Ceratium is a unicellular organism and a member of the family Ceratiaceae, commonly known as dinoflagellates. Dinoflagellates are usually considered algae but mostly are marine planktons. Ceratium genus comprises a rather small number of about 7 freshwater dinoflagellate species. The characteristics of Ceratium dinoflagellates include their horns, two flagella and armoured plates.
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Species of Ceratium
There are 7 species of Ceratium categorised to date. These include:
Ceratium species are known to exhibit both plant and animal characteristics and as such, their classification as algae is contentious.
Occurrence of Ceratium
Ceratium species are generally seen in fresh water and at times in saltwater and their occurrence is widespread from the arctic to the tropical regions of the earth. They are, however, more commonly found in temperate regions. In the water bodies, Ceratium species are generally found in the upper regions where there is an abundance of light for them to carry out photosynthesis.
Ceratium Structure and Metabolism
Cell wall: The unicellular Ceratium is identified by the unique shape of their cell wall. Its cell wall is made of polysaccharides which impart a characteristic shell or armour like appearance. The shell or the pellicle is made from vesicles and the cell membrane. Vesicles have cross-linked cellulose that forms the plates. The pellicle gets divided into two structures - epicone and hypocone. These lie above and below the transverse groove. The epicone and the hypercone are surrounded by two rows of plates in a particular pattern (used for identifying Ceratium species) and can be inherited by offsprings.
Horns: The armour also known as the theca is composed of many textured plates that form one anterior and two posterior horns usually. The horns of Ceratium, also known as arms can be attributed as their most distinguishing features. The shape and size of the arms vary among the different species of Ceratium and the species C.monoceras has a single arm located apically. The arms of the Ceratium allow it to float all the while preventing them from moving too quickly. The morphology of the horns is dependent upon the temperature and the salinity of the surrounding environment. The horns in species of Ceratium are generally shorter and thicker in salty cold water while in warmer, less-salty waters, they are longer and thinner.
Flagella: Ceratium species are characteristic dinoflagellates and as such, have two flagella that are unlike each other. The flagella remain wound sound the cell body and have different shapes and movements. While the transverse flagellum in Ceratium beats in a spiral motion, the longitudinal flagellum pulses through in waves.
Ceratium contains small plasmids and peridium and most of the species contain chloroplasts as well. The chloroplasts contain chromatophores comprising green, yellow and brown pigments. Certain species of Ceratium are also bioluminescent. Ceratium species possess the ability to encyst and protect themselves under adverse conditions. The cell of Ceratium fusus is long and fusiform. The epitheca of the Ceratium fusus cell terminates in a long, straight apical horn, while the hypotheca terminates in a fully developed left antapical horn and a reduced right antapical horn (Montagnes 2006). The right antapical horn has been reduced to the point that it resembles a hump, giving the cell a gentle curve.
Metabolically, Ceratium species are mixotrophs as they can obtain food by both phagocytosis and photosynthesis. They consume other planktons by means of phagocytosis. A unique adaptation seen in Ceratium dinoflagellates is that they are able to store compounds in a vacuole which they can use for growth under instances when nutrients become scarce. Their growth is also assisted by another adaptation that allows them to extend their chloroplasts containing appendages during the day. The chloroplasts then absorb light for photosynthesis. These appendages get retracted during the night which allows the Ceratium to move into the deeper layers of the water bodies.
Reproduction in Ceratium
Ceratium species exhibit alternation of generation and zygotic meiosis. Their reproduction is either sexual or asexual.
Asexual Reproduction in Ceratium dinoflagellates is characterised by the pulling apart of the pellicle or the shell exposing the naked cell. The cell then gradually increases in size and divides to create 4 to 8 daughter cells. Each of the daughter cells has two flagella. Throughout the process of asexual reproduction, the nuclear membrane is present while the centrioles are absent. The nuclear membrane undergoes division only when the organism constricts.
Sexual Reproduction occurs in Ceratium when the cells of two unique organisms couple close to their longitudinal groove or sulci. This is followed by meiosis allowing the chromosomes from the haploid parents to get paired. The resulting diploid offspring called ‘swarmer’ is released into the water.
Ceratium in Ecosystem
Ceratium species are known to be relatively harmless as they are non-toxic in nature. They are necessary for the food web as they form a crucial part of the plankton found in the waters of the temperate zone. They serve as necessary components of their habitats by not only serving as nutrients for larger organisms but at the same time keeping smaller organisms in check through their predatory activities.
However, in presence of continuous favourable conditions, Ceratium species can cause water blooms and red tides due to excessive blooming. This red tide though non-toxic in nature can cause the resources of the environment to deplete creating a strain on the ecosystem.