Haptophyte diversity in the Skagerrak

(Coccolithus pelagicus)

Haptophyta is a division of algae with a few unique features such as two flagella, both of which are smooth, and a unique organelle called a haptonema, which is superficially similar to a flagellum but differs in the arrangement of microtubules and in its use. Probably the best-known haptophytes are coccolithophores, which have an exoskeleton of calcareous plates called coccoliths (see image).

Marine haptophytes occur in all seas as major components of the smallest plankton and carry out key processes in global biogeochemical cycles. Some of the colony forming representatives can form toxic blooms that are harmful to marine biota in general, farmed fish in particular, and to fisheries and tourism through the production of scum and acrylic acid. On the other hand some haptophytes are economically important (e.g. Pavlova lutheri and Isochrysis sp.) as they are widely used as food source in the aquaculture industry.

Despite their importance haptophyte species are difficult to identify using microscopy only because they are small and fragile. They may change form drastically upon fixation for microscopy, and may lose appendages and scales essential for morphological identification. 

About 300 species have been described so far but investigations of environmental water samples using molecular methods have revealed a large diversity of novel haptophyte sequences which suggests that the total number of species is much higher, particularly among the pico-haptophytes. Currently, ca. 650 unique, full-length or partial 18S rDNA haptophyte sequences are included in the Protist Ribosomal Reference Database (PR2). Some of these environmental sequences may represent novel deep-branching haptophyte lineages.

Researchers from the University of Oslo were interested in the diversity of haptophytes in the Skagerrak and they used 454 pyrosequencing of the 18S ribosomal DNA V4 region (= metabarcoding), and supplemented it with electron microscopy. 

The work revealed higher species richness of haptophytes than previously observed in the Skagerrak by microscopy. From ca. 400,000 reads we obtained 156 haptophyte operational taxonomic units (OTUs) after rigorous filtering and 99.5% clustering. The majority (84%) of the OTUs matched environmental sequences not linked to a morphological species, most of which were affiliated with the order Prymnesiales. Phylogenetic analyses including Oslofjorden OTUs and available cultured and environmental haptophyte sequences showed that several of the OTUs matched sequences forming deep-branching lineages, potentially representing novel haptophyte classes. Pyrosequencing also retrieved cultured species not previously reported by microscopy in the Skagerrak. Electron microscopy revealed species not yet genetically characterised and some potentially novel taxa. 

According to the authors some closely related species have identical 18S V4 rDNA regions, which in turn means that the true richness may be even higher. What strikes me is the fact that new lineages range from species to class level once again highlighting how little we know about the large group of diverse eukaryotic, mainly unicellular microorganisms called protists.