Diatom of the Month May 2019 - Isolating Haslea: the blue diatom

Up until now we thought diatoms were all yellowish-brown.
Anusuya Willis* present us a diatom with different colour: blue!

Diatoms are single-celled algae characterized by ornate silica shells. Diatoms have a centric or pennate form with thousands of different shapes and sizes. One unique diatom has an additional feature that makes it stand out: it is blue. Diatom species of the genus Haslea Simonsen produce a water-soluble blue pigment called ‘marennine’ that make the inside of the cell vivid blue. Species of Haslea are found in benthic habitats all over the world, such as on sand and rocks on the bottom of coastal waters. These diatoms are particularly well known in France because when Haslea grows in high enough numbers within oyster ponds, its blue-pigment can be released into the seawater, or the cells can be consumed by the oysters, which causes their gills to turn green (Figure 1). These green oysters or “huitres vertes” are a delicacy in French cuisine. The name of the pigment, “marennine,” comes from Marennes-Oleron, a region in France with extensive oyster production. Marennine is a biologically active pigment with antioxidant properties that may affect other diatoms in the phytoplankton community and/or improve the health of the oysters.

Figure 1. “Greening” of shellfish exposed to the blue pigment produced by the diatom Haslea (reproduced with permission from Gastineau et al. 2014).

Oyster greening is less common in Australia, but Haslea does occur in the coastal waters and I recently found some in a Pipe Clay Lagoon in Tasmania, Australia. At the Australian National Algae Culture Collection we collect, isolate, and culture algae from around Australia and supply them to aquaculture producers and researchers around the world for a variety of purposes (e.g. feed stock, molecular diversity studies). The first step in growing a culture of a new species is finding it in the wild. Just as land plants grow and thrive in spring and summer so do algae, and so this is the best time to look for new and/or interesting species. A few weeks ago, I collected a benthic sample and took it back to the laboratory for microscope analysis: I was lucky enough to find some Haslea cells attached on sand grains (Figure 2). Then, I transferred a single Haslea cell with a micro-pipette to a well in a multi-well plate. Diatoms and some other algae divide asexually, and this allows development of a monoculture in the laboratory (Figures 3 and 4). Such monocultures can then be used to answer research questions about their biology and ecology.

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Figure 2. a) One Haslea cell gliding between grains of sand from a lagoon in Tasmania, Australia; b) a high-resolution image of Haslea (scalebar= 50 µm; author’s own photos).

Figure 3. A monoculture of Haslea obtained by isolating a single cell and allowing asexual reproduction to occur.

Figure 4. Asexual reproduction resulted in these two Haslea cells.

The new Haslea culture, grown from a single cell found in Tasmania, will now be maintained at the Australian National Algae Culture Collection and will be available for researchers to use in their studies. Although Haslea grow readily in the wild and our Haslea culture’s health is good, it is not usually one of the easiest diatoms to maintain successfully in the laboratory. The reasons for this are not clear, but, in general, maintaining long-term diatom cultures can be challenging.

Haslea is just one of many types of algae that oysters eat. Aquaculture systems feed oysters a well-balanced diet of different algae species selected for their nutritional quality and omega-3 oils. Omega-3 oils are known for their health benefits to people via trophic interactions of invertebrates, oysters and fish. Although fish are known as a good source of these healthy omega-3 oils, these oils actually come from algae and only accumulate in the fish when they consume the algae.

The diversity of diatoms is indeed spectacular! As an algal molecular and cell biologist, my research interests are in understanding phytoplankton diversity, their environmental adaptation and population dynamics. I am interested in how genome variation and differential gene expression in response to environmental stimuli leads to population changes, variable cell physiology and adaptation. The more I look, the more I love finding these unique organisms gliding between the grains of sand (Figure 5).

 

Figure 5. Two Haslea sp. cells amongst other benthic diatoms (authors’ own photo).

 

*Australian National Algae Culture Collection, CSIRO, Hobart, Tasmania, Australia.

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