“ProFuture”: an EU-funded project on microalgal proteins for the food and feed of the future

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CULTIVATING SUSTAINABLE AND NUTRITIOUS MICROALGAE
Microalgae are the ancestors of higher plants. They are tiny, often single-celled microorganisms that can grow photoautotrophic by use of sunlight, nitrogen and CO2. They can also be grown in fresh, brackish, or marine water in a species-dependent manner. The systems used for microalgae cultivation are usually of two types: closed photobioreactors (no interaction with the environment) and open ponds in which, on the contrary, the culture is exposed to the environment. Closed photobioreactor systems can be tubular, flat‐plat, or vertical columns. In all systems the light is supplied either directly by the sun or via artificial sources such as LEDs (1).
Four major modes of microalgae cultivation can be adopted, i.e., photo-autotrophic, heterotrophic, photo-heterotrophic, and mixotrophic. In photo-autotrophy, microalgae are grown by fixing dissolved CO2 and absorbing solar energy for photosynthesis. Heterotrophic species use organic compounds in the growth medium as carbon and energy sources and do not need light as additional energy source. In photo-heterotrophy, cells use light as an energy supplier, while nitrogen and organic matter as a carbon source are fixed instead of CO2. Mixotrophic cultivation is the growth mode whereby microalgae simultaneously use inorganic CO2 and organic carbon sources (e.g. glucose, glycerol, and acetate) in the presence of light. CO2 is fixed through photosynthesis, while organic compounds are assimilated through aerobic respiration (2). Mixotrophic cultivation offers higher growth rates, produces higher quantities of biomass (3) and its production lines can be built on non-arable land avoiding competition with crop plants (4). Due to the controlled growth conditions, some microalgae species can be produced and harvested all year round. Since they avoid complex differentiation processes, as it is the case in plants that form different types of tissues, the energy wastage in microalgae is low. Additionally, they show a higher (8 – 10%) photosynthesis efficiency compared to higher plants (3%) (5). The nutritional composition of microalgae strongly depends on the phylum, growth stage and culture conditions (6). While marine microalgae are known to have high ash content (up to 25%) (7), other species contain up to 30% lipids with a high share in mono and poly unsaturated fatty acids (MUFA, PUFA, up to 80% in Chlorella vulgaris) or up to 50% carbohydrates (including fibres) (8, 9).