Petalomonas

Petalomonas is a genus of phagotrophic, flagellated euglenoids.^[1] Phagotrophic euglenoids are one of the most important forms of flagellates in benthic aquatic systems, playing an important role in microbial food webs.^[2] The traits that distinguish this particular genus are highly variable, especially at higher taxa.^[2] However, general characteristics such as a rigid cell shape and single emergent flagellum can describe the species among this genus.

Petalomonas was first described by Dr. Friedrich Stein, a zoologist at the University of Prague, in 1859.^[3]

Petalomonas is a cosmopolitan genus, most abundant in fresh water with a few species observed in marine environments.^[1][4] These euglenoids mainly reside in muddy sediments as benthic organisms.^[5] The cells are phagotrophic, feeding on bacteria, and/or osmotophic, assimilating nutrients from its surroundings.^[1][6]

These non-metabolic, colourless cells range in size from 8–45 um, with a general flattened, leaf-like shape.^[1] The posterior end is rounded or truncate and the anterior end is narrowed; however, cells can span from ovoid, to fusiform or triangular, to elongately oval.^[1][4] A distinguishing feature of the euglenoids is the presence of proteinaceous pellicle strips that are underlined with microtubules.^[7] In Petalomonas, cells are covered with approximately a dozen thickly, fused pellicle strips making the cell very rigid and possibly resistant to surface ice crystal formation that can disrupt the cell.^[7] These pellicle strips, unlike most euglenoids, are lacking grooves or troughs; however, species specific pellicle features, such as pleat-like thickenings at the joints of pellicle strips, that characterize P. cantuscygni, can distinguish certain species.^[5] Strong ribs or keels are also evident in these cells, which can be arranged spirally or relatively straight, ranging in width.^[1][4] Some species may contain furrows that vary in size and depth, and can be located dorsally and/or ventrally on the body of the cell.^[4] The cells also have an abundance of paramylon bodies, typically used for the storage of starch, that are observed in all species.^[1][4]

The feeding structure, not visible under light microscopy, is relatively simple consisting of a pocket-like cavity ending with a cytostome, lined with microtubules for phagocytosis.^[8][5] The cells within this genus are also defined by one emergent flagellum extending from a sub-apical opening, directed anteriorly when swimming.^[1][7][4] The movement of this flagellum is very minimal with some vibration at the tip; however, some species are observed to have vigorously, whipping flagellum that result in rapid rotation and oscillation of the cell body.^[4] These euglenoids have also been observed to glide forward using the body, while the flagellum is used to contact the substrate.^[7][4] The nucleus is located centrally to the left side of the cell.^[4]

In euglenoids, sexual reproduction is unknown; however, asexual reproduction has been observed to occur in this genus through longitudinal fission, where the division occurs very quickly, starting at the anterior end of the cell.^[6]