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Picozoa, Picobiliphyta, picobiliphytes, or piliphytes are protists of a phylum of marine unicellular 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. The phylum currently contains a single species, Picomonas judraskeda. They probably belong in the Archaeplastida as sister of the Rhodophyta.
They were formerly placed within the cryptomonads-haptophytes assemblage.
Apart from the unfamiliar gene sequences, the researchers also detected . 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 , (2025). 9783540686927, Springer. ISBN 9783540686927 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)". 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.
In 2013, 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, unusual movement, and heterotrophic mode of nutrition, substantiate their unique phylogenetic position. No traces of viral or bacterial particles were found inside these heterotrophic cells, which prompted these authors to suggest that they feed on small organic particles.
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