Desmids
Desmids are tiny, single-celled green algae that live in fresh water. Many are made of two mirror-image halves joined in the middle, and they come in beautiful shapes such as stars, crescents and snowflakes, which makes them a favorite of people who explore pond water with a microscope.
Scientific Classification & Quick Facts
| Common name | Desmid |
| Scientific name | Desmidiales |
| Kingdom | Plantae |
| Phylum | Charophyta |
| Class | Conjugatophyceae |
| Type | Single-celled green alga |
| Diet | Makes food by photosynthesis |
| Habitat | Fresh water: ponds, lakes, bogs and acidic marshes |
| Special feature | Two mirror-image halves joined by a thin bridge |
What Are Desmids?
Desmids belong to a group of green algae called the charophytes, which are the closest algal relatives of land plants. The true desmids make up the order Desmidiales, and scientists have described thousands of species in around 40 genera.
Desmids are often split into two kinds based on their cell walls:
- Placoderm desmids, the “true” desmids, have a cell wall made of two halves, often with a narrow waist in the middle. Micrasterias, which looks like a lacy green star, is a famous example.
- Saccoderm desmids have a simpler, smooth, one-piece cell wall. Scientists today place many of them in a separate, closely related group.
Habitat
Desmids live almost entirely in fresh water. They can be found in lakes, ponds, ditches and slow rivers, floating in the water as plankton, living on the bottom, or clinging to underwater plants.
They are especially common in clean, slightly acidic water that is low in nutrients. Peat bogs, moorland pools and wet patches of sphagnum moss are some of the richest places to find them. A single drop of bog water can contain dozens of different kinds.
Most desmids are single cells, but a few species join together in long chains or threads.
Desmids are rarely common in polluted water. When water is full of nutrients from fertilizers or sewage, other algae grow quickly and crowd them out. Because of this, scientists sometimes use desmids as a sign of clean, healthy water. In general, the more varied and undisturbed a wetland is, the more kinds of desmids it holds.
Anatomy and Locomotion
Photo by: Derek Keats
In a typical desmid, the cell is divided into two matching halves called semicells, joined by a narrow bridge called the isthmus. The cell’s single nucleus sits right in the middle, in the isthmus.
Each semicell contains a large, often folded or ridged chloroplast, where photosynthesis happens. Inside the chloroplast are one or more small round bodies called pyrenoids, which help the cell build starch to store energy.
Desmids come in an amazing range of shapes: flat and star-like, long and crescent-shaped, round, or covered in spines. Scientists use these shapes to tell the species apart. Most are microscopic, though the very largest can just about be seen as tiny green specks with the naked eye.
Some crescent-shaped desmids, such as Closterium, have a small bubble at each end containing tiny crystals of barium sulfate. Under a microscope you can see the crystals jiggling about constantly, a kind of random movement called Brownian motion.
Desmids have no flagella, so they cannot swim. Instead, many can move slowly by squeezing out a slimy, jelly-like mucilage through tiny pores in their cell walls. This lets them glide along surfaces and turn toward the light. The mucilage can also form a protective coat around the cell.
Reproduction
Most of the time, desmids reproduce simply by splitting in two. The cell divides at the isthmus, and each half keeps one of the old semicells. It then grows a brand-new matching half, so for a while the young desmid has one old half and one smaller new half.
Desmids can also reproduce sexually, by a process called conjugation. Like other members of their group, they have no swimming sperm. Instead, two cells line up next to each other, often inside a shared coat of mucilage. Their cell walls open and the contents of the two cells come together and fuse.
This forms a single cell called a zygote, or zygospore, which grows a thick, tough wall. The zygospore can survive hard times, such as drying out or freezing. When conditions improve, it divides by a special kind of cell division called meiosis to produce new desmids.
For most of their life, desmid cells carry just one set of chromosomes. The zygote is the only stage with two sets.
Desmid FAQs
What are desmids?
Desmids are a group of mostly single-celled green algae. Their cells often have two matching halves joined by a thin bridge, and they come in star, long and round shapes.
Where do desmids live?
Desmids mostly live in fresh water such as rivers, ponds and lakes. They are especially common in bogs and in marshes among sphagnum moss.
Are desmids plants?
They are green algae, and they are close relatives of land plants. Each half of the cell has a big chloroplast for photosynthesis.
Can you see desmids without a microscope?
Mostly no, but the biggest ones can be seen with the naked eye.
How do desmids reproduce?
Usually a desmid splits at the thin bridge to make two new cells. Two cells can also join together in a process called conjugation.
Randi Jones is the owner and founder of the Nature Gift Store, an online retailer specializing in nature-themed gifts, educational science projects, live animal nature kits and real butterfly artwork. A biology graduate who fell in love with insects, she founded the company in 2001 to make active learning about nature fun, accessible and engaging for children and adults. Her store offers real butterfly frames, home and garden decor and educational live animal kits, including ant farms, worm farms, tadpoles, ladybugs and hermit crabs, and she runs a bug museum in Washington featuring live animals like Eddie the Iguana alongside other reptiles and insects.
