Microalgae are small because small cells grow fast. A Chlorella cell is only 2 to 10 micrometres across, and a Spirulina filament is about 6 to 12 micrometres wide. At that size, a cell has a lot of surface area compared with its volume, which lets it take in light, carbon dioxide and nutrients very efficiently. Size also decides what can eat the algae, how they stay suspended in water, and how you collect them.
How small is small?
| Thing | Approximate size |
|---|---|
| Human hair (width) | 50 to 100 µm |
| Grain of fine sand | 100 to 500 µm |
| Rotifer | 100 to 400 µm |
| Spirulina filament (length) | Up to a few hundred µm |
| Chlorella cell | 2 to 10 µm |
| Bacterium | 0.5 to 5 µm |
One micrometre is a thousandth of a millimetre. A drop of healthy Chlorella culture can hold millions of cells, which is why it looks like a smooth green fluid rather than a collection of particles.
The surface area advantage
As a cell gets smaller, its surface area shrinks more slowly than its volume. A small cell therefore has more membrane per unit of contents, so nutrients and gases pass in and out quickly and every part of the cell is near the light. This is why microalgae can double their number in a day or less, while a large seaweed takes weeks to add a visible amount of tissue.
Why size matters when feeding animals
Most animals eat only what fits their mouth. Rotifers and Daphnia are filter feeders that capture particles in a range of a few micrometres, which is just the size of Chlorella. Fish larvae are eating at a later stage, usually at 100 to 300 micrometres, so they need that intermediate step. Microalgae feed the small animals that feed the larvae. See feeding rotifers with Chlorella for the practical side.
Size also matters to the diet. A cell that is too large, or has a thick wall, can pass through a small animal without being digested. This is the reason Chlorella is a good feed for filter feeders but its wall limits direct use for other purposes, explained in algae cell walls.
Why they stay suspended and why they eventually settle
Very small particles sink extremely slowly, and tiny movements of water keep them in suspension. Gentle stirring or aeration keeps a culture evenly mixed. If you stop moving the water, cells settle to the bottom over hours or days, which is a simple way to concentrate them.
How size affects harvesting
- Spirulina forms spiral filaments large enough to be caught by a fine cloth or mesh of about 30 to 50 micrometres. It is easy to harvest at home.
- Chlorella cells pass through ordinary cloth. Harvesting needs settling, flocculation, a centrifuge or a very fine filter.
The harvest methods are compared in how to harvest algae from a healthy culture.
Seeing them for yourself
A basic microscope at 400x is enough to see Chlorella as clear green dots and Spirulina as a coiled thread. Seeing microalgae under a microscope explains the setup, and the Spirulina vs Chlorella comparison shows how the shapes differ.
Size and speed: the scaling logic
If you take a sphere and double its diameter, its surface area increases four times but its volume increases eight times. Put differently, the ratio of surface to volume halves. A cell must bring in everything it needs through its surface and supply everything inside its volume, so a larger cell is slower at feeding itself relative to its size. A Chlorella cell of 5 micrometres has about 1,000 times more surface per unit of volume than a 5 millimetre-wide seaweed frond, which is why a seaweed has to be built from many flat sheets and thin layers to catch light.
This scaling is the root of why microalgae are fast. The same logic explains why they must be small to float: in still water, a cell sinks at a rate that depends on the square of its radius, so a 5 micrometre cell sinks hundreds of times slower than a 100 micrometre particle of the same density.
How algae cope with being small
- They make themselves less dense. Some cyanobacteria carry gas vesicles to float to the light. Others store oils, which are lighter than water.
- They are motile. Many flagellates swim, and Spirulina glides slowly to find better light.
- They form chains or colonies. Chains such as Spirulina’s trichomes are harder for grazers to swallow, which is a defence worth the extra size.
The food chain depends on size
In aquatic food webs, each step is typically 10 to 100 times bigger than the one below. Algae of a few micrometres feed ciliates and rotifers of 50 to 300 micrometres; those feed copepods and larvae of 1 to 5 millimetres; and those feed small fish. Hatcheries mimic this stepwise ladder: algae to rotifers to larvae. A step that is too big or small breaks the chain, which is why mouth size appears in every hatchery protocol. See feeding rotifers with Chlorella and choosing microalgae for live feed.
Measuring tiny things at home
You can estimate cell size without special equipment. Use a microscope slide with a printed millimetre scale, or the diameter of the field of view: at 400x a typical school microscope shows a field of roughly 0.4 to 0.5 millimetres across. If a Chlorella cell fills about one hundredth of the field, it is around 4 to 5 micrometres. Photograph the slide and compare. For the set-up, see seeing microalgae under a microscope.
Why it matters for you
Whenever you choose a culture, a filter, a feed or a harvest method, the size of the cell is the number to ask about. It decides whether your net catches the culture, whether your rotifers can eat it and whether your pump will clog. It is a small number that settles many practical questions.
Common questions
Can you see microalgae without a microscope?
Not individual cells. You see them collectively as green or blue-green colour in water.
Are bigger algae better?
Not for growth. For some uses, such as harvesting, bigger filaments are easier. For feeding small animals, smaller cells are better.
Why does a Chlorella culture look like paint?
Because millions of cells are suspended and evenly distributed, so the liquid appears uniform and opaque.
What is the smallest alga?
Some marine picoplankton are under 2 micrometres across. Common culture species are a little larger.
Why do bigger algae exist at all?
Larger algae, such as seaweeds, trade speed for structure and shelter from grazers, and they can reach deeper nutrients and strong light near surfaces.
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Chlorophyll in Algae: How Microalgae Capture Light
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