Phacus

Phacus are photosynthetic unicellular organisms, meaning that they are capable of producing their own food. Although the genus primarily receives their nutrients through photosynthesis, they are also capable of feeding on certain kinds of alga and bacteria using a feeding apparatus located on their underside.[10] Many species of Phacus are known to be prey for a variety of marine and freshwater genera. The best known predators of the genus are planktonic crustaceans, such as species of Diaptomus, Tropocyclops, Epischura, Daphnia, Diaphanosoma, and Holopedium.

Phacus consists of organisms that are microscopic photosynthetic Euglenoids, which are a group of single celled, chloroplast-containing autotrophs within the phylum Euglenozoa, though the majority of genera within the phylum actually do not contain chloroplasts and are colourless. Generally these species are small, free swimming and exhibit a vibrant green colour. What sets the genus apart from other photosynthetic species is the presence of its rigid cytoskeleton (although some species have semi-rigid or plastic cytoskeleton) made up of pellicular strips and its predominantly flat, leaf-shaped structure.[6] Many different species express secondary fusion of these pellicular strips and many of the strips have a variety of shapes including S-shaped, A-shaped, M-shaped or plateau.[11]

The pellicles forms a shell around the cytoskeleton covering the whole cell and fuses together around the microtubule reinforced-pocket (MTR).[10] This pocket acts as a sort of cytostome or ingestion organelle, allowing the organism to feed when bacteria enter inside. The microtubules are arranged in a peculiar doublet and triplet pattern in the upper canal. In certain species of Phacus, the MTR is a microtubule organizing center and is connected to a reservoir membrane by a striated fiber.[10] Many species also possess an elongated caudal process with extended pellicle strips. Throughout the cytoplasm of the cell, chloroplasts are spread out fairly evenly and in high numbers. They are typically discoid and are regularly disposed of and replaced.[7] Chloroplasts that are present in large numbers are typically smaller, are without pyrenoids and species containing fewer chloroplasts tend to have much larger ones. Phacus, like all photosynthetic euglenoids, obtained their plastids through secondary endosymbiosis, where the ancestral phagocytic euglenoid engulfed a green alga, and the resulting organism became the plastid.[12]

Most of these organisms also possess a semi-rectangular eyespot, often reddish in color, and a single flagellum, although some species have two, which emerges anteriorly. The flagellum is responsible for cell movement by gyrating in the direction of travel allowing the cell to glide and swim in the water. Furthermore, some flagella vary in length from short all the way to the length of the cell in other species. Although many members of the genus have two flagella, only one is ever used for movement. The other one is usually too short and does not exit the invagination of the posterior area known as the flagellar pocket. They are located within a posterior structure called the flagellar apparatus, also known as the basal body complex.

Aside from the flagella, the flagellar apparatus also contains two basal bodies connected by a striated fiber, three asymmetric microtubular roots, and other connective fibers. The genus only has one large anisotropic body unlike many other euglenid genera, which commonly have two.[13] This anisotropic body is referred to as paramylon and functions as a storage substance. The paramylon is a carbohydrate energy store reserve and it is quite different from other algal carbohydrate stores due to the high amounts of crystallization present. The nucleus of the cell is generally positioned towards the middle of the cell and is adjacent to the paramylon reserve. Within the nucleus are permanently condensed chromosomes, which can easily be viewed under a light microscope. Just like other euglenids, several species of Phacus also possess many contractile vacuoles and have a red-pigmented eyespot.

Although the general morphology of the genus is considered to be well established, given the large number of species there are critical morphological differences observed worth noting.[6] The sulcus for example, in many species is shallow (Phacus viridioryza), and in others it is deep and longitudinal (Phacus hordeiformis). As well, the shape of the cell in some species are completely flat, while many have also been described as helically twisted, straight or curved. Phacus helikoides is actually helical in shape throughout the entire cell as opposed to flat and leaf-shaped like most Phacus organisms.

Metaboly, which is the ability of some organisms to alter their shape, is not possible in Phacus due to the fusion of the genus’ pellicular strips. These varied morphological shapes make defining the genus as symmetrical or asymmetrical rather difficult. Scientists who study the genus Phacus Dujardin, have also observed the presence of ellipsoid, tiny disk-shaped or flat-shaped paramylon grains. In some species of Phacus, a single plate dominates the interiors of the cell (Phacus orbicularis), and in others there are multiple plates with different morphologies that exist (Phacus curvicauda).[14]

Other differences among species include: the presence or lack of haplopyrenoids within the chloroplasts, position of the nucleus, a large or small endosome, shape of the cytoskeleton, few to several paramylon discoid grains, the presence of lateral caps and presence of oblique truncated poles. In addition, the morphology of the caudal process in many species of Phacus is extremely varied. Phacus parvullis and Phacus pusillus have very a blunt caudal process while Phacus segretti and Phacus stokesii actually lack a caudal process entirely. Those species are described as having rounded posteriors in place of the caudal process. Studies show that morphological changes observed in the genus are possibly due to the level of organic enrichment in their freshwater habitats.[9] These morphological differences, given the massive size of the genus, have led to certain confusion in Phacus taxonomy.

Phacus and other euglenids reproduce asexually. They do so by dividing their cells longitudinally, from the apex of the cell to the base. Until cytokinesis is fully complete, the cells remain attached to one another, forming what looks like a “two-headed” organism. Prior to cytokinesis, the amount of pellicle strips each cell has is doubled in order to have an equal number between each daughter cell. In addition, each daughter cell will contain half the number of the newly formed strips and half the number of the old strips present prior to cell division.

Phacus is a member of the family Phacaceae,[15] the order Euglenales, class Euglenoidea, and finally the phylum Euglenozoa. Certain ancestral information regarding Phacus has been debated. Many studies looked at the genes of its many species by examining small subunit rRNA (SSU) sequences and arranging certain species into clades.[16] What those scientists were trying to determine is what the phylogenetic tree of Phacus looks like based on molecular factors. Certain molecular phylogenetic analyses have described Phacus as being monophyletic depending on the clade studied, but the general consensus through multiple repeated analyses is that the genus is polyphyletic.[16] This possible discrepancy has caused certain scientists to suggest possibly redefining this genus as two separate genera. In accordance with that, it is believed that Phacus divided early on in the history of photosynthetic euglenids.

A major genetic change in the genus occurred in its chloroplast genome throughout its evolution. This resulted in a genome reduction - possibly due to gene loss or transfer to the nucleus, an increase in the number of introns, and large genomic rearrangements.

The literature mentioned above have also looked into the evolutionary history of the genus’ morphology. It is believed that the rigidity of the cells has evolved numerous times, and that was determined by comparing the semi-rigid pellicles of ancestral species with the rigid pellicles of their descendants.[17] Another feature believed to have evolved, are the longitudinal strips that appear on most species. It appears that the number of those strips has either increased or decreased over time depending on certain species and that their arrangement (either helical or longitudinal) has also changed throughout evolution.[17] Furthermore, it has been argued that certain behavioral and locomotor traits which previously existed for predatory feeding have no longer been selected. This seemed to have had an effect on the number of strips species of Phacus generally have. The changing numbers of strips and the clustered patterns associated are not actually adaptive themselves, but may have evolved due to the cell becoming more flat and more rigid over time. Those traits are believed to have evolved in order to adapt to a more planktonic lifestyle.[17]