Picobiliphyte

Picobiliphytes or Picobiliphyta are a group of eukaryotic algae, discovered in 2007,^[1] which are found among the smallest members of photosynthetic picoplankton.

In some subsequent publications they were renamed "biliphytes" when sampling with greater statistical strength (than in the original publication) revealed these cells were larger than 3 µm in size. In this later research the conclusion that biliphytes were photosynthetic was also more tentative.^[2][3]

Discovery

At the end of the 1990s with the European project "Picodiv" it would be 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. 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

Researchers investigated gene sequences of the 18S ribosomal RNA gene, common to all cells. The identity of new organisms can be 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 Dr Klaus Valentin and Dr. Linda Medlin, co-authors of the study and molecular biologists at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven. 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 phycobiliproteins.^[4] 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,^[5] 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".

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)".^[6] A study later in 2011, conducted by researchers at Rutgers University and Bigelow Oceanographic Labs, 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.^[7][8]

Some sources group picobiliphytes within the cryptomonads-haptophytes assemblage.^[9]

Most recently, Dr. Seenivasan working in conjunction with Professors 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 organic particles. see link to Picozoa

See also

• Microphyte
• Picoeukaryote
• Biliphyta
• Picozoa
• Picobiliphytes have been considered autotrophic algae, there is, however a strong question mark over this fact. In a recent study there has been a thorough search for genes from plastid origin in three different picobiliphytes that has been unsuccessful. In this analysis several sequencing techniques have been applied and, with the help of comparative genomic tools, the authors have demonstrated that Picobilihytes have complex relationships with other microorganisms including virus, but they could not find any trace of plastid encoded genes. These results strongly suggest that Picobiliphytes are most likely heterotrophic rather than autotrophic. They have now been formally described as a heterotrophic nanoflagellate phylum, PIcozoa.

External links

• Neue Algengruppe entdeckt: Picobiliphyta (German)
• New Group Of Algae Discovered: Picobiliphytes (English)
• Bizarre New Form of Life Found in Arctic Ocean, Scientists Announce
• Tree of Life: Biliphytes

References

Plankton
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