There are several algae which calcify, but the ones in this experiment are known as coccolithophores. One such algae, Emiliania huxleyi, can be found almost anywhere, and is the most abundant organism on the planet.
One-celled marine plants covered with calcified plates (coccoliths) that live in large numbers throughout the upper layers of the ocean. Many types of coccolithophores exist, and the scales can be extremely elaborate. The coccolithophore that will fly in space on the ISS is Pleurochrysis carterae. Coccolithophores are very important in the carbon cycle of the earth, so NASA has been studying them and have published the NASA Earth Observatory articles.
An ESA gravitational biology payload to be operated during a two year period on board the US lab. You can read more detailed information at the European Space Agency.
A species of phytoplankton which is a freely drifting, photosynthesizing microscopic organism that lives in the upper, sunlit layers of the ocean. More information can be found at the National Oceanography Centre, Southampton.
Directed locomotory response to gravity. When an object tends to swim or move against the gravity vector, they are said to be negatively gravitaxic. Moreover, when an object tends to move or swim with the gravity vector, they are positively gravitaxic.
A unicellular marine alga, covered with calcite-rimmed scales (coccoliths).
Photo courtesy of Dr. Mary Marsh, UTSD, and UT Research Office
Scanning electron micrograph of a Pleurochrysis carterae cell. Each scale, or coccolith, has a rim of calcium carbonate crystals. The oblong shape of the cell is thought to play a role in its response to gravity. The production of coccoliths, the amount of calcium per cell, the cell's shape, and its response to gravity will all be studied on the International Space Station.
Photo courtesy of D. Montufar-solis and UTSD
From the picture above, you can see that P. carterae swims using two flagella. The coccoliths are visible as a ring surrounding the cell and their weight may be used in their directionality in swimming.