Can algae fulfill their reputation as the green gold? That’s a question a lot of researchers have aimed to answer in the last years. Algae form a diverse group of photosynthetic organisms which accumulate different useful substances. These components are used for a wide range of application such as pharmaceuticals, food supplements, animal feed, chemicals and energy. The “green gold” reputation originates from their ability to accumulate large amounts of lipids which can be converted to biofuels. As they can also reach relatively high growth rates, they have the potential to provide an alternative to the traditional fossil fuels. However, this is not a new story as algae received already a large amount of research attention in the past decades. So what happened to the hype? Can algae live up to their promise? Or has the green gold lost his glitter?
The original algae hype almost exclusively focused on the production of biofuels. With the fossil-fuel-related problems such as global warming and security of supply, the search for an alternative and more sustainable fuel was pressing. The first biofuels on the market were made from food crops, such as corn or wheat. However, the production of these biofuels induced a competition with the production of food and caused an increase in food prices. As this violates the third pillar of sustainability, the social dimension, the sustainability of these biofuels is still highly controversial. The second generation biofuels circumvented this problem by introducing new feedstocks for biofuels, such as lignocellulosic biomass or agricultural residues. However, this generation still faces technical difficulties. Algae, which are often considered as the third generation of biofuels, can reach higher lipid productivity rates than the other biofuel crops and have therefore a huge potential as a biofuel.
Besides the social dimension, also the two other pillars of sustainability need to be taken into account: the environmental and the economic dimension. Algae require CO2 for their growth and have therefore the potential to act as carbon capturing organisms, not only avoiding CO2 emissions by replacing fossil fuels, but also actively decreasing the CO2 reserves in the atmosphere. Moreover, they can be grown on degraded lands and do therefore not compete for land with food crops. However, all that glitter is not gold. Although algae have the potential to provide a sustainable alternative for fossil fuels, the current technologies aren’t there yet. Algae are cultivated in water and cannot reach very high concentrations. Accordingly, a large amount of energy is needed to mix this water during cultivation and to harvest and dry the algae afterwards. The production of this energy has a carbon footprint which should be taken into account as well. Moreover, the burning phase of biofuels does also include CO2 emissions, which makes a negative carbon footprint unlikely. The CO2 concentrations in the atmosphere are also not sufficient to achieve high algae growth rates. Therefore, additional CO2 needs to be supplied. Researchers have suggested the use of industrial flue gas, however, this put a limit on the possible scale of algae production as industrial flue gas is not supplied indefinitely. Another bottleneck may be the use of fertilizers as nutrients for algae cultivation. The production process of these fertilizers is often not that environmentally friendly. An alternative source of nutrients which is under research is the use of wastewater. Thus, although the use of microalgae as a feedstock for biofuels is not per definition environmentally-friendly, there is still a lot of potential to validate their “green” reputation.
A more tricky point in the “green gold” hype is the gold. How economically viable is the production of microalgae for biofuels? To compete with fossil fuels, the price of algae-based biofuels should be in the same range. However, this is currently not yet achievable. The main bottlenecks for the achievement of a cheap algae-based biofuels are the large energy costs during cultivation and harvesting.
The solution to this problem does not have to be something completely anew. If we look at the history of the human-algae relationship, the story started with the use of algae as food. Moreover, there are currently commercially viable companies, which are cultivating algae for this purpose. As their products are sold at much higher prices than biofuels, the costs of algae cultivation is compensated. Logically, it also makes much more sense to primarily commercialize high-value products before starting to develop low-value applications such as biofuels. The combination of valorizing different products out of one biomass feedstock is also known as the biorefinery concept. However, the ideal biorefinery has not yet been developed as multiple problems still exist. For example, how can you commercialize products which widely differ in the scale of their production volume such as fuels and pharmaceuticals? How can you sustain the “green” reputation of algae if you cannot use waste products such as flue gas for CO2 and wastewater for nutrients, which would be quite a challenge for food products approval?
The ideal commercial concept for algae production has thus not yet been found. However, Rome wasn’t built on a day either.