Phacus

Phacus is a genus of unicellular excavates, of the phylum Euglenozoa (also known as Euglenophyta), characterized by its flat, leaf-shaped structure, and rigid cytoskeleton known as a pellicle. These eukaryotes are mostly green in colour, and have a single flagellum that extends the length of their body. They are morphologically very flat, rigid, leaf-shaped, and contain many small discoid chloroplasts. Phacus are commonly found in freshwater habitats around the globe and include several hundred species that continue to be discovered to this day. Currently, there are 564 species of Phacus in the database, but only 171 have been accepted taxonomically. The genus was established in 1841 and since then major discoveries have led it to become an extremely large group containing hundreds of species with varied physiological characteristics. Contemporary studies agree that Phacus is not monophyletic or holophyletic, but is actually polyphyletic. Unfortunately, the phylogenetic relationships within the genus are currently poorly understood. The genus name is believed to have originated from the Greek word Phakos, meaning lentil or lens. This may be due to the general round or oval shape of the many species that are part of this genus. Its origins date back to the nineteenth century, in France, where it first received publication and establishment by Dujardin. The genus name is currently treated in literature as masculine. The genus was first established by the French biologist Felix Dujardin in 1841. Dujardin first published collective findings of the genus in the scientific journal Histoire naturelle des Zoophytes, Infusoires in 1841, separating them from the genus Euglena. The reason behind the separation was in order to create a group that correctly organizes their established morphological characteristics such as their rigid, flat, leaf-shape and small discoid chloroplasts with absent pyrenoids. Christian Ehrenberg was one of the first scientists to discover and classify members of the genus; his discovery of Phacus longicauda (Ehrenberg) was one of the first species of the genus to be discovered (1830) and was used as the holotype species when describing the genus. Ehrenberg, Hubner, Dujardin, and Klebs were some of the earliest researchers of Phacus, but they all had different diagnostic criteria when classifying these organisms. Originally, Ehrenberg tried to classify three species he discovered as Euglena because of their color, but ultimately were moved to the genus Phacus by Dujardin due to the similarity they exhibited in other more prominent morphological characteristics such as shape and strip arrangement. Since the establishment of the genus, there has been difficulty in classifying organisms due to morphology because many species of Phacus have physiological traits that closely resemble other related genera. Even the most minor difference in certain physiological properties caused great confusion in determining the proper classification for many species. This occurred more prominently during the time of Dujardin because the methodology and technology used at the time in studying these organisms were far more simplistic compared to today, leading to greater difficulty in determining critical differences between organisms. Since its inception, Phacus has undergone constant reclassification and considerable re-evaluation of different taxa in accordance with new morphological and molecular information. Phacus are commonly found in freshwater habitats all over the world. Many species of this genus have been discovered in several countries, including Japan, the United States, Portugal, Brazil, Korea and the Philippines. Different members of the genus have been found in temperatures ranging from 11.4-21.6 oC, and a pH between 6.2 and 7.5. Phacus organisms are found in a range of freshwater environments (some more acidic or alkaline than others), prefer cooler temperatures, and on average exist in more neutral pH aquatic habitats. Many species of Phacus are considered to be euplanktonic (free-floating organisms or open water plankton) because they are commonly found together with other genera of Euglenophyta. These species include members of the genera Lepocinclis, Trachelomonas, Euglena, and many different kinds of algae, which are typically found in similar aquatic habitats. The planktonic environments Phacus are generally found include swamps, ditches, trenches, ponds and even in many rice fields throughout North America and around the world. They are a small part of phytoplankton communities, but do serve important purposes such as nutrient cycling and food web stability. Their keen ability to colonize in huge numbers makes them able to survive and thrive in areas where certain algae cannot. However, Phacus are not common inhabitants of stagnant environments because those areas often do not have proper organic composition. 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. 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, Daphnia, Diaphanosoma, and Holopedium.