An array of diverse chemical compounds from algae, including alkaloids, polyketides, peptides, polysaccharides, phlorotannins, diterpenes, sterols, quinones, lipids, and glycerols, have been found to exhibit antibacterial action [14,31]. However, in some instances, the specific compounds with antibacterial properties are not fully elucidated, and the activity is collectively attributed to algal extracts. Also, as most studies focus on the antibacterial properties of algal extracts on human pathogens, information on plant pathogens is scarce but is constantly being enriched. An excellent example of algal extracts with antibacterial action comes from the brown marine alga Sargassum wightii (Table 1).

The methanolic extracts of this species showed maximum antibacterial activity against the plant pathogen Pseudomonas syringae, to prevent leaf spot disease of the medicinal herb Gymnema sylvestre [32]. Similar in vitro activity against the phytopathogenic bacterium Xanthomonas oryzae was observed by the macroalgae Gracilaria edulis, Sargassum wightii, and Enteromorpha flexuosa [16]. The compound that was responsible for this action in the brown alga Sargassum wightii was the sulphoglycerolipid 1-O-palmitoyl-3-O(6′-sulpho-a-quinovopyranosyl)-glycerol (Figure 1) from the methanolic extract [33]. The plant pathogens Xanthomonas campestris and Erwinia carotovora were also inhibited in vitro by the methanol-soluble and insoluble extracts of Ulva fasciata [34]. The spray application of aqueous extracts from the brown algae Cystoseira myriophylloides and Fucus spiralis significantly reduced Crown gall disease incidence that was caused by the bacterial pathogen Agrobacterium tumefaciens in greenhouse tomato plants (Solanum lycopersicum) [18]. The response was related to oxidative burst mechanisms since the treated plants exhibited significantly greater activity levels of the plant defense enzymes polyphenol oxidase and peroxidase. The methanolic extract of the marine brown algae Sargassum latifolium, Hydroclathrus clathratus, and Padina gymnospora showed antibacterial activity against the soil-borne phytopathogenic bacteria Ralstonia solanacearum and Pectobacterium carotovorum. The extract of Padina gymnospora, which showed the most potent effect, was dominated by palmitic and oleic acids (Figure 1) [35].

In most documented cases, the antibacterial activity is against human or animal pathogens. In terms of agricultural practices, this is important for pathogenic bacteria that are associated with livestock. However, such agents could be further tested as potential candidates for plant protection applications, leaving ample room for future research [19,36,37,38,39,40,41,42,43,44,45,46,47]; such are the cases of the tetrabrominated diphenyl ether-like 2-(2′,4′-dibromophenoxy)-4,6-dibromoanisole (Figure 2) that was isolated from the green alga Cladaphora fascicularis that showed bactericidal action against Escherichia coli, Bacillus subtilis, and Staphylococcus aureus [38], and tetracyclic brominated diterpenes such as 3-hydroxy-1-deoxy-bromotetraspaerol (Figure 3) from the red alga Sphaerococcus coronopifolius that exhibited antibacterial activity against a panel of S. aureus strains [36]. Interestingly, ethyl acetate extracts from two cyanobacterial species, Anabaena variabilis and A. circinalis, were effective against bacterial fish pathogens belonging to the genus Aeromonas, proving to be useful in aquaculture applications [48]. Moreover, as most of the published studies focus on macroalgae or cyanobacteria’s antimicrobial activity, microalgae could constitute another exciting prospect for future research.

Plant viruses are a serious threat to agricultural crops, affecting product quality and yields and resulting in severe economic losses [30]. Their management is heavily dependent on synthetic chemical products, but natural compounds are continuously gaining ground. In this respect, natural compounds from algae that exhibit antiviral properties could be valuable resources. Among them are various polysaccharides, such as laminarins, agarans, alginate, carrageenans, and sulphated fucans, that can function as elicitors of defense mechanisms, as well as proteins, lipids, tannins, and terpenoids [10,13,30,50,51]. Polysaccharides are the most common compounds in algal extracts that induces antiviral responses in plants. For example, sodium alginate (Figure 4) from marine algae exhibited strong inhibitory activity against the tobacco mosaic virus (TMV) that was isolated from systemically-infected leaves of Nicotiana tabacum L. var bright yellow [21] (Table 2).

The degree of inhibition increased with alginate concentration and was higher when the alginate polymer had a lower mannuronate to guluronate ratio [21]. Another polysaccharide, i.e., kappa/beta-carrageenan (Figure 4) from red marine alga Tichocarpus crinitus, reduced tobacco mosaic virus (TMV) infection in Xanthi-nc tobacco leaves by 87 percent [52]. The same compound stimulated lytic processes against PVX particles in the leaves of Datura stramonium [22]. Methanolic extracts from six algal species also showed inhibitory effects against PVX by more than 80%. Among them, the extract from Fucus gardneri contained the polysaccharide alginate, which showed 95% success in suppressing PVX infection via aggregation of the virus particles [23].

