## 3 Microalgal/Cyanobacterial Metabolites and Phytohormones with Potential Interest for Agriculture
### Phenolic Compounds  
Phenolic compounds correspond to aromatic hydrocarbons bonded to a hydroxyl group. These compounds can be broadly categorised into phenols and polyphenols, with phenols being sub-divided into phenolic acids, flavonoids and phenylpropanoids [2; 7; 19; 21]. In microalgae, phenols are considered secondary metabolites and their production and release is determined by several physical, chemical and environmental conditions. Phenolic compounds present in microalgae and cyanobacteria are associated with their antioxidant properties [18; 55], playing an important role in the growth, reproduction and protection against several stress conditions [55]. [PERSON], et al. [18] assessed the antioxidant activity of 32 microalgal strains and compared the obtained results with total phenolics and carotenoids contents, concluding that, after carotenoids, phenolic compounds had a significant contribute to the antioxidant capacity of the studied microalgae. Among the studied microalgal species, _Tetraselmis suecia_, _Botryococcus braunii_, _Necochloris oleoabundans_, _Isochrysis_ sp., _Chlorella vulgaris_ and _Phaeodactylum tricornutum_ presented the highest antioxidant capacities, and the highest phenolic contents (between 1.70 and 4.57 mg g\({}^{-1}\) DW, dry weight basis). More recently, [PERSON], et al. [17] evaluated the relationship between the antioxidant activity of six algal species (five microalgae and one macroalgae) and the presence of bioactive compounds (e.g., carotenoids, phenolic acids, fucoxanthin and fatty acids profile). In this study, the authors concluded that _Chaetocros calcitrans_ and _Isochrysis galbana_ presented the highest antioxidant activity, followed by _Odonttella sinensis_ and _Skeletonema costatum_, with moderate antioxidant activity, and _P. tricornutum_ and _Saccharina japonica_ with the lowest antioxidant properties. Regarding the influence of bioactive compounds in the antioxidant activity of these algal species, through Pearson correlation and multiple linear regression  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Metabolites** & **Examples** & **Microalgal/Cyanobacterial Sources** & **Biological Activity** & **Role in Agriculture** & **Refs. ** \\ \hline \multirow{4}{*}{ \begin{tabular}{c} Phenolic compounds \\ \end{tabular}  } & & _Botryococcus braunii_, _Chlorecens_ & **Cros’ protection against** & **Cros’ protection against** \\  & & _acidisms_, _isochrysis gallbana_, _Isochrysis sp., _Necollis oleoabundans_, _Mometella_ & **Anticbacterial** & **Pathogens or other biotic** & [2; 7; 16; 17; 18; 19; 20; 21] \\  & Polyphenols; phenolics & _Necollis oleoabundans_, _Mometella_ & **Anticodistant** & **Anticodistant** & **Cros’ protection against** \\  & acids; Flavonoids; & _Necollis oleoabundans_, _Mometella_ & **Anticodistant** & **Anticodistant** & **Pathogens or other biotic** & [2; 7; 16; 17; 18; 19; 20; 21] \\  & phenyl Trypanoids & _Susemiemi_, _Phaeudaryum tricornutum_, & **Saccharina japonica**, _Skeleton_ & **Anticodistant** & **Anticodistant** \\  & & _catcatrum; Trianchis suecia_ & **Anticodistant** & **Anticodistant** & **Cros’ protection against** \\  & Hemiterepenses; & _Phaeudaryum tricornutum_, & **Saccharina japonica** & **Anticodistant** & **Cros’ protection against** \\  & monoterpenses; & _Phaeudaryum tricornutum_, & **Saccharina japonica** & **Anticodistant** & **Anticodistant** \\  & sesquiterpenenses; & _Iztpughlin_, _Pourieria hrenmann_; & **Anticodistant** & **Stimulation of preliminary** & [2; 7; 22; 23; 24; 25; 26; 27; 28] \\  & dipterpenses; & _Pyeathathathanebana_ sp. _Sphanococcus_ & **Anticodistant** & **Anticodistant** & **Growth and development** \\  & & _coronpifolia; Synchocystis sp.