"The most fascinating feature of Micrasterias algae is their sheer beauty", enthuses Monika Engels, a phycologist at the University of Hamburg who grows desmids in laboratory cultures. They are more closely related to land plants than to other algae, but their beauty is best appreciated when looking through a microscope: the mostly spherical or disc-shaped unicellular organisms have a very characteristic cell wall and usually consist of two identical, symmetrical compartments which are linked by a narrow connection point or bridge, the so-called isthmus. "Some Micrasterias species even have a radial symmetry and therefore even more than just one axis", explains Engels. Most Micrasterias species are very small, about the diameter of a human hair, between 0.1 und 0.3 millimeter. This means the different types and species can only be distinguished and identified by using a microscope.
Micrasterias indicates clean water
Algae belonging to the genus Micrasterias are commonly used in environmental protection and management as bioindicators: Micrasterias is the most important genus of desmids for indicating water quality because these algae are extremely sensitive and respond to their immediate environment. For example, different species can be found in water bodies with different pH: only very few species can exist in acid waters, however each of these may be present in high numbers. By contrast, many species occur in moderately acidic waters, but usually in only small numbers. Some Micrasterias species occur only in very nutrient-poor waters and can therefore be used to identify unpolluted water bodies.
Survival is uncertain
Most of the nearly 800 groups of desmids that occur in Germany are threatened by extinction. "More than 500 species are already listed as endangered in the Red Data Book of endangered species", says Wolf-Henning Kusber of the Botanical Museum in Berlin-Dahlem, whose job it is to regularly update the list of endangered and threatened species of German desmids. Desmids are under threat because they live in nutrient-poor bogs; due to peat extraction and agricultural development, more and more of our bogs have been lost. Some species of Micrasterias have only been recorded by algal researchers a couple of times after they had initially been described. Several species have been lost already from regions near Hamburg and Berlin. But even in the few remaining bogs the „little stars“ have become very rare because nutrients are often washed even into remote boggy areas by rainfall. As the algae are unable to adapt to changing environmental conditions, the only way to protect them is to protect the habitats they live in.
Resistant cell walls
Desmids belonging to the Micrasterias group can either float in open water bodies, live on the bottom of lakes, ponds and streams, or grow on other aquatic plants. Because their cell walls contain a very resistant polymer, Micrasterias species can survive periods of desiccation at the water’s edge, or can, on a trail of mucilage they can excrete, move towards damper areas or water, or towards the light.
Micrasterias is immortal
Sometimes Micrasterias species are described as „immortal“ because they can reproduce and multiply by a type of asexual reproduction that works by simple cell division: to reproduce, the two halves of the algae move away from the centre of the cell. After the nucleus (which is located in the isthmus) has been duplicated, the constriction in the centre of the cell extends, the two halves separate, and for each remaining half cell, a new half re-grows. The two new algal cells thus are identical copies of the original organism and each new cell now consists of two halves of different ages, one newly grown, and one derived from the mother cell.
Micrasterias can also reproduce sexually but this is quite rare and this occurs by what is called "sexual conjugation": two differently determined cells arrange themselves under a mucilaginous envelope and connect by means of a conjugation channel. The protoplasts of the two cells merge into a zygote with a very smooth outer surface. For a short time the four half-cells remain connected. As Micrasterias species contain only a single set of chromosomes (haploid), it takes two cell divisions to develop their typical cell walls, which allows them to adopt their characteristic, species-specific shapes. The newly formed young algae now contain re-mixed genetic material derived from the two parent cells, but in a new combination.