When do I add nutrients to my algae culture?

Hello AlgaeLab,

I just joined the Algae Lab community. I’m an engineering student (and an algae enthusiast) doing a project on algae. I’ve been working on algae for about a month now. I’m trying to optimize the growth conditions of a local algal strain at lab-scale. I’m working on the physical factors (light, temperature, CO2, pH, salinity). I can’t figure out how often should I add more nutrients in my culture. What indicators should I look for, to know its the right time to feed my algae?

Thank you in anticipation. =)

Hello and welcome to the community… this sort of question comes up a lot. It is easy to throw some nutrients into water, add algae, and get it growing (though this can take some tweaking to get it right), but eventually the algae consume most or all of one or more critical nutrients, and growth slows or stops; if left alone long enough, nutrient-starved algae will die, though some species will form long-lived cysts. This sort of growth process, where the algae are taken through a sort of life cycle, is called batch growth.

The alternative is called continuous growth; the goal is to maintain the algae at a constant density – generally the density at which they have the highest production rate – while keeping the level of every nutrient in the optimum range for maximal growth. This is the most productive way to grow algae, as they are always growing and reproducing at their fastest rate. It is a lot more work, though, as you must harvest and add nutrients continuously at a rate exactly matched to the growth rate, and watch the levels of all the nutrients. Nutrient levels are measured using colorimetric chemistry – adding chemicals that change color depending on the concentration of the substance you are interested in; this color is then measured exactly by a device called a colorimeter (most popular models are made by Hach). Probes can also be used (pH for CO2 concentration, dedicated sensors for ammonia, nitrate, etc.). But this is generally expensive and (excepts for pH) above the level of most DIY-ers.

For our kits we try to strike the best possible balance between robust growth and ease of use; as the nutrients absorbed from the medium can be calculated from the weight of the algae you harvest, we simply add nutrients back to the culture in proportion to the amount harvested. We call this mix of nutrients “Make-Up Mix”; you cannot use Starter Mix (used to mix the initial culture medium) for this purpose, as it contains a number of ingredients that create the right salinity and alkalinity, but which are not consumed by the algae. For our Spirulina make-up mix, happily this works out to a very simple ratio: one teaspoon of mix per tablespoon of harvested Spirulina. Once these nutrients are back in the culture, your Spirulina will have just the right nutrients to keep reproducing rapidly until your next harvest. If you harvest frequently, this is somewhat like continuous production; if you harvest in frequently, it is more like batch growth.

Hope that is helpful.

From the heart of the chloroplast,
Dr. Baum

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