Biliphytes

Introduction

Biliphytes are an enigmatic eukaryotic group so far only known by SSU rDNA sequence and FISH-FITC images. Their divergent 18S rRNA gene sequences were first noted as a separate group (i.e. Rosko II) in 2004 (Romari and Vaulot 2004). In 2007, they were proposed as a putative algal group "picobiliphytes", as the first evidence of the group was found in the fraction of picoplankton i.e., cells going through 3 µm sieve (Not et al. 2007a). However, a recent environmental sequence study revealed that the group includes larger cells as well. Therefore, "biliphytes" is also used to refer to the group (Cuvelier et al 2008). Because these organisms remain uncultured, studies to date have relied solely on molecular evidence and some FISH (fluorescence in situ hybridization). Without conventional morphological data or information on actual pigment composition this taxon remains an enigmatic group.

Characteristics

As the name suggests, biliphytes appear to have phycobiliproteins, based on orange autofluorescence seen in FISH-FITC labeled cells. This pigment would most likely be maintained as an antenna pigment along with chlorophyll a (although only residual chlorophyll has been observed in these cells as the FISH protocols employed use an alcohol dehydration which removes chlorophyll) in its plastid (which has also not yet been observed, as it requires higher resolution microscopy than has been done to date).

The only micrographs of biliphytes available are the FISH images showing cytozolic ribosomes (green; labeled with DNA probe), nearby putative plastid (orange, phycobilin like fluorescence), a nucleus and a smaller nucleomorph-like structure (blue; labeled with DAPI). Nucleomorphs are a miniaturized secondary endosymbiont's nucleus still present in the secondary plastid of cryptophytes and chrorarachniophytes. Nucleomorphs give us an insight into the evolutionary process involved in the establishment of new plastids. The biliphyte nucleomorph was reported in the first description of these cells (Not et al. 2007a), but a report soon thereafter (Cuvelier et al. 2008) was unable to confirm the finding using similar (although not identical) epifluorescence microscopy techniques. Assuming there is DNA associated with the phycobilin containing structure, it is still unclear if the DNA is in a nucleomorph compartment or the plastid DNA (Cuvelier et al 2008). In fact, it is uncertain if the phycobilin containing structure would be an "established" plastid or a specific but temporary symbiont (e.g. the one in the katablepharid Hatena arenicola) or a kleptoplastid (a plastid stolen from another algal cell; e.g. the ones in dinoflagellates Amphidinium spp. and Dinophysis spp.), as no culture strain has been available to date.

Discussion of Phylogenetic Relationships

The phylogenetic position of biliphytes is still unsettled. Molecular phylogeny inferred from SSU rDNA has revealed that biliphytes may be a sister group of the cryptophytes+katablepharids clade, but bootstrap support is not consistently found. Recently cryptophytes and katablepharids have been recognized to be members of Hacrobia (Okamoto et al 2009). Thus biliphytes are probably a member of this supergroup.

Known Habitat

Biliphytes were originally reported primarily in cold water samples in the Arctic and Atlantic (Not et al. 2007a, Not et al. 2007b). However, a more recent environmental sequence study revealed that they are widespread in tropically influenced, subtropical and as well as temperate waters and the Arctic (Cuvelier et al 2008).

Other Names for Biliphytes

• Picobiliphytes

References

Cuvelier ML, Ortiz A, Kim E, Moehlig H, Richardson DE, Heidelberg JF, Archibald JM, Worden AZ (2008) Widespread distribution of a unique marine protistan lineage. Environmental Microbiology 10:1621-34

Not F, Valentin K, Romari K, Lovejoy C, Massana R, Töbe K, Vaulot D, Medlin L (2007a) Picobiliphytes: a marine picoplanktonic algal group with unknown affinities to other eukaryotes. Science 315:253–5

Not F, Gausling R, Azam F, Heidelberg JF, Worden AZ (2007b) Vertical distribution of picoeukaryotic diversity in the Sargasso Sea. Environmental Microbiology 9: 1233 - 1252

Okamoto N, Chantangsi C, Horák A, Leander BS, Keeling PJ (2009) Molecular Phylogeny and Description of the Novel Katablepharid Roombia truncata gen. et sp. nov., and Establishment of the Hacrobia Taxon nov. 4:e7080.

Romari & Vaulot (2004) Composition and temporal variability of picoeukaryote communities at a coastal site of the English Channel from 18S rDNA sequences. Limnol Oceanogr 49:784-98