## 3 Microalgal/Cyanobacterial Metabolites and Phytohormones with Potential Interest for Agriculture
### Phytolhormones  
Phytohormones are considered signal molecules that are responsible for the regulation of several cellular processes in plants. Typically, these molecules are produced in small amounts and their presence has been firstly identified in higher plants [11]. However, the booming in algal research in the last decades, especially microalgal genomic studies, has demonstrated that these signalling molecules are also produced by a wide diversity of microalgae and cyanobacteria. In fact, the most popular phytohormones (i.e., auxins, abscisic acid, cytokins, ethylene and gibberellins) have already been identified in multiple algal lineages, such as brown, red and green algae, cyanobacteria and diatoms [10; 11]. Although research studies on the physiological responses promoted by microalgal phytohormones are still very scarce, the similarities found between phytohormones profiles in higher plants and algae suggest that their roles may be very similar. In higher plants, auxins, especially indole-3-acetic acid, are responsible for the regulation of growth and development [11]. In microalgae, these molecules also act as growth regulators, as auxins from _C. vulgaris_, _D. salina_, _Nannochloropsis oceanica_, _Scenedesmus obliquus_ and _Scenedesmus quadrica_ have promoted an increase in cell size and chlorophyll and protein contents, as well as cell division, which resulted in higher biomass productivities [61; 62; 63; 64; 65]. abscisic acid is commonly associated with the cellular response to environmental stress factors, such as high salinity levels, drought and nutrients scarcity [10; 11]. Typically, this molecule induces cellular mechanisms to increase the plants' tolerance to these stress conditions and avoid their harmful effects. In higher plants, it has been demonstrated that abscisic acid can improve the tolerance to temperature and salt stresses through the control of water and ions uptake [66]. In microalgae and cyanobacteria, this molecule is quite effective in the control of cells' response to salt, osmotic, oxidative, drought and nutrients' stresses [67; 68]. Common responses induced by the activation of the abscisic acid pathway include the production of carotenoids, which typically act as protecting agents against oxidative stress [62]. Regarding cytokins, these molecules mediate several processes in higher plants, namely cell division, growth, biogenesis, chloroplasts differentiation, regulation of seed dormancy, among others [10]. Therefore, the role of these hormones in plants' growth and development is crucial. Besides these activities, cytokins also improve the plants' tolerance to adverse environmental conditions [10; 11]. In microalgae and cyanobacteria, the role of these hormones is very similar, as they also stimulate cell division and photosynthetic activity and protect cells from environmental stress conditions [69]. For example, cytokins from _Acutodesmus obliquus_, _C. vulgaris_ and _Gracilaria caudata_ have been associated with increased pigments' contents in these organisms, which resulted in higher photosynthetic rates and, hence, higher biomass productivities [70; 71; 72]. Additionally, a cytokin produced by _D. salina_, kinetin, induced the accumulation of the photo-protective pigment beta-carotene in this microalga [62]. Ethylene is known for its multiple responses in plants, which range from the regulation of plants' growth and ageing, to the improvement of plants' tolerance to both biotic and abiotic stress factors [10]. In microalgae and cyanobacteria, the role of ethylene is not well documented. However, some studies have evidenced that this molecule may act as a growth stimulation agent in _Chlamydomonas reinhardtii_, _Chlorella_ sp. and _Spirogyna platensis_[73; 74; 75]. Gibberellins from plants and microalgae/cyanobacteria have similar functions, which are mainly related with growth stimulation and induction of pigments and proteins accumulation [10; 11]. In the case of microalgae and cyanobacteria, gibberellins from _C. ellipsoidea_, _Chlorella pyrenodiosa_, _C.