A batch culture of algae passes through five phases: lag, exponential growth, linear growth, stationary, and decline. Knowing which phase your jar is in tells you when to wait, when to harvest and when to split. The best time for both harvesting and starting a new culture is usually late exponential growth, when cells are numerous but still young and healthy.
The five phases
| Phase | What happens | What you see |
|---|---|---|
| 1. Lag | Cells adjust to new medium, light and temperature | Little visible change for hours to days |
| 2. Exponential | Cells divide at a constant rate, doubling regularly | The colour deepens noticeably each day |
| 3. Linear (slowing) | Light and nutrients start to limit; self-shading | Still thickening, but more slowly |
| 4. Stationary | Growth equals loss; density plateaus | Dense, steady colour, sometimes paling |
| 5. Decline | Nutrients run out; cells die | Colour fades or browns, smell turns |
How long each phase lasts
It depends on species, temperature, light and how much starter you used. A Chlorella culture inoculated at 10 to 20% might show a short lag of a day, a week of exponential growth, and reach a plateau within two to three weeks. Spirulina is slower, with doublings taking one to three days and a total of several weeks. Your own log gives the best numbers for your setup.
Why the lag happens
A lag is normal after a transfer. Cells need to adjust, especially if the new jar differs in light, temperature or medium. A longer lag means a bigger shock, or a culture that was thin or old. Starting with a dense, healthy inoculum and matching conditions keeps it short; see how to split and scale up.
Why growth slows
As cells increase, they shade each other, and the light that reaches the middle of the jar drops sharply. Growth shifts from exponential to linear because light, not cell division, is the limit. Later, nutrients or carbon become scarce, and a culture reaches stationary phase. Stirring, a stronger lamp, or adding nutrients can extend growth, but only to a point.
What to do in each phase
- Lag: be patient, do not add more nutrients, and check temperature.
- Exponential: a good time to monitor daily and prepare the next jar.
- Late exponential / early linear: split or harvest. Cells are healthy, and the culture is dense.
- Stationary: harvest or dilute into fresh medium. It will hold for a while, but quality begins to drop.
- Decline: do not feed it to animals; use a reserve or restart. See how to tell if your algae culture is healthy.
Semi-continuous cultures
If you harvest a portion every day or two and replace it with fresh medium, you hold the culture in late exponential growth indefinitely. That is the basis of production systems described in small-scale algae production.
Plotting your own growth curve
Drawing a growth curve for your jar takes five minutes a day and teaches you more than any article. You need only a clear container, a white card and a camera or a colour score.
- Inoculate a fresh jar with a measured amount of starter and note the date.
- Each day at the same time, photograph the jar in front of the white card, with the same light.
- Score the colour from 1 (pale) to 10 (very dark) by comparing the photographs side by side.
- Plot the score against the day number on graph paper or in a spreadsheet.
On a simple chart, a healthy batch makes an S shape: a flat start, a steep climb, a slow bend and a plateau. If you log the temperature as well, you will see how the climb steepens on warm days.
Mathematics of doubling, without the maths
Exponential growth looks slow and then suddenly fast. A culture that doubles every day and starts at 1 unit passes through 2, 4, 8, 16, 32, 64 and 128 in a week. The last three days make up most of the final amount. That is why a jar can look nearly unchanged for several days and then be dark within a few. It also explains why you should not give up on a thin culture too early, and why splitting at the right moment gives you more than waiting a week.
| Day | Relative density if it doubles daily |
|---|---|
| 0 | 1 |
| 2 | 4 |
| 4 | 16 |
| 6 | 64 |
| 8 | 256 (in practice, light limits it well before this) |
What limits the plateau
- Light. Cells in the middle of the jar are shaded; stronger lamps or thinner containers raise the limit.
- Carbon. Air-fed cultures run short of CO2; see how microalgae take up carbon dioxide.
- Nutrients. Nitrogen often runs out first; nutrients and culture media explain how to restore it.
- Self-produced inhibitors. Dense cultures accumulate waste products and a high pH that slow growth.
Keeping a culture in the sweet spot
The most efficient way to get algae is not to let a jar run to its plateau, but to harvest regularly and refill, so the culture lives in the steep, fast part of the curve. This semi-continuous approach produces more biomass per week than a batch that is left until it is dense. It also keeps cells young and healthy, which matters when feeding animals. See how to harvest algae from a healthy culture and how to split and scale up.
Common questions
Why has nothing happened for three days?
It is probably lag, which can be several days for a thin or cold culture. Check warmth and light.
When is the best time to harvest?
Late exponential or early stationary, when density is high and cells are young.
Can I stop a culture reaching decline?
Yes, by harvesting and replacing medium regularly, or by splitting it.
Is a longer lag phase a bad sign?
Not by itself. A cold or thin start has a long lag. It is only a warning if it lasts more than a week with good conditions.
Do all species follow the same curve?
The shape is the same; the speed differs. Chlorella moves through the phases faster than Spirulina.
Ready to grow your own?
Live starter cultures, grown with care and shipped ready to thrive.
Spirulina PlatensisChlorella VulgarisVisit the shopGet new guides by email, and 10% off
Subscribe for 10% off your first order, plus our latest articles by email. No spam, unsubscribe any time.
Keep reading
What Is Phycocyanin? The Blue Pigment in Spirulina
Chlorophyll in Algae: How Microalgae Capture Light
Leave a Reply