Occurrence of Microalgae in Aquatic Ecosystems with a Description of Three New Species, Prototheca fontanea, Prototheca lentecrescens, and Prototheca vistulensis.

species are unicellular, nonphotosynthetic, saprophytic, and occasionally pathogenic, microalgae, with an extensive environmental reservoir. This study explores, for the first time, the occurrence of in aquatic ecosystems by using a molecular profiling approach. A total of 362 samples were collected from 80 natural and artificial waterbodies at 88 sampling sites in 26 localities across Poland during a 1.5-year period. The overall isolation rate of from water environments was 14.1%. were most prevalent in rivers of urbanized areas, indicating that the algae are primarily adapted to lotic ecosystems with a high input of organic matter. Interestingly, it is not the amount of organic matter but its quality that seems to shape the habitat potential of the protothecae. The two most frequently isolated species were P. wickerhamii and P. pringsheimii, representing a third and a fourth of the strains, respectively. Additionally, three novel species were described, namely, P. fontanea, P. lentecrescens, and P. vistulensis. The high species diversity of the genus may reflect the complexity of water ecosystems along with ecological and functional adaptations of the algae to such environments. For further investigations, the study provides a revised scheme for identification of all 18 species currently recognized. The study investigates the occurrence of very rare and poorly studied microalgae of the genus , potentially pathogenic to humans and animals, in different water environments. Given the potential hazard to human and animal health from exposure to water-inhabiting protothecae, the prevalence of the algae in aquatic habitats deserves an insightful examination. The study is the first since the 1980s to explore the aquatic habitat of spp. and the first ever performed to do this by molecular methods. Although the isolation rate was low, a high species diversity was observed. The algae appear to represent allochthonous microflora, brought into waterbodies from various anthropogenic sources. Large rivers of urbanized areas were the most -abundant. The study provides a description of three new species, namely, , , and . The study also delivers a new identification scheme for all species currently recognized.