Picozoa

Picozoa, Picobiliphyta, Picobiliphytes, or Biliphytes are protists of a phylum of marine unicellular heterotrophic eukaryotes with a size of less than about 3 micrometers. They were formerly treated as eukaryotic algae and the smallest member of photosynthetic picoplankton before it was discovered they do not perform photosynthesis.[2] The first species identified therein is Picomonas judraskeda.[1] They probably belong in the Archaeplastida as sister of the Rhodophyta.[3][4][5]

Picozoa
Movement of a Picomonas judraskeda cell
Animation of the 3D structure of Picomonas judraskeda
Scientific classification Edit this classification
(unranked): Archaeplastida
Phylum: Picozoa
Seenivasan, Sausen, Medlin, Melkonian, 2013[1]

They were formerly placed within the cryptomonads-haptophytes assemblage.[6]

Discovery

At the end of the 1990s the European project "Picodiv" clarified which organisms occur in picoplankton. 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.[7] Picobiliphyta were found particularly within the nutrient-poor ranges from cold coastal seas, where they can constitute up to 50 percent of the biomass.

Affinities to other organisms
Picomonas judraskeda

Picozoa were first detected using 18S ribosomal RNA genes in 2007.[8] 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, co-authors of the study and molecular biologists at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven.[9] 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.[9]

Apart from the unfamiliar gene sequences, the researchers also detected phycobiliproteins.[10] In red algae, for example, these proteins occur as pigments. But in this newly discovered group of algae, the phycobiliproteins appear to be contained inside the plastids,[11] where the photosynthesis 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".[8]

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)".[12] A study later in 2011, conducted by researchers at Rutgers University and Bigelow Laboratory for Ocean Sciences, 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.[13][14]

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.[1] 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.[1]

See also

References
  1. Seenivasan R, Sausen N, Medlin LK, Melkonian M (2013). Waller RF (ed.). "Picomonas judraskeda gen. et sp. nov.: the first identified member of the Picozoa phylum nov., a widespread group of picoeukaryotes, formerly known as 'picobiliphytes'". PLOS ONE. 8 (3): e59565. Bibcode:2013PLoSO...859565S. doi:10.1371/journal.pone.0059565. PMC 3608682. PMID 23555709.
  2. Moreira D, López-García P (May 2014). "The rise and fall of Picobiliphytes: how assumed autotrophs turned out to be heterotrophs". BioEssays. 36 (5): 468–474. doi:10.1002/bies.201300176. PMC 4133654. PMID 24615955.
  3. Burki F, Kaplan M, Tikhonenkov DV, Zlatogursky V, Minh BQ, Radaykina LV, et al. (January 2016). "Untangling the early diversification of eukaryotes: a phylogenomic study of the evolutionary origins of Centrohelida, Haptophyta and Cryptista". Proceedings. Biological Sciences. 283 (1823): 20152802. doi:10.1098/rspb.2015.2802. PMC 4795036. PMID 26817772.
  4. Lax G, Eglit Y, Eme L, Bertrand EM, Roger AJ, Simpson AG (December 2018). "Hemimastigophora is a novel supra-kingdom-level lineage of eukaryotes". Nature. 564 (7736): 410–414. Bibcode:2018Natur.564..410L. doi:10.1038/s41586-018-0708-8. PMID 30429611. S2CID 205570993.
  5. Cavalier-Smith T, Chao EE, Lewis R (December 2015). "Multiple origins of Heliozoa from flagellate ancestors: New cryptist subphylum Corbihelia, superclass Corbistoma, and monophyly of Haptista, Cryptista, Hacrobia and Chromista". Molecular Phylogenetics and Evolution. 93: 331–362. doi:10.1016/j.ympev.2015.07.004. PMID 26234272.
  6. Keeling P, Leander BS (28 October 2009) [8 September 2000]. "Eukaryotes". Tree of Life Web Project. Retrieved 2009-06-17.
  7. Massana R, Guillou L, Díez B, Pedrós-Alió C (September 2002). "Unveiling the organisms behind novel eukaryotic ribosomal DNA sequences from the ocean". Applied and Environmental Microbiology. 68 (9): 4554–4558. Bibcode:2002ApEnM..68.4554M. doi:10.1128/AEM.68.9.4554-4558.2002. PMC 124113. PMID 12200313.
  8. Not F, Valentin K, Romari K, Lovejoy C, Massana R, Töbe K, et al. (January 2007). "Picobiliphytes: A Marine Picoplanktonic Algal Group with Unknown Affinities to Other Eukaryotes" (PDF). Science. 315 (5809): 253–255. Bibcode:2007Sci...315..253N. doi:10.1126/science.1136264. PMID 17218530. S2CID 22285394.
  9. "New Group Of Algae Discovered: Picobiliphytes". ScienceDaily. January 15, 2007. Retrieved 2021-11-01.
  10. Hearn K (11 January 2007). "Bizarre New Form of Life Found in Arctic Ocean, Scientists Announce". National Geographic News. Archived from the original on 11 February 2008. Retrieved 2009-06-17.
  11. Aronsson H, Sandelius AS (2008). The Chloroplast: Interactions with the Environment (Plant Cell Monographs). Berlin: Springer. p. 9. ISBN 978-3-540-68692-7.
  12. Kim E, Harrison JW, Sudek S, Jones MD, Wilcox HM, Richards TA, et al. (January 2011). "Newly identified and diverse plastid-bearing branch on the eukaryotic tree of life". Proceedings of the National Academy of Sciences of the United States of America. 108 (4): 1496–1500. Bibcode:2011PNAS..108.1496K. doi:10.1073/pnas.1013337108. PMC 3029697. PMID 21205890.
  13. Yoon HS, Price DC, Stepanauskas R, Rajah VD, Sieracki ME, Wilson WH, et al. (May 2011). "Single-cell genomics reveals organismal interactions in uncultivated marine protists". Science. 332 (6030): 714–717. Bibcode:2011Sci...332..714Y. doi:10.1126/science.1203163. PMID 21551060. S2CID 34343205.
  14. Worden AZ, Dupont C, Allen AE (June 2011). "Genomes of uncultured eukaryotes: sorting FACS from fiction". Genome Biology. 12 (6): 117. doi:10.1186/gb-2011-12-6-117. PMC 3218834. PMID 21722350.
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