Increasing Accessibility of Spirulina by Growing in Seawater Medium

Klara Zietlow
7 min readDec 16, 2021


Healthy, nutrition packed spirulina I Source

Spirulina microalgae are packed with health benefits and full of nutrients, yet few people know about or consume them regularly. One of its biggest barriers to adoption is its high cost compared to other food sources and out of its cost, up to 25% is taken up by the price of growth medium.

The most common growth medium is called the Zarrouk medium and is made with a list of ingredients dissolved in freshwater. Not only does the amount of nutrients added restrict who has access to grow spirulina, but the fact that freshwater is required in itself is already a limiting factor.

Freshwater is used for so many other vital purposes that especially in an environment where resources are scarce it would be greatly beneficial to have a food source not reliant on unstable freshwater availability.

Seawater, on the other hand, is much easier to access and as a result also cheaper. Despite representing 97.5% of all water on the planet, it isn’t used in agriculture because of its high salinity. Microalgae has the amazing ability to tolerate and even thrive in seawater.

If you’ve ever heard of plankton being in the ocean, spirulina are a type of phytoplankton I Source

So what exactly happens when you put spirulina in seawater?

There’s a lot that changes. It would be like drinking ocean water instead of tap water every single day. You’d expect your body to respond in some weird ways!

Biochemical Changes

One of the most interesting side effects of culturing spirulina in seawater is that the carbohydrate content of it skyrockets while the protein content drops significantly.

This can be explained by the microalgae redirecting their metabolism towards producing and accumulating low molecular weight carbohydrates such as sucrose and trehalose. These carbs are made because they’re osmoregulating, which helps them acclimate to their saline environment and high concentrations of Na+ and Cl- ions.

One study observed that spirulina grown in seawater with no added nutrients had 203.5% more carbs than spirulina cultured with Zarrouk medium.

Because so much of their energy is going towards making carbs, cultivating spirulina in seawater will leave you with algae lower in protein than if they were cultivated with Zarrouk medium and as one study found, lower in lipids as well.

The same effects happen when nitrogen and phosphorus are reduced as well, which occurs if fewer nutrients are added to the seawater than would be present with Zarrouk.

Why does this matter?

So yes, if you want to eat this spirulina on a keto diet, that may not be great ;) but there are many purposes where having a high carbohydrate content can be beneficial. For example, biofuels and bioplastics.

Plus, don’t let all the anti-carb marketing get to you–carbohydrates are an important source of fuel for our bodies to run on. In the end, less protein and fat doesn’t look too bad when you look at all the benefits of culturing in seawater.

Growing spirulina in seawater without any added nutrients grew 52% faster than the control culture with freshwater and Zarrouk. When grown in a seawater medium containing 2 parts seawater to 1 part freshwater, it showed a lower doubling time than the control group (2.7 days compared to 3.0 days).

Not to mention, the cost of growing spirulina is reduced significantly with seawater and opens up more places for spirulina to be grown — places without access to our ever-diminishing amounts of freshwater. If you don’t need to pay for water, and only need to use a fraction of the nutrients, if any, you can imagine the costs would plummet.

Spirulina growing in the New Mexico desert I Source

Plus, when you grow spirulina in seawater, it allows you to make more money selling the valuable pigments found in spirulina. And these pigments are expensive. Lutein, a yellow pigment that’s also taken as a supplement, is increased significantly when grown in diluted seawater.

The presence of other pigments found naturally in spirulina, including phycocyanin, beta carotene, and chlorophyll a, is slightly reduced but the cheaper cost of production and high profit margin make up for it.

The cost of food-grade phycocyanin can be as high as $355/kg and beta carotene costs over $500/kg!

Nutrients Still Needed

On its own, spirulina already contains trace compounds of carbon, lithium, barium, manganese, iron, zinc, aluminum, copper, and selenium plus dissolved gasses nitrogen (N), oxygen (O2), and carbon dioxide (CO2).

Even so, it does benefit the growth of spirulina if small amounts of nitrogen (NaNO3), phosphorus (K2HPO4), iron (FeCl3.6H2O), and disodium EDTA are added. This can be just 25% of the normal amount used in Zarrouk medium and it will still greatly benefit the health of the microalgae.

Sterilizing the seawater before adding the culture is good if the spirulina grown is to be used as human food.

Since calcium and magnesium ions present in seawater will precipitate with phosphate and carbonate in the nutrients added forming little pieces of sediment, sodium bicarbonate can be added before to intentionally precipitate it and then remove it to prevent this from happening while the algae is growing in it.

Having a turbid, sediment-filled medium can inhibit light absorption and therefore photosynthesis.

Nitrogen Source

Nitrogen is one of the most important nutrients to add to the medium and although NaNO3 is usually used, urea could be a great alternative.

Urea is colorless when dissolved, tasteless, and odorless I Source

Not only is urea half the cost, you also only need half the amount of it because each molecule has two nitrogen atoms compared to one. It’s so powerful that if more than 2g/L of it is used, nothing will grow because it decomposes into ammonia, which is toxic to the photosynthesis and growth of many algae. A good amount is 0.2g/L.

Adding Digestate

Anaerobic digestion is a method that valorizes organic matter and produces renewable energy at the same time. The byproducts of it are still rich in valuable macro-and micronutrients making it important to find good uses for them.

This is what dried digestate looks like. It can be blended up into a powder for easier distribution I Source

Turns out, if you put 5% digestate in artificial seawater, the little microalgae like it a lot. The digestate provides nitrogen in a form easily accessible to the cells but in low enough quantities that it’s not toxic. Any lower and it doesn’t have any effect, but any higher and there’s a lot of photon flux reduction–meaning light penetration inhibition.

Rate of growth between spirulina cultured in different mediums (Zarrouk best, 5% digestate second) I Source

In terms of getting light through to the algae, since putting digestate in your medium is like putting little grains of sand in it, there’s lots of sediment like what I said would happen if you didn’t treat your seawater with sodium bicarbonate before. But, there’s another side to the argument I didn’t mention earlier: turbidity protects against photoinhibition.

When there’s too much NaCl in seawater, it can increase the susceptibility of the osmotically stressed cells to photoinhibition so reducing the amount of light able to get to the cells may actually help to protect them. Of course, there’s a fine line between protection and total blocking so that’s why 5% is a nice number for this.

Surprisingly, unlike regular seawater, the protein content of spirulina grown in 15% digestate is higher than when grown in Zarrouk. The carbohydrate content though remains higher. With 15% digestate, the amount of phycocyanin is also higher than in the Zarrouk control.

Variations between the biochemical profile of algae fed different media I Source

I hypothesize that this is due to the increased levels of nitrogen, which may be better to just provide via urea because it doesn’t come with the added turbidity.


Whether due to its low cost or wide availability, seawater is a highly promising growth medium to grow spirulina in. Studies that changed mediums’ nutrient content ratios also showed the important concept that by changing the inputs different biochemical compositions of the algae could be achieved without the need for genetic modification or selective breeding.

Spirulina for all ❇ I Source


  • Using seawater as a culture medium for spirulina is a cheap and effective way of making the microalgae more accessible
  • There are some biochemical changes that occur in the algae such as increased carbohydrate content and decreased protein + lipid content along with altered pigment levels
  • The performance of spirulina in seawater can be boosted by adding additional nutrients such as urea

Thanks for reading! If you’d like to learn more about me, you can visit my personal website or LinkedIn. Make sure to follow me here on Medium too for updates on articles I publish.

I’ve written a four-part series on everything you need to know about spirulina, so if you’d like to read that, check out article one!



Klara Zietlow

Passionate about the future of food and the environment. Likes animals too :)