Picozoa
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Picozoa, Picobiliphyta, Picobiliphytes, or Biliphytes are
protists A protist ( ) or protoctist is any Eukaryote, eukaryotic organism that is not an animal, Embryophyte, land plant, or fungus. Protists do not form a Clade, natural group, or clade, but are a Paraphyly, paraphyletic grouping of all descendants o ...
of a
phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...
of marine unicellular
heterotroph 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 ...
ic
eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s with a size of less than about 3 micrometers. They were formerly treated as
eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
and the smallest member of
photosynthetic picoplankton Photosynthetic picoplankton or picophytoplankton is the fraction of the photosynthetic phytoplankton of cell sizes between 0.2 and 2 μm (i.e. picoplankton). It is especially important in the central oligotrophic regions of the world ocea ...
before it was discovered they do not perform photosynthesis. The first species identified therein is ''Picomonas judraskeda''. They probably belong in the
Archaeplastida The Archaeplastida (or kingdom Plantae '' sensu lato'' "in a broad sense"; pronounced ) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group glaucophytes. It als ...
as sister of the
Rhodophyta Red algae, or Rhodophyta (, ; ), make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest phyla of algae, containing over 7,000 recognized species within over 900 genera amidst ongoing taxonomic revisions. ...
. They were formerly placed within the
cryptomonads-haptophytes assemblage The cryptomonads-haptophytes assemblage is a proposed but disputed monophyletic grouping of unicellular eukaryotes that are not included in the SAR supergroup. Several alternative names have been used for the group, including Hacrobia (derived ...
.


Discovery

At the end of the 1990s the European project "Picodiv" clarified which
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s occur in
picoplankton Picoplankton is the fraction of plankton composed by cell (biology), cells between 0.2 and 2 μm that can be either prokaryotic and eukaryotic phototrophs and heterotrophs: * photosynthetic * heterotrophic They are prevalent amongst microbial p ...
. In addition, for a period of two years, samples were taken in the Atlantic, in the Mediterranean, before the coast of Scotland, Alaska and Norway. Picobiliphyta were found particularly within the nutrient-poor ranges from cold coastal seas, where they can constitute up to 50 percent of the
biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...
.


Affinities to other organisms

Picozoa were first detected using
18S ribosomal RNA 18S ribosomal RNA (abbreviated 18S rRNA) is a part of the ribosomal RNA in eukaryotes. It is a component of the Eukaryotic small ribosomal subunit (40S) and the cytosolic homologue of both the 12S ribosomal RNA, 12S rRNA in mitochondria and the 1 ...
genes in 2007. The identity of new organisms was deduced from a comparison of familiar and unfamiliar gene sequences. “The gene sequences found in these algae could not be associated with any previously known group of organisms”, explain Klaus Valentin and
Linda Medlin Linda Karen Medlin is a molecular biologist known for her work on diatoms. She is an elected member of the Norwegian Academy of Science and Letters. Education and career Medlin has a B.S. from the University of Texas at Austin (1970), and an ...
, co-authors of the study and molecular biologists at the Alfred Wegener Institute for Polar and Marine Research in
Bremerhaven Bremerhaven (; ) is a city on the east bank of the Weser estuary in northern Germany. It forms an exclave of the Bremen (state), city-state of Bremen. The Geeste (river), River Geeste flows through the city before emptying into the Weser. Brem ...
. The algae in this study were found in plankton samples originating from various regions of the North Atlantic and the Mediterranean. The scientists have discovered a group of organisms which, despite being completely new to science, have a wide distribution. “This is a good indication for how much there is still to discover in the oceans, especially using molecular tools”, says Valentin. Apart from the unfamiliar gene sequences, the researchers also detected
phycobiliprotein Phycobiliproteins are water-soluble proteins present in cyanobacteria and certain algae (rhodophytes, cryptomonads, glaucocystophytes). They capture light energy, which is then passed on to chlorophylls during photosynthesis. Phycobiliproteins are ...
s. In
red algae Red algae, or Rhodophyta (, ; ), make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest Phylum, phyla of algae, containing over 7,000 recognized species within over 900 Genus, genera amidst ongoing taxon ...
, for example, these proteins occur as pigments. But in this newly discovered group of algae, the phycobiliproteins appear to be contained inside the
plastid A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...
s, where the
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
occurs. Hence, it provides a clear indication that the researchers are dealing with previously unidentified group of algae. Referring to their small size and the presence of phycobiliproteins, the researchers named the new group "Picobiliphyta". Two studies published in 2011 found the hypothesis that biliphytes, or picobiliphytes, were photosynthetic was likely to be false. A 2011 study by an international team from the Monterey Bay Aquarium Research Institute, Dalhousie University and the Natural History Museum London found that cells in the Pacific Ocean did not have fluorescence indicative of photosynthetic pigments, and concluded "...biliphytes are likely not obligate photoautotrophs but rather facultative mixotrophs or phagotrophs, whereby transient detection of orange fluorescence could represent ingested prey items (e.g., the cyanobacterium ''Synechococcus'')". A study later in 2011, conducted by researchers at
Rutgers University Rutgers University ( ), officially Rutgers, The State University of New Jersey, is a Public university, public land-grant research university consisting of three campuses in New Jersey. Chartered in 1766, Rutgers was originally called Queen's C ...
and
Bigelow Laboratory for Ocean Sciences Bigelow Laboratory for Ocean Sciences, founded in 1974, is an independent, non-profit oceanography research institute. The Laboratory's research ranges from microbial oceanography to the large-scale biogeochemical processes that drive ocean eco ...
, used whole genome shotgun sequence data from three individual picobiliphyte cells to show absence of plastid-targeted or photosystem proteins within the fragments of nuclear genome sequence they reconstructed. This again suggested that picobiliphytes are
heterotrophs 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 ...
. Most recently, Seenivasan working in conjunction with Michael Melkonian (University of Cologne) and Linda Medlin (Marine Biological Association of the UK) formally described the picobiliphytes as the heterotrophic nanoflagellate phylum, Picozoa, and published thin sections of the cells. Several unique features in the cell, such as a feeding organelle, substantiate their unique phylogenetic position, an unusual movement, and heterotrophic mode of nutrition. No traces of viral or bacterial particles were found inside these heterotrophic cells, which prompted these authors to suggest that they feed on very small organic particles.


See also

* Microphyte * Picoeukaryote * Picocyanobacteria *
Picoplankton Picoplankton is the fraction of plankton composed by cell (biology), cells between 0.2 and 2 μm that can be either prokaryotic and eukaryotic phototrophs and heterotrophs: * photosynthetic * heterotrophic They are prevalent amongst microbial p ...
*
Biliphyta Biliphyta is an obsolete polyphyletic subkingdom of algae. It includes Glaucophyta The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environme ...


References


External links

* *
Neue Algengruppe entdeckt: Picobiliphyta



Tree of Life: Biliphytes
{{Taxonbar, from=Q10640455 Hacrobia Protist phyla Diaphoretickes phyla Articles containing video clips