Diatoms are key and highly diversified primary producers

Diatoms constitute a group of eukaryotic microalgae, which are relatively easy to specifically identify because of their distinctive morphology. Each cell has a wall composed of two silicified valves that are held in place by silicified girdle bands, and fit together like a Petri dish, to form the frustule. Diatoms constitute the major component of photosynthetic planktonic as well as microbenthic communities, and are ubiquitous in all marine and freshwater ecosystems, as well as in many terrestrial habitats. They are also the most species-rich group of algae; their biodiversity is assumed to be the highest, close to that of higher plants, possibly reaching tens of thousands of species. Diatoms account for ca. 40% of the total marine primary production, thus these organisms play a central role in the biogeochemical cycles of carbon and nitrogen, and silica.

Diatoms way of life presents unique features

Diatoms are also characterized by unique features of their mitotic cell division and life cycles. When a diatom undergoes mitosis, each daughter cell receives one of the two valves of the frustule from the mother cell. The inherited valve is used as the outer valve, and the daughter cell synthesizes a new inner valve. Hence, one of the daughter cells is identical in size to the parental cell, the other is smaller, a phenomenon which usually leads to a reduction in the average cell size of the population. The species-specific maximum cell size (that of the ‘initial cells’) is usually restored by a phase of sexual reproduction, when the zygotes turn into specialized nursery cells, known as auxospores, which expand in volume and form large initial cells. Diatoms are generally divided into two groups based on valve symmetry and their mode of reproduction: centric forms and pennate forms. Centric diatoms display radial symmetry and are oogamous (i.e., they produce small motile male gametes and large nonmotile female gametes), whereas pennate diatoms are rather boat-shaped, display bilateral symmetry and are aplanogamous (they do not release flagellate gametes.

The first 'blue-diatom' Haslea ostrearia

Up to now the term ‘blue diatom’ has referred specifically to the pennate marine diatom Haslea ostrearia (Fig. 1), a tychopelagic - benthic organism, known for its ability to produce a water-soluble blue pigment, called marennine. Marennine is produced during algal growth and ageing, and accumulates mainly at the apical regions of the cells, before being released in the culture medium. In oyster ponds of western France, where H. ostrearia may become dominant year after year, marennine is released to the seawater and it is responsible for the ’greening’ of oysters (Fig. 2). Oyster’ gills, which are rather yellow, turn green when the blue pigment is fixed upon them. This phenomenon has a noticeable local economical impact (added value to the bivalves), that has long been known, as well as the direct implication of a microscopic alga as a possible causative factor. This microalga was first described as Vibrio ostrearius, then as Navicula ostrearia and finally as H. ostrearia. The name ‘marennine’ originates from the Marennes-Oléron region, an area dedicaded to bivalve production in south-western France.

Marennine, a specific and valuable blue pigment

For decades, the biological function of marennine in H. ostrearia cells remained mostly hypothetical, with the exception of a demonstrated light filtering effect, which induces changes in light quality through the water column. This would likely effect the competition between microalgae in oyster ponds. Some allelopathic and biological activities have been observed, as water-soluble crude extracts of H. ostrearia containing marennine inhibited in vitro growth of four marine diatom species. Crude extracts also have shown in vitro antiviral and anticoagulant activities, antiproliferative and antitumoral activity. The extracts also found applications in cosmetology and personal care products, despite the fact that marennine chemical structure and properties were unknwon. Several hypotheses, sometimes contradictory, regarding marennine chemical nature and structure have been proposed in the literature, but none of them were supported by strong experimental evidences. In spite of this lack of knowledge, for several years, H. ostrearia was grown industrially under indoor controlled conditions to produce marennine, mainly for the intensive greening of oysters (transfer of knowledge from University of Nantes to SOPROMA, Bouin, France), but this production unit ceased its activity about 6 years ago. Nevertheless, a demand exists, at least from the cosmetic or seafood industry to exploit this renewable biological resource, despite recurrent uncertainties about its molecular structure, and the fact that it could be absorbed on a more or less regular basis, by the digestive system of thousands of oysters’ consumers, with effects unknown.

Current developments on marennine

Our knowledge of marennine chemical nature recently increased with the publication of a method for its extraction and purification. Preliminary characterization has shown that marennine is a polyphenolic compound of medium molecular weight (ca. 10 kDa), but its chemical structure has not yet been established. Purified marennine demonstrates variable biological activities, but in agreement with the previous studies made using crude extracts of H. ostrearia. Indeed purified marennine has been shown to inhibit in vitro the growth of some benthic diatoms encountered in oyster-ponds, involving both allelopathic mediation and light filtration. Marennine has also been shown to display antioxidant properties, as well as antibacterial activity. Furthermore, it has long been known that marennine synthesis in this diatom is enhanced by nutrient limitation as well as by light intensity and quality (enhancement by high irradiance and blue radiation). However little is known about other factors that could control marennine production by H. ostrearia, such as the relation with life and cell cycle events, or the genetic variability between clones.

Geographic occurrences of H. ostrearia

The diatom H. ostrearia has long been considered the only microalga known, worldwide in distribution, to produce a blue pigment like marennine. In the literature, diatoms with blue tips described as H. ostrearia were reported in almost all seas and oceans in northern and southern hemispheres, e.g., on all French coasts, from the Channel to the Mediterranean Sea, in Italy, in the Adriatic Sea. In the north-western Europe it has been observed in the coasts of Great-Britain, Norway, Sweden, Denmark, and in the Baltic Sea. In northern Atlantic Ocean, bluish diatoms identified as H. ostrearia has been observed in different places along the North American coasts: North Carolina and Rhode Island, Virginia, New Jersey, and possibly in Honduras. In the northern Pacific Ocean, diatoms identified as H. ostrearia have been observed in San Juan Islands, between Washington state and British Columbia, and in Japan. In the southern hemisphere, H. ostrearia has been observed in the Indian Ocean, in northern and eastern Australia, thus presuming a worldwide distribution of this peculiar organism. However it is worth noting that usually the presence of H. ostrearia was deduced either directly from observation of diatoms with bluish apices, or indirectly from a greening effect of bivalves. In most cases, a careful identification of the ‘blue diatom’ at the species level did not rely on detailed studies of morphology and morphometrics.

New 'blue diatoms' from Ukraine and beyond

In Spring 2008, phytoplankton samples taken on the rocky shores of the Karadag Natural Reserve in Crimea (Ukraine), contained pennate diatoms exhibiting morphological similarities to H. ostrearia: they had bluish apices, but in which the pigment seemed slightly darker than marennine (Fig. 2). Some of these cells were isolated and different monoclonal strains were cultured in the laboratory. Preliminary studies show that the pigment produced by the Ukrainian diatom presents spectroscopic similarities to, but also differences from the pigment produced by H. ostrearia. Although different from marennine, the pigment produced by the Ukrainian diatom can colour oyster gills (Fig. 2). The protocol used for the induction of the sexual reproduction in H. ostrearia was successfully applied to the Ukrainian diatom. Auxospores and F1 hybrids resulted from mating Ukrainian clones interclonally, but the Ukrainian diatom and the French clones of H. ostrearia have not interbred. Morphological and molecular characterization demonstrated that French and Ukrainian ‘blue diatoms’ belong to two separate species of the genus Haslea. The Ukrainian species has been named H. karadagensis. From a more global perspective, the discovery of the Ukrainian diatom raised a concern about occurrences of ‘blue diatoms’ and their identification as H. ostrearia in other marine environments. This concern is the cornerstone of the program BIOVADIA.