Picoeukaryotes are

picoplankton Picoplankton is the fraction of plankton composed by cells between 0.2 and 2 μm that can be either prokaryotic and eukaryotic phototrophs and heterotrophs: * photosynthetic * heterotrophic They are prevalent amongst microbial plankton communit ...

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

organisms 3.0 µm or less in size. They are distributed throughout the world's marine and

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

ecosystems An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...

and constitute a significant contribution to

autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...

communities A community is a Level of analysis, social unit (a group of living things) with commonality such as place (geography), place, Norm (social), norms, religion, values, Convention (norm), customs, or Identity (social science), identity. Communiti ...

. Though the SI prefix

pico- A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pre ...

might imply an organism smaller than

atomic size The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the Atomic nucleus, nucleus to the outermost isolated electron. Since the boundary is not a well-defined physica ...

, the term was likely used to avoid confusion with existing size classifications of

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

Characteristics

Cell structure

Picoeukaryotes can be either

autotrophic An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...

and

heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

, and usually contain a minimal number of

organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...

s. For example, ''

Ostreococcus tauri ''Ostreococcus tauri'' is a unicellular species of marine green alga about 0.8 micrometres (μm) in diameter, the smallest free-living (non- symbiotic) eukaryote yet described. It has a very simple ultrastructure, and a compact genome. As a c ...

'', an autotrophic picoeukaryote belonging to the class

Prasinophyceae The prasinophytes are a group of unicellular green algae. Prasinophytes mainly include marine planktonic species, as well as some freshwater representatives.Sym, S. D. and Pienaar, R. N. 1993. The class Prasinophyceae. In Round, F. E. and Chapma ...

, contains only the

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...

, one

mitochondrion A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

and one

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...

, tightly packed within a

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

. Members of a heterotrophic class, the Bicosoecida, similarly contain only two mitochondria, one food

vacuole A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic mo ...

and a nucleus.

Distributions

These organisms are found throughout the water columns. Autotrophic picoeukaryotes are restricted to the upper 100–200 m (the layer that receives light) and are often characterized by a sharp cell maximum near the Deep Chlorophyll Maximum Layer (DCML)Campbell, Lisa and Daniel Vaulot
Photosynthetic picoplankton community structure in the subtropical north pacific ocean near Hawaii (station ALOHA).
Deep-Sea Research Part I, Vol. 40, No. 10, pp. 2043-2060 (1993). Accessed April 30, 2008. and decrease significantly below.Hall, J.A. and W.F. Vincent
Vertical and horizontal structure in the picoplankton communities of a coastal upwelling system.
Marine Biology 106, 465-471 (1990). Accessed April 30, 2008. Heterotrophic groups are found at greater depths and for example, in the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

, they have been found in the vicinity of

hydrothermal vents A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...

at depths up to 2000–2550 m. Some heterotrophic lineages are found, unstratified, at all depths from the surface down to 3000 m. They show high phylogenetic diversityMassana, R. et al
Unveiling the Organisms behind Novel Eukaryotic Ribosomal DNA Sequences from the Ocean.
Applied and Environmental Microbiology, 4554-4558 (2002). Accessed April 30, 2008Moon-van der Staay, S. et al

Nature 409, 607-610 (2001). Accessed April 30, 2008. and high variability in global cell concentrations, ranging from 10⁷ to 10⁵

liter The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3). ...

⁻¹.

Diversity

Autotrophic An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...

picoeukaryotes commonly found in nature are members of groups such as the

(a kind of

green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

) and the

Haptophyceae The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta (named for ''Prymnesium''), are a clade of algae. The names Haptophyceae or Prymnesiophyceae are sometimes used instead. This ending implies classification at ...

. Despite their small size, these organisms have been found to contribute >10% of the total global aquatic net

primary productivity In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...

