Supplément 3.4: Les algues dangereuses (en anglais)

Harmful Algae

There are four main groups of algae that form Harmful Algal Blooms (HABs):

Flagellates (includes dinoflagellates), diatoms, blue-green algae, and coccolithophores. Their concentration can have large variations: A bloom can be made up of 200 or 2 million algal cells per litre of sea water. It could even be an increased concentration of algal cells that are attached to the substrate, like a blade of seagrass or a piece of seaweed.

1. Flagellates / Dinoflagellates

Flagellates are single-celled organisms that usually have organic walls. They move about with whip-like appendages and can move up and down in the water column covering up to 15-20 meters in a day.

Dinoflagellates make up approximately 70% of all HAB species. They can bloom in concentrations of more than a million cells per millilitre. Many of these organisms contain coloured pigments that cause the water to appear colored when these organisms bloom, leading to the terms "red tide" or "brown tide," for example. However there are some dinoflagellate species that are colourless.

Red Tide caused by Dinoflagellates off the Scripps Institution of Oceanography Pier, La Jolla California.
Photo: Alejandro Díaz / Wikimedia Commons

2. Diatoms

Diatoms are unicellular algae but the yalso occur attached in chains. Their cell walls are composed of silica, a glass-like material, which comprises a shell-like structure. Diatoms can float at the water's surface or sink to the depths of the ocean.

The diatom Chaetoceros dichaeta. A dominant species during an algae bloom artifically caused by enhancing iron concentrations (EIFEX).
Photo: Alfred-Wegener-Institut

3. Blue-green algae

Blue-green algae (also called Cyanobacteria) are among the oldest types of bacteria found on Earth and are believed to have played a major role in the addition of oxygen to the Earth's early atmosphere. Up until today they are a significant component of the marine nitrogen cycle.

Cyanobacteria still have plant pigments like other algae but do not have the internal microstructure like that of the diatoms and flagellates. Individual cells, filaments, or colonies often float at the sea surface and generally appear blue-green in color.

Summer Bloom in the Baltic Sea.
Source: NASA

More information about this image ↓↑

The Baltic Sea blooms with life two times per year, once in the spring and once in the late summer. This photo-like Moderate Resolution Imaging Spectroradiometer (MODIS) image shows the deep green swirls of the summer bloom around the Swedish island of Gotland.
The summer bloom is usually caused by blue-green algae growing on the surface of the water. The algae thrive when ocean waters are warm and winds are calm. Strong winds would churn the ocean, stirring the plants down into the water?s depths. Different types of blue-green algae grow in these waters, at least one of which is toxic, but it is impossible to tell what kind of algae covers the sea from this image alone. Quite possibly, many different kinds of algae make up this bloom. Large summer blooms can contain toxic algae that are dangerous for humans and other animals.
MODIS flying on board NASA's Aqua satellite captured this image on July 5, 2005.

4. Coccolithophores

Coccolithophores are single-celled algae covered with calcium carbonate plates called coccoliths. The coccoliths give the ocean a milky white or turquoise appearance during intense blooms. The long-term flux of coccoliths to the ocean floor is the main process responsible for the formation of chalk and limestone.

Envisat's MERIS reduced-resolution image from 3 May 2007 showing a bloom of phytoplankton (primarily coccolithophores) along the edge of the continental shelf south west of Ireland and to the west of the English Channel, and suspended sediments in the Irish Sea, Bristol Channel and English Channel.
Image: ESA