Early detection and the importance of environmental drivers for the invasive cyanobacterium Raphidiopsis raciborskii

Freshwater habitats often experience species loss, with the greatest pressures coming from land use alterations and the establishment of non-native species as they become invasive. One species of concern is the tropical invasive freshwater cyanobacterium Raphidiopsis raciborskii, currently expanding its geographic distribution towards northern regions of Europe. As a toxin-producing, bloom-forming species, population growths of R. raciborskii can negatively affect local biodiversity and ecosystem services. Hence, in this study, I wanted to evaluate and validate (i) the geographical extent of suitable habitats, (ii) its competitive capacities with a native phytoplankton assemblage and (iii) its adaptive capabilities when moved from ideal to colder temperatures. To identify suitable colonization areas, I used Species Distribution Models (SDMs), which revealed suitable habitats in areas where the invasive cyanobacterium has not been recorded, i.e. several parts of Europe including south-eastern Sweden. However, there were some inconsistencies between SDM results and the field studies used to validate the SDM prediction, where only a few of the European screened lakes matched with the SDM prediction. To better understand the competitive success of R. raciborskii we performed a laboratory study using different nutrient and temperature conditions and, as competitors, an assembly of phytoplankton species typically found in Scandinavian freshwaters. Results confirmed the importance of temperature on R. raciborskii growth and its ability to withstand suboptimal environments. The physiological changes and gene expressions of R. raciborskii, when transferred from optimal to suboptimal temperatures, were tested through a differential transcriptomic experiment. Results showed a difference in gene expression in different treatments, with a downregulation of genes in the suboptimal temperature. Although R. raciborskii has not been recorded in Sweden, it is a well-established invasive species in many European lakes. Combined results from this thesis show that R. raciborskii meets the climatic conditions matching with the south-eastern parts of Sweden. If able to surpass physical and ecological barriers, due to its ability to withstand a wide-range of environments, it might be able to survive in lakes with less favorable conditions and bloom when conditions become ideal. To be able to prevent and contain the expansion of this invasive species in northern European lakes, proactive monitoring programs with high temporal and spatial frequency are needed in combination with broad information campaigns to minimize further spread to other freshwater system