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

This study presents the first occurrences of the diatom species Discostella asterocostata in Europe’s second largest river, the Danube River and the earliest occurrence in European freshwater ecosystems. Samples collected between 2016 and 2022 in Croatia and Hungary were analysed using light and scanning electron microscopy (SEM) along with molecular techniques to confirm the presence of the species. Morphometric comparisons with known populations from Asia and the USA revealed no significant morphological differences. Molecular analyses facilitated the detection of D. asterocostata even in samples with low abundance and confirmed the genetic similarity with already sequenced populations. Environmental correlations showed that D. asterocostata had a significant positive correlation with temperature and a negative correlation with total organic carbon and nitrogen forms, suggesting that it favours warmer waters with lower nutrient forms. Given the ongoing increase in water temperatures due to climate change, the occurrence and potential spread of this species in European rivers may be related to changing environmental conditions. Our results emphasise the importance of integrating traditional and molecular approaches for the early detection of non-indigenous diatom species. The establishment of D. asterocostata in Europe raises questions regarding its ecological role and long-term impact on native phytoplankton communities and emphasises the need for continuous monitoring and research on the responses of large river ecosystems to global environmental change.