When to Harvest When Growing Spirulina at home?
When growing spirulina it’s important to keep a fairly concentrated population in your tank and not harvest too much of it, because spirulina is much more fragile when the biomass is low (the bio-mass, is the total mass of what’s living (bio); here the spirulina) and it could be damage by the sun and lyse.
This is why, after each harvest (or after each dilution if you are increasing the culture medium), you have to measure the biomass to make sure you keep it to the recommended 0.5g/l.
There are many different ways to measure the concentration of spirulina in you medium: from ultra-sophisticated with complete digital and automated equipment, to the simplest one, using chop stick and a white plastic disk… and they all work! The use of optical density (OD) is the most usual and widespread method in laboratories but also at home for the estimation of the biomass.
What does it mean when growing spirulina at home?
Well, basically, it means that you can assess the concentration of the spirulina by looking at the color of your aquarium. Pretty cool isn’t it?
At home, the most basic, but actually fairly accurate, tool to measure the biomass is a with Secchi disk (invented in 1865 by Angelo Secchi – check it out in Wikipedia). This Secchi disk is a super easy tool to make and it’s absolutely necessary for your culture. All you need is a straight ruler with a small white disk attached at one end. You can use a chopstick with 1 cm graduations as ruler (use a knife to mark the graduations on the chopstick) and cut a disk (diameter: ~2 inch / ~5 cm) in some plastic material (a yogurt container will do, a gallon of milk won’t if you can see through it). Make a small hole in the middle of your white plastic disk and insert the ruler.
All you have to do now is plunge the Secchi disk in the culture medium to the point where the disk is not visible anymore. Now read the depth (from the graduation on the ruler) at which the Secchi disk is not visible any more. And TA DA! There you have it, the number you have is directly related to the spirulina in your tank.
The biomass (or concentration) of spirulina is roughly linear for the depths between 1.5 and 3.5 cm.
Typically, a Secchi disk at about 2 cm equals a concentration of ~ 0.5 g/l, and your culture is ready for production. If the Secchi disk remains visible beyond 5 cm (for a concentration of ~0.3 g/l), your culture is very diluted and you should let it grow before doing anything else. Don’t forget that a diluted culture is very fragile. No full sun exposure, no violent chocks!
When growing spirulina at home in a commercial farm, you really should try to not let the biomass drop under that threshold of 0.3 g/l. On the other side of the ladder, the concentration may go up to 10 g/l. (N.B.: the weight of spirulina always refer to contained dry matter).
You also have more complex way to read the spirulina density in your tank at home for the geeks (like me :0) ). you can use an electronic density reader.
One of our readers – Tim, growing spirulina in the Netherlands – sent us some pictures of the density reader he made at home. So you’re in for some DIY-digital-spirulina-density-meter-building! What a treat, thanks Tim!
Here is step-by-step guide to make the same at home:
1. The first thing he did was to connect 2 LED lights to the base of a small canister. The canister Tim used was transparent, which would void the readings; this is why he covered the canister with black duct tape. Duck tape was also used to connect the canister to the LED lights.
2. The LED lights are connected to a small transformer to assure reliability and constancy of the readings over time.
3. A standard digital Luxmeter is used to measure the amount of light that goes through the solution from the base up to the top of the canister.
4. The apparatus is then calibrated with clear water – in Tim’s case: 300 Lux.
5. Precisely 20ml of culture medium is added to the canister.
6. You can now read the density on your Luxmeter!
Which method do you prefer to use? The Secchi disk or the digital reader and why??