A New Algivorous Heterolobosean Amoeba, Euplaesiobystra perlucida sp. nov. (Tetramitia, Discoba), Isolated from Pilot-Scale Cultures of Phaeodactylum tricornutum.

The diatom Phaeodactylum tricornutum is regarded as a prospective "cell factory" for the high-value products fucoxanthin and eicosapentaenoic acid (EPA). However, contamination with grazing protozoa is a significant barrier to its commercial cultivation. Here, we describe a new species of heterolobosean amoeba, Euplaesiobystra perlucida, which caused the loss of in pilot-scale cultures. Morphological and molecular characteristics distinguish from the other species in the genus . is 1.4 to 3.2 times larger than other species in terms of average length/width and maximum length/width of the trophozoites. Unlike Euplaesiobystra salpumilio, has no cytostome; lacks a flagellate stage, whereas Euplaesiobystra hypersalinica and both display a flagellate stage in their life cycle. The small-subunit rRNA gene sequence of shared only 88.02% homology with that of its closest relative, Euplaesiobystra dzianiensis, and had two distinctive regions. Its phylogenetic branch was clustered with one uncultured heterolobosean clone (bootstrap support/posterior probability = 100%/1.00). Results of feeding experiments demonstrated that could graze on various unicellular and filamentous eukaryotic microalgae (chlorophytes, chrysophytes, euglenids, and diatoms) and cyanobacteria. 's ingestion rate declined exponentially with increasing size of unicellular prey, and attained the highest growth rates on . On the basis of its strong ability to graze on microalgae, capacity to form large populations in a short period of time, and capacity to form resistant resting cysts, this contaminant has the potential to cause severe problems in large-scale microalgal culture and merits further attention. Heteroloboseans have garnered considerable interest because of their extraordinary ecological, morphological, and physiological diversity. Many heteroloboseans have adapted to various extensive habitats, including halophilic, acidophilic, thermophilic, psychrophilic, and anaerobic habitats. Most heteroloboseans are bacterivores, with a few algivorous species reported. In this study, a new species of algivorous heterolobosean amoeba, , is described as a significant grazer that causes losses in outdoor industrial cultures. This study provides phenotypic, feeding, and genetic information on a previously unknown heterolobosean, emphasizes the impact of contaminating amoebae in commercial microalgal cultures, and will contribute to the management strategies for predicting this kind of contaminant in large-scale microalgal cultivation.