Picoeukaryotes are picoplanktonic

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. Bact ...

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 in ...

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 syst ...

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", ...

ic communities. Though the SI prefix pico- might imply an organism smaller than atomic size, the term was likely used to avoid confusion with existing size classifications of

plankton 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 cr ...

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,'' t ...

s. For example, '' Ostreococcus tauri'', an autotrophic picoeukaryote belonging to the class Prasinophyceae, 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 Nucl ...

, 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 use ...

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 ...

, 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 (the ...

. Members of a heterotrophic class, the

Bicosoecida Bicosoecida ( ICZN) or Bicosoecales/Bicoecea (ICBN) is an order of Bikosea, a small group of unicellular flagellates, included among the heterokonts. Informally known as bicosoecids, they are a small group of unicellular flagellates. The cells ...

, 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 ...

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 contine ...

, 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 hotspo ...

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 Prasinophyceae (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 alg ...

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

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 biom ...

is due to cells in the pico-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 b ...

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 interconne ...

, and Acantharea. 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 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 (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 cruc ...

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 (, "le ...

. 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