.Fouilland, E. et al
Productivity and growth of a natural population of the smallest free-living eukaryote under nitrogen deficiency and sufficiency.
Microbial Ecology 48, 103–110(2004). Accessed April 30, 2008. Although much less abundant than cyanobacterial

Photosynthetic picoplankton Photosynthetic picoplankton or picophytoplankton is the fraction of the phytoplankton performing photosynthesis composed of cells between 0.2 and 2 µm in size ( picoplankton). It is especially important in the central oligotrophic regions of ...

they have been shown to be as important in terms of biomass and primary production than picocyanobacteria. In more oligotrophic environments, such as Station ALOHA, researchers believe that approximately 80% of the

chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...

biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...

is due to cells in the

pico Pico may refer to: Places The Moon * Mons Pico, a lunar mountain in the northern part of the Mare Imbrium basin Portugal * Pico, a civil parish in the municipality of Vila Verde * Pico da Pedra, a civil parish in the municipality of Ribeir ...

-size range. and picoeukaryotes are now known to make up a large fraction of the biomass and productivity in this size fraction in open ocean environments and even in exported carbon in the North Atlantic Bloom. Analysis of rDNA sequences indicate that heterotrophic oceanic picoeukaryotes belong to lineages such as the Alveolata,

stramenopiles Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...

choanoflagellates The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates, having a funnel shaped collar of intercon ...

, and

Acantharea The Acantharea (Acantharia) are a group of radiolarian protozoa, distinguished mainly by their strontium sulfate skeletons. Acantharians are heterotrophic marine microplankton that range in size from about 200 microns in diameter up to several ...

. In these lineages, many groups do not have cultured representatives yet. Grazing experiments have demonstrated that novel stramenopile picoeukaryotes are bacterivorous.

Ecology

Since the size of these organisms determines how they interact with their environment, it is no surprise that they are not known to form significant sinking organic aggregates. Their contribution to

carbon cycling The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major component ...

is difficult to assess because they are difficult to separate by techniques such as filtration.Worden, A. Z. ''et al.'' (2004). Assessing the dynamics and ecology of marine picophytoplankton: The importance of the eukaryotic component. ''Limnology and Oceanography'' 49: 168-79. Recent

fluorescent in situ hybridization Fluorescence ''in situ'' hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent probes that bind to only particular parts of a nucleic acid sequence with a high degree of sequence complementarity. It was developed ...

(FISH) experiments have shown that picoeukaryotes are fairly abundant in the

deep sea The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combin ...

. Increased resolution with the development of better FISH techniques indicates that study and detection should become easier. Additionally, qPCR has been a valuable approach for delineating and quantifying the different species, e.g. oceanic and coastal ''Bathycoccus'' and ''Ostreococcus'' species. Research has also shown that picoeukaryotes have a strong correlation with chlorophyll concentrations in both meso-autotrophic reservoirs and hypereutrophic reservoirs. Moreover, nitrogen enrichment experiments suggest that picoeukaryotes have an advantage over larger cells when it comes to acquiring nutrients because of their large surface area per unit volume. They have exhibited more effectiveness in the uptake of photons and nutrient from low-resource environments.

Biological characteristics

Photosynthetic picoeukaryotes, much like other

planktonic Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

species in the ocean photic zone, are exposed to light variations during the diel cycle and due to vertical displacement in the mixed layer of the water column. They have specialized biological reactions to help them deal with excessive densities of light, such as the

Xanthophyll cycle Xanthophylls (originally phylloxanthins) are yellow pigments that occur widely in nature and form one of two major divisions of the carotenoid group; the other division is formed by the carotenes. The name is from Greek (, "yellow") and (, "l ...

. However, there are also many types of non-photosynthetic picoeukaryotes that extend into the deep ocean and do not have these biochemical pathways.

Notes

External links

MicrobeWiki
A site on a biology Wiki run by Kenyon College {{microorganisms Microscopic eukaryotes Biological oceanography Planktology Aquatic ecology