Ostreococcus tauri
Encyclopedia
Ostreococcus tauri is a unicellular
species
of green alga about 0.8 micrometre
s (μm) in diameter, the smallest free-living eukaryote
yet described. It has a very simple ultrastructure
, and a compact genome
.
As a common member of global oceanic picoplankton
populations, this organism has a major role in the carbon cycle
in many areas. Recently, O. tauri has been the subject of studies using comparative genomics
and functional genomics
due to its compact genome and green lineage.
, in an year-long study of the picoplankton population of the lagoon using flow cytometry
. O. tauri was found to be the main component of the picoplankton population in the lagoon, and images of cells produced by transmission electron microscopy
revealed the smallest yet described free-living eukaryotic
cells. O. tauri was immediately placed in the class Prasinophyceae
based on the presence of characteristic chlorophyll pigments and Chlorophyceae
-related carotenoids, and this classification was confirmed by further work.
), averaging about 1 μm long by 0.7 μm wide. The cell's ultrastructure is very simple, lacking a cell wall
and consisting of a nucleus
, a single mitochondrion
, a single chloroplast
, and a single Golgi apparatus
. Cells also lack flagella.
Initially described as containing 14 chromosomes, it is now known that the nucleus contains 20 chromosomes, in all about 33 fg of DNA
.
); larger eukaryotic species of alga and many predators of smaller algae are preferentially consumed by the molluscs, which are filter feeder
s. The second consideration selects against cyanobacteria, as O. tauri is thought to coupe better with "adverse conditions". The excess copper in the lagoon is thought to originate from agricultural chemicals used by surrounding vinyards.
such as cell division and/or genome sequencing studies".
Unicellular organism
A unicellular organism, also known as a single-celled organism is an organism that consists of only one cell, in contrast to a multicellular organism that consists of multiple cells. Historically simple single celled organisms have sometimes been referred to as monads Prokaryotes, most protists,...
species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of green alga about 0.8 micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s (μm) in diameter, the smallest free-living eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
yet described. It has a very simple ultrastructure
Ultrastructure
Ultrastructure is the detailed structure of a biological specimen, such as a cell, tissue, or organ, that can be observed by electron microscopy...
, and a compact genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
.
As a common member of global oceanic picoplankton
Picoplankton
Picoplankton is the fraction of plankton composed by cells between 0.2 and 2 μm that can be either :* photosynthetic * heterotrophic Some species can also be mixotrophic....
populations, this organism has a major role in the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
in many areas. Recently, O. tauri has been the subject of studies using comparative genomics
Comparative genomics
Comparative genomics is the study of the relationship of genome structure and function across different biological species or strains. Comparative genomics is an attempt to take advantage of the information provided by the signatures of selection to understand the function and evolutionary...
and functional genomics
Functional genomics
Functional genomics is a field of molecular biology that attempts to make use of the vast wealth of data produced by genomic projects to describe gene functions and interactions...
due to its compact genome and green lineage.
History
O. tauri was discovered in 1994 in the Thau lagoon, FranceFrance
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
, in an year-long study of the picoplankton population of the lagoon using flow cytometry
Flow cytometry
Flow cytometry is a technique for counting and examining microscopic particles, such as cells and chromosomes, by suspending them in a stream of fluid and passing them by an electronic detection apparatus. It allows simultaneous multiparametric analysis of the physical and/or chemical...
. O. tauri was found to be the main component of the picoplankton population in the lagoon, and images of cells produced by transmission electron microscopy
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...
revealed the smallest yet described free-living eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
cells. O. tauri was immediately placed in the class Prasinophyceae
Prasinophyceae
In taxonomy, Prasinophytes are a class of the Division Chlorophyta. These are primitive eukaryotic, marine green algae. Its best known genus is Ostreococcus , which is considered to be the smallest free-living eukaryote and which has been detected in marine samples around the world...
based on the presence of characteristic chlorophyll pigments and Chlorophyceae
Chlorophyceae
The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. For example the chlorophycean CW clade, and chlorophycean DO clade, are defined by the arrangement of their flagella. Members of the CW clade have flagella that are displaced...
-related carotenoids, and this classification was confirmed by further work.
Anatomy
Cells are roughly spherical (coccoidCoccus
Coccus can be used to describe any bacterium that has a spherical shape. It is one of the three distinct types of bacteria shapes, the other two being bacillus and spirillum cells...
), averaging about 1 μm long by 0.7 μm wide. The cell's ultrastructure is very simple, lacking a cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
and consisting of a nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, a single mitochondrion
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
, a single chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
, and a single Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
. Cells also lack flagella.
Initially described as containing 14 chromosomes, it is now known that the nucleus contains 20 chromosomes, in all about 33 fg of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
.
Ecology
O. tauri is the dominant algal species, by cell abundance, in the Thau Lagoon in the south of France. The conditions that are thought to lead to this dominance are firstly that the Lagoon is used for intensive mollusc cultivation, and secondly that copper levels in the Lagoon are high. The first consideration selects for smaller cells (picoplanktonPicoplankton
Picoplankton is the fraction of plankton composed by cells between 0.2 and 2 μm that can be either :* photosynthetic * heterotrophic Some species can also be mixotrophic....
); larger eukaryotic species of alga and many predators of smaller algae are preferentially consumed by the molluscs, which are filter feeder
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
s. The second consideration selects against cyanobacteria, as O. tauri is thought to coupe better with "adverse conditions". The excess copper in the lagoon is thought to originate from agricultural chemicals used by surrounding vinyards.
Use as a Model Organism
As early as 1998, O. tauri was identified as "a good candidate for biological modelsModel organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...
such as cell division and/or genome sequencing studies".