Apart from polysaccharides, lectins (carbohydrate-binding proteins) that were isolated from the marine alga Ulva pertusa also showed antiviral action against tobacco mosaic virus (TMV) [53,54]. Similarly, lipids that were extracted from 11 algal species displayed antiviral activity against TMV on Nicotiana tabacum cv. Xanthi-nc. The brown alga Cystoseira balearica and the red alga Lophocladia lallemandii showed the highest inhibitory activity [19]. Aqueous and ethanolic extracts from the brown alga Durvillaea antarctica suppressed damage that was caused by TMV in tobacco leaves [17]. Moreover, an exciting application in livestock animals refers to the use of sulphated polysaccharides from the algae Ulva clathrata and Cladosiphon okamuranus in inhibiting Newcastle Disease Virus (NDV) infection in poultry [55].

Research for algal compounds with antiviral effects in plants has yielded significant results in the past years and remains promising for discovering new biopesticide products. However, as expected, algal antiviral research is mainly concentrated around human pathogens. Algal extracts have been studied extensively for their inhibitory action against important human viruses, such as human immunodeficiency virus (HIV), human papilloma virus (HPV), hepatitis B virus (HBV), herpes simplex virus types 1 and 2 (HSV-1, HSV-2), and diverse strains of Dengue virus (DENV-2) [56,57,58,59,60,61,62,63,64], suggesting that there are still many research opportunities in the field of algal antiviral products.

Natural algal compounds are constantly gaining ground in modern agricultural practices in controlling fungal infection, one of the most common types of disease in cultivated plants. They are mostly preferred over synthetic products, due to lower environmental impact, high specificity, and performance [14,51]. Algal powders and a large variety of extracts, such as aqueous, methanolic, ethanolic, diethyl ether, acetone, ethyl acetate, benzene, and chloroform, have proven to be effective in protecting plants against pathogenic fungal species [17,18,20,24,35,65,66,67,68,69,70,71,72,73,74,75,76,77]. For instance, ethanolic extracts of the cyanobacterium Nostoc strain ATCC 53,789 inhibited the growth of nine fungal plant pathogens (Table 3).

The extract, which was directly applied on tomato plants, completely inhibited the growth of the fungus Sclerotinia sclerotiorum [20]. The aqueous extracts of three brown algae Cystoseira myriophylloides, Laminaria digitata, and Fucus spiralis, that were applied as spray or drench, had a protective effect against Verticillium wilt disease that was caused by the fungus Verticillium dahliae, in greenhouse tomato seedlings [18]. In tomato leaves that were infected with Botrytis cinerea, organic extracts from the brown alga Lessonia innamomic reduced the frequency and extent of necrotic lesions, while aqueous and ethanolic extracts from the red alga Gracillaria chilensis were effective against Phytophthora innamomic, showing dose and time-dependent responses [17]. Methanolic extracts of the sea brown algae Sargassum latifolium and Padina gymnospora showed antifungal activity against phytopathogenic fungus species such as Fusarium solani and Rhizoctonia solani in addition to antibacterial activity [35]. Notably, the antifungal activity of alkaline extracts of Ulva lactuca, Sargassum filipendula, and Gelidium serrulatum has been linked to the increase in plant defense enzymes activity and the overexpression of key marker genes of plant defense pathways [49].

De Corato et al. [68] performed in vitro and in vivo tests of extracts from two brown and three red macroalgae against three phytopathogenic fungi. The compounds that were identified in the extracts included twenty fatty acids, three types of polysaccharides (laminarans, fucoidans, and alginates) (Figure 4), and three types of phlorotannins (phlorethols, fucophloretols, and eckols) (Figure 2). Regarding lipids, palmitic acid, linoleic acid, and arachidonic acid were found at the highest concentrations in most extracts. The quantity of fatty acids corresponded to most of the total dry weight of the crude extracts and could be linked to the antifungal activity [68]. Lipids that were extracted from 10 algal species inhibited the germination of Phoma tracheiphila, a pathogenic fungus that causes a disease known as Mal secco on citrus trees. More specifically, the inhibition effect of the brown alga Cystoseira balearica and the green alga Codium effusum reached 100% [19].

