Table 1.

A summary of representative bioactive compounds in algal extracts and their effects on plants under salinity. GABA: γ-aminobutyric acid, IAA: indole-3-acetic acid, IAM: indole-3-acetamide, IBA: indole-3-butyric acid, ROS: Reactive Oxygen Species.

Class	Metabolite	Algae	Discussed Effects on Plants	Ref.
Amino acids	D-homocysteic acid, GABA, ornithine, citrulline, hydroxyproline	Chlorophyceae, Phaeophyceae, Rhodophyceae	Nitrogen storage, stress response, osmolytes, pH buffering, antioxidants	[79,80]
Amino acids	Mycosporine-like amino acids	Rhodophyceae	Protection from UV radiation and oxidative damage, osmolytes	[90,91]
Amino acids	Phosphoserine	Phaeophyceae	Phosphoserine-containing peptides lower lipid peroxidation, increase intracellular glutathione and expression of antioxidant enzymes	[82,83]
Amino acid	Proline	Trebouxiophyceae (Stichococcus)	Osmolyte, antioxidant, cellular protectant against saline stress	[84]
Amino acid	Taurine	Rhodophyceae (Porphyra)	Antioxidant activity	[81,92]
Betains	Glycine betaine, γ-aminobutyric acid betaine and proline betaine	Entire algal kingdom	Osmolytes, ROS scavengers, macromolecules protectans	[28,32,33,89]
Brassinosteroids	Brassinolide, castasterone, typhasterol	Chlorophyceae, Trebouxiophyceae	Promote plant growth, increase crop yield and resistance to biotic and abiotic stresses	[62,93]
Carbohydrate	Trehalose	Chlamydomonas, Chlorella, Scytonema	Osmolyte, carbon reserve, and salt stress protectant	[88]
Carotenoids	E-fucoxanthin	Nannochloris, Tetraselmis, Nannochloropsis, Phaedactylium	Radical scavenger and iron chelator	[70,94]
Flavonoids	Catechin and epicatechin	Ankistrodesmus, Spirogyra, Euglena, Caespitella	ROS scavengers, metal ion chelators, induction of antioxidant enzymes, inhibition of pro-oxidant enzymes	[69]
Phenolic acids	Gallic, syringic, protocatechuic, and chlorogenic acids	Ankistrodesmus, Spirogyra, Euglena, Caespitella	High antioxidant capacity, inhibition of generation as well as scavenging of free radicals, upregulation of antioxidant enzymes	[69]
Phenolic acids	Protocatechuic acid	Nannochloris, Tetraselmis, Nannochloropsis, Phaedactylium	Superoxide anion radical and hydroxyl radical scavenger, metal ion chelator	[70,95]
Phytohormones	Abscissic acid	Entire algal kingdom	Involvement in stress response	[73,74]
Phytohormones	Auxins and cytochinins	Chlorophyceae, Trebouxiophyceae, Ulvophyceae, Charophyceae	Increase of plant growth, yield and defense response against abiotic stress	[63]
Phytohormones	Auxins (IAA, IAM, IBA)	Rhodophyceae, Phaeophyceae, Ulvophyceae	Stimulation of rooting and root growth, increase of resources use efficiency, stress resistance	[34,72]
Phytohormones	Isopentenyladenine, cis-zeatin	Chlorophyceae, Phaeophyceae, Rhodophyceae	Stimulation of seed germination, transition between vegetative to generative phases, inhibition of senescence, response to abiotic stresses	[38,96,97]
Phytohormones	Gibberellins	Chlorophyceae, Trebouxiophyceae, Ulvophyceae, Charophyceae	Promote plant growth and resistance to salinity by inducing the degradation of the nuclear family of DELLA TFs and the increase of salicylic acid	[62,98]
Polyamines	Putrescine and spermidine	Chlorophyceae, Charophyceae, Rhodophyceae	Regulation of the cell cycle and increased cell proliferation, stress tolerance	[85,86]
Polyols	Sorbitol and mannitol	Platymonas, Stichococcus	Osmoprotectans	[88]
Polyphenols	Phloroglucinol, eckol, and dieckol	Phaeophyceae (Ascophyllum, Fucus)	More efficient ROS detoxification due to the higher number of phenolic rings.	[65,67,68]
Polyphenols	Phlorotannins	Phaeophyceae	Response to both biotic and abiotic stresses, ROS scavenging	[66]
Polysaccharides	Agars, alginates, carrageenans, and fucans	Chlorophyceae, Phaeophyceae, Rhodophyceae	Elicitors of hormonal stress signals (i.e., SA, JA, ethylene) and inductor of resistance to biotic stresses	[77,78]
Tertiary sulphonium compound	3-dimethylsulfoniopropionate	Entire algal kingdom	Osmoprotectant	[32]