## 2 Microalgae as a Source of Biofertilisers, Biostimulants and Biopesticides  
Microalgae and cyanobacteria are an important source of biologically-active compounds, such as phenolic compounds, polysaccharides, hormone-like substances and proteins, known for their benefits as antioxidant agents, plant-growth promotors, among others. Moreover, living organisms, both prokaryotes (e.g., nitrogen-fixing cyanobacteria) and eukaryotes (e.g., microalgae and macroalgae/seaweeds), are broadly recognised for their role in soils' fertilisation and plant growth stimulation [3]. Besides these important characteristics, the production of microalgal/cyanobacterial biomass can be quite advantageous, when compared to the production of other biological resources [3; 4]: (i) production of these photosynthetic organisms can be performed in non-arable areas, thus not competing with the areas intended for food production; (ii) microalgae and cyanobacteria can be grown in low-quality waters, such as wastewaters, thus reducing the requirements for freshwater and nutrients (mainly nitrogen and phosphorus); (iii) the wealth composition of microalgal/cyanobacterial biomass can be fully exploited towards the induction of multiple responses in the same crop; and (iv) when growing autotrophically, microalgae and cyanobacteria uptake CO\({}_{2}\) from the atmosphere, thus reducing the carbon footprint of agricultural practices. Although several products can be used in the improvement of crops' productivities, it is important to understand that different biological compounds may improve the agricultural productivities through different modes of action: (i) soils' improvement; (ii) crops' protection against biotic and abiotic stress factors; and (iii) direct growth stimulation. Considering these roles, microalgal/cyanobacterial products and biomass can be classified as biofertilisers, biostimulants and biopesticides. Figure 1 summarises the main activities associated to these biologically-based products in the development of agricultural practices, highlighting their action mode and effect on crops' production. Biofertilisers are biologically-based compounds that promote an improvement in crops' productivities through their activity at the soil level. Typically, these compounds and/or microorganisms are responsible for the improvement of soil properties and soils' fertility, providing the essential nutrients (e.g., nitrogen, phosphorus and potassium) for plants' growth [4; 8; 12; 13]. Depending on their beneficial effects and on the microorganisms integrating these formulations, bioofertilisers can be classified as [8; 14]: (i) plant-growth-promoting bacteria; (ii) compost; (iii) nitrogen-fixators; (iv) phosphate- and potassium-solubilising biofertilisers; and (v) phosphorus- mobilising biofertilisers. The use of biofilersers in agriculture presents several advantages, such as [14; 15]: (i) increased crop productivities per unit of area and time; (ii) reduced energetic requirements; (iii) control and maintenance of adequate soil properties and fertility; (iv) lower risks of soil and water contamination; and (v) crops' protection against pathogenic organisms. Biostimulants promote crops' productivity by acting directly on the plant. These compounds are responsible for improving respiration, photosynthetic activity, nucleic acid synthesis and ion uptake, enhancing plants' metabolism and, hence, plants' growth [9]. The stimulatory activity of biostimulants can be observed under both optimal and adverse conditions, meaning that these compounds can play an important role in the improvement of plants' resistance and tolerance against stress conditions [3; 4]. The application of biostimulants in agriculture has, therefore, the following advantages [3; 4]: (i) increased crop productivities; (ii) increased nutrients' utilisation efficiencies; and (iii) enhanced crops' quality.