Friday, January 12, 2007

 

Picobiliphytes: A marine picoplanktonic algal group with unknown affinities to other Eukaroytes

An international group of researchers has succeeded in identifying a previously unknown group of algae. As currently reported in the scientific journal Science, the newly discovered algae are found among the smallest members of photosynthetic plankton - the picoplankton*. On account of the minute size of the organisms (no more than a few thousandth of a millimetre) and the appearance of phycobili-proteins, researchers have termed the new group Picobiliphytes.

Approximately 50 percent of global photosynthesis is conducted in the world's oceans where it is dominated by microscopic algae, the so-called phytoplankton. Scientists estimate that up to 90 percent of phytoplanktonic species are currently unidentified. In the present study, scientists used molecular techniques to investigate the smallest members of the plankton, the picoplankton. Because picoplankton algae are so extremely small, they are almost impossible to study by means of microscopy.

Researchers investigated gene sequences of the 18S 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 molecularbiologists at the Alfred Wegener Institute* 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 extracted phycobili-proteins from the algae. In red algae, for example, these proteins occur as colour pigments. However, in the newly discovered algal group, the phycobili-proteins are found at the sites for photosynthesis, i.e. in the plastids. This location of pigment proteins represents a novelty in that it has not been reported for any other algal species. Hence, it provides a clear indication that the researchers are dealing with a previously unidentified species of algae. Referring to their small size and the presence of phycobili proteins, the researchers named the new group Picobiliphytes.

Original press release (dated 12.01.07) from the Alfred Wegener Institute is available via this link

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Based on the journal Science paper:

Picobiliphytes: A Marine Picoplanktonic Algal Group with Unknown Affinities to Other Eukaryotes

Klaus Valentin and Linda Medlin et al.

Science 12 January 2007:
Vol. 315. no. 5809, pp. 253 - 255
DOI: 10.1126/science.1136264

Abstract

Environmental sequencing has revealed unimagined diversity among eukaryotic picoplankton. A distinct picoplanktonic algal group, initially detected from 18S ribosomal DNA (rDNA) sequences, was hybridized with rRNA {lambda}-targeted (rRNA-targeted) probes, detected by tyramide signal amplification-fluorescent in situ hybridization, and showed an organelle-like body with orange fluorescence indicative of phycobilins. Using this fluorescence signal, cells were sorted by flow cytometry and probed. Hybridized cells contained a 4',6'-diamidino-2-phenylindole-stained organelle resembling a plastid with a nucleomorph. This suggests that they may be secondary endosymbiotic algae. Pending the isolation of living cells and their formal description, these algae have been termed picobiliphytes.

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An earlier post on the Alfred Wegener Institute: "Antarctic research within the International Polar Year IPY 2007/2008"

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*A 2001 paper on picoplankton from the journal of Applied and Environmental Microbiology:

Beatriz Diez, Carlos Pedros-Alio, and Ramon Massana

Applied and Environmental Microbiology, July 2001, p. 2932-2941, Vol. 67, No. 7
0099-2240/01/$04.00+0 DOI: 10.1128/AEM.67.7.2932-2941.2001

Study of Genetic Diversity of Eukaryotic Picoplankton in Different Oceanic Regions by Small-Subunit rRNA Gene Cloning and Sequencing

Abstract

Very small eukaryotic organisms (picoeukaryotes) are fundamental components of marine planktonic systems, often accounting for a significant fraction of the biomass and activity in a system. Their identity, however, has remained elusive, since the small cells lack morphological features for identification.

We determined the diversity of marine picoeukaryotes by sequencing cloned 18S rRNA genes in five genetic libraries from North Atlantic, Southern Ocean, and Mediterranean Sea surface waters. Picoplankton were obtained by filter size fractionation, a step that excluded most large eukaryotes and recovered most picoeukaryotes.

Genetic libraries of eukaryotic ribosomal DNA were screened by restriction fragment length polymorphism analysis, and at least one clone of each operational taxonomic unit (OTU) was partially sequenced. In general, the phylogenetic diversity in each library was rather great, and each library included many different OTUs and members of very distantly related phylogenetic groups. Of 225 eukaryotic clones, 126 were affiliated with algal classes, especially the Prasinophyceae, the Prymnesiophyceae, the Bacillariophyceae, and the Dinophyceae. A minor fraction (27 clones) was affiliated with clearly heterotrophic organisms, such as ciliates, the chrysomonad Paraphysomonas, cercomonads, and fungi. There were two relatively abundant novel lineages, novel stramenopiles (53 clones) and novel alveolates (19 clones).

These lineages are very different from any organism that has been isolated, suggesting that there are previously unknown picoeukaryotes. Prasinophytes and novel stramenopile clones were very abundant in all of the libraries analyzed. These findings underscore the importance of attempts to grow the small eukaryotic plankton in pure culture.

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