For a few decades, microalgae have been of special interest to researchers all over the world due to their content in valuable nutrients and the promise of a sustainable alternative over conventional sources of proteins, such as soy crops and meat.

Some, such as Spirulina, have even been nicknamed as “super foods” for being rich sources of vitamins, minerals and anti-oxidants, known to help promoting health.

But what happens before the colourful powders reach the supermarket’s shelves or are turn into other food and feed ingredients? In fact, where does it all start, if we are talking about microscopic organisms?

In this article, we’ll take a sneak peek of the production of Tetraselmis at the facilities of one of ProFuture’s partners: Necton.

Tetraselmis production

Tetraselmis chui is one of the many species Necton cultivates under the Portuguese sun; and one of the few - alongside Spirulina, Chlorella and Odontella aurita - legally allowed to be used in food production in Europe.

Before we delve into microalgae production, it’s important to note that, depending on the species, microalgae can grow under different conditions (read more here).

For example, while Spirulina and Chlorella can only grow in freshwater, Tetraselmis requires saltwater. This gives it a sustainable advantage, as it eliminates the need for freshwater use. More so, Tetraselmis can grow relatively faster - compared to other species - thus allowing to continuously scale up the production process which is of great interest for the food industry.

So, what are the different stages underpinning microalgae production?

Microscopic view of Tetraselmis chui - © Inês Maia

Stage 1: Production of clean cultures in the lab

The first step in the production of microalgae, is also one of the most important since it defines the success of the following stages. In contrast to plant crops, the first steps to produce a new generation of microalgae do not take place in the field but in the laboratory.

In the lab, producers select species they want to cultivate, making sure that all hygienic measures are taken to protect the microalgae cultivation from contamination. Otherwise, the new established algae cultures could be invaded by other strains or organisms that could destroy it or make it inappropriate to be used in food and animal feed.

In this first step, cells of the microalgae species are cultivated in small glass balloons of 1 or 6 litres each. These balloons contain a mixture of water and nutrients - the so called “growth medium” - which provides the cells with the optimal conditions for them to grow and reproduce. At the same time, air (containing CO2) is constantly being injected into the cultures to mix them and provide carbon to the cells. The microalgae use the carbon from the air, together with the other atoms and small molecules present in the growth medium, to create their building blocks: nutrients such as carbohydrates, lipids and proteins.

For all this to happen, microalgae also need light. The balloons are illuminated with natural light or LEDs so that the cells can do the photosynthesis (the process by which they form new molecules and grow). When the cells reach the maximum density for the water volume – or in other words, when they can no longer get enough light to continue growing – they are diluted to more balloons.

This first stage of cultivation usually lasts a couple of weeks for Tetraselmis.

Cultures of different microalgae species - ©Benjamin Schmid

Stage 2: The “Green Wall”

When there’s enough microalgae cells to populate bigger volumes of water, it’s time to transfer them from the inside production in the lab to an outside facility. The microalgae are then put in 80-100 litters bags, also called “flat panel green walls” which are structures made of thin plastic bags up to one metre tall held by a metal grid.

There, the microalgae are directly exposed to sunlight and are mixed with a constant supply of bubbling air. This helps the water circulate and avoids that the microalgae settle or accumulate at the surface, which would block the sunlight and prevent other microalgae cells to do the photosynthesis and grow. At the same time, CO2 is injected into the culture to regulate the pH and provide carbon to the microalgae.

When the culture is dense enough, the whole volume is transferred to a bigger Green Wall (up to 500-1000 litters of capacity). This process is repeated until enough volume is reached for the next step: the transfer to a tubular photobioreactor.

“Green Wall” at Necton’s facility – © Benjamin Schmid

Stage 3: Tubular photobioreactor

When the microalgae reach a high concentration in the “green wall”, they are moved to one final structure – the tubular photobioreactor – which at Necton can take up to 19.000 litres of microalgae solution. With the help of a pump, the microalgae are moved around the tubes of the system in a circular cycle, allowing them to continuously (and evenly) receive sunlight and nutrients.

Tubular photobioreactors are more productive than Green Walls, allowing the cultures to reach higher concentrations of microalgae cells. This is possible because the tubes are thinner than the Green Walls, and have a larger surface exposed to the sun, both factors allowing the microalgae cells to get more light in the tubular photobioreactors. Because of this, the microalgae quickly reach high concentrations in the tubular photobioreactor And once the optimal concentration is reached, the culture is harvested.

Tubular reactor at Necton - ©Benjamin Schmid

Stage 4: Collecting & Harvesting

When microalgae reach a high enough concentration (which depends on the species and season), the collection process can start. At Necton’s tubular photobioreactors, this usually happens when Tetraselmis cultures reach 1-2 grams of microalgae per litre of water.

Various methods can be used to harvest microalgae, or in other words, to collect the microalgae biomass from the water in the tubular reactor. Necton uses a so called “semi-continuous method”, where between 10-30% of the full-grown microalgae culture is collected and the rest is diluted with new water. This approach helps the recycling of the cultures and enables a continuous biomass production. Usually, it takes only 1-3 days for the diluted culture to reach the optimal concentration and be harvested again.

Stage 5: Separation from water & drying

The harvested microalgae culture needs then to be “dewatered”, which means removing the highest possible amount of water. This can be achieved with membrane filtration systems and/or by centrifugation.

After centrifugation, the microalgae biomass reaches a very high concentration but still contains a lot of water - up to 70-80% of the total volume. At this stage, the biomass is called microalgae “paste” because of its highly viscous and dense texture. Depending on the desired product, microalgae can then be processed differently. For example, the microalgae paste can be stored and sold as a frozen product, keeping most of the nutritional value of the microalgae.

Alternatively, it can be sold as a dried product which is easier to transport and store thanks to its lower volume and weight. In this case, the water in the paste needs to be removed to levels below 5%, which is the minimum value that protects the microalgae from spoiling.

The drying process needs to be fast but should not use high temperatures, or else many thermosensitive nutrients (nutrients that can be destroyed or inactivated by heat, such as vitamins and lipids) are lost.

There are many ways to dry microalgae, being the most frequent used techniques include “freeze drying” (or lyophilization) and “spray drying”. Freeze drying, is a drying technique that helps to keep many of the thermosensitive nutrients in microalgae, meaning that it protects the nutritional value of the final product.

In turn, spray drying is a more affordable technique, which allows to dry larger amounts of microalgae, but at the expense of some nutrients being lost. The ProFuture project will test several innovative ways to dry microalgae, being one of those the solar dryer. You can read more about this drying process here.