, M., Kiss, K. T., Trábert, Z., Udvari, Z., Bíró, T., Vadkerti, E., & Ács, É. (2025). From Asia through America to Europe: Molecular and morphological evidence for a non-indigenous diatom species, Discostella asterocostata (B.Q.Lin, S.Q.Xie & S.X.Cai) Houk & Klee in Hungary and Croatia. Metabarcoding and Metagenomics, 9, Article e154667. https://doi.org/10.3897/mbmg.9.154667

Image Credits: AI Generated

DOI: 10.3897/mbmg.9.154667

Keywords: diatoms, invasive species, Danube River, metabarcoding, climate change, freshwater ecology, phytoplankton, non-indigenous species, Asia, America, Europe, Molecular