**1. Introduction: threat to freshwater environment**

*Didymosphenia geminata* (Lyngb.) M. Schmidt (*D. geminata*) has attracted considerable attention as an invasive nuisance species in natural habitats, in different country, like Chile, Argentina, Canada, Poland and New Zealand [1, 2]. The species also show nuisance effect in the southern hemisphere [3, 4]. Biological invasions are a threat to freshwater environments and the ecosystem services they provide [5, 6]. *D. geminata* is a large diatom species (~100 μm long) that produces sulphated polysaccharide stems and forms nuisance strands that can grow up to 10 cm thick with 100% coverage in streams [7]. This microalga, belonging to the brown algae diatom family, is rich in specific molecules such as antioxidants, which include polyphenols and diadinoxanthin [8]. What is the nuisance effect of this plague? The most significant impacts that this diatom presents to aquatic systems

can be observed through its physical changes, substantial increases in algal biomass, stagnation of fine deposits, hydrodynamic change and their secondary effects on biogeochemical states and processes, such as redox conditions, pH and nutrient cycling in the benthic layers [9]. Recently, *D. geminata* has been molecularly identified in the rivers of southern Chile [10] and was compared with different samples; in fact, Argentinian and Chilean *D. geminata* are the same species, but the authors did not describe their origin [11]. This observation is important because the movements of *D. geminata* can improve our understanding of the species' dispersion; in fact, an essential work in New Zealand proved the invasion of *D. geminata* and gave us clues about the source of these diatoms [12].

Few studies have focused on environmental impacts, although these were mentioned indirectly in nutrient recycling mechanism and flow channel attenuation studies [13–15]. Other studies have indicated that *D. geminata* alters microenvironments and reduces fish populations [16] by altering fish community diets [17]. Also, it disturbs aquatic invertebrate communities and the filters of drinking water systems [18, 19], although it is unknown whether the contamination effect is direct. Recently, the toxic effects of microalgae on contaminated river communities [14] and the effects on *Salmo salar* spermatozoon activation time have been described [20]. Despite extensive research on *D. geminata* in recent years [21] with results on ecosystem effects, further advances on the toxicological effects are still needed.