Various algal oligo- or polysaccharides have demonstrated antifungal activity against plant pathogens either directly or indirectly, by activating plant defense mechanisms [78,79,80,81,82,83,84]. The cell wall polysaccharide laminarin (Figure 4) that was purified from the brown algae Laminaria digitata induced a defense response in grapevine leaves against the fungus Botrytis cinerea, effectively reducing the infection. In addition, grapevine plants that were sprayed on the leaves with laminarin were protected against the fungus Plasmopara viticola [85]. Polysaccharides from Anabaena sp., Ecklonia sp., and Jania sp. showed inhibitory action against Botrytis cinerea, effectively protecting strawberry fruits from infection in in vitro experiments [86]. Similar polysaccharide-rich extracts from green (Ulva lactuca and Caulerpa sertularioides) and brown (Padina gymnospora and Sargassum liebmannii) macroalgae induced resistance in tomato plants against the necrotrophic fungus Alternaria solani [87]. The water-soluble heteropolysaccharide ulvan (Figure 4) from Ulva sp. extracts significantly reduced the severity of Glomerella leaf spot (GLS) disease, caused by the fungus Colletotrichum gloeosporioides on the leaves of apple plant seedlings (Malus domestica). The induced resistance was associated with increased peroxidase activity, revealing that although ulvan does not exhibit antimicrobial activity, its action is associated with plant defense mechanisms [88].

Extracts and powders of the green seaweeds Ulva fasciata and Enteromorpha flexuosa inhibited the growth or affected the microsclerotia formation of the soil-borne fungi, Macrophomina phaseolina and Fusarium solani, that infect cucumber plants. The identification of iron-monocarbonyl and their functional groups, such as amine and ether in the extracts, could suggest a potential antifungal role for these compounds [89]. Similar compounds were also identified in the chloroform extracts from the red macroalgae Gracilaria confervoides that exhibited inhibitory action against three soil-borne pathogenic fungi of cucumber: Rhizoctonia solani, Fusarium solani, and Macrophomina phaseolina [75].

The methanolic extract of the brown alga Sargassum vulgare contained phenolic acids and flavonoids that may be responsible for the antifungal action that was observed against Pythium aphanidermatum, the causative agent of Pythium leak disease in potato [90]. Phenolic acids and phytoalexins were among the 18 compounds that were detected in the acetone extract of the brown alga Sargassum wightii. The extract exhibited antifungal activity against Rhizoctonia solani, the causative agent of rice sheath blight [91].

The extracts and compounds from micro- and macroalgae are also effective against plant-parasitic nematodes that are responsible for the annual loss of 10–25% of worldwide crop production [92] (Table 4).

Brominated diterpenes that were isolated from the marine red alga Jania rubens were effective against Allolobophora caliginosa [93]. Different concentrations of methanolic extracts from the Nostoc strain ATCC 53,789 either killed or slowed the development of the nematode C. elegans [20]. Dry powders from three macroalgae species, Spatoglossum variabile, Stokeyia indica, and Melanothamnus afaqhusainii displayed a suppressive effect on the root-knot nematode Meloidogyne incognita by reducing the gall formation and preventing nematode penetration in the roots of eggplant and watermelon [65]. Similar effects against the nematode Meloidogyne javanica were identified in okra, sunflower, and tomato after treatment with aqueous and ethanolic extracts of the marine macroalgae Sargassum tenerrimum, S. swartzii, S. wightii, Spatoglossum variabile, Melanothamnus afaqhusainii, and Halimeda tuna [24,94]. Algal extracts showed relatively similar suppressive effects with a carbofuran nematicide; however, the best result was obtained when extracts from Spatoglossum variabile were applied together with the synthetic product [24]. The marine alga Stoechospermum polypodioides was also effective against Meloidogyne javanica, causing 80% mortality, the strongest effect among 21 species of algae that were examined for their nematocidal action [95]. A reduction in root-knot nematode infestation using macroalgal extracts has been previously reported in tomato [96]. There are two commercially available products that are derived from the marine macroalgae Ascophyllum nodosum and Ecklonia maxima, that affected the hatching and sensory perception of the root-knot nematodes Meloidogyne chitwoodi and M. hapla. The alkaline extract from the brown marine alga A. nodosum showed a stronger inhibitory effect compared to the extract from the brown alga E. maxima, which in certain cases enhanced the infectivity of the nematodes [97]. Extracts of A. nodosum have shown inhibitory action against other nematodes, such as Radopholus similis, Meloidogyne incognita, and Belonolaimus longicaudatus, infecting citrus, tomato, and centipede grass, respectively [98,99,100,101].

Marine macroalgae extracts exhibit insecticidal/acaricidal activity and can be used in integrated pest management applications as environmentally friendly approaches for arthropod population control. These botanical biopesticides are safer than synthetic products and are equally or more effective since they display different modes of action. Therefore, their targets are less likely to develop resistance against them [102,103]. There are a large number of publications describing the pesticidal or repellent action of algal extracts against different arthropods that are either agricultural pests or related to human and animal health [25,26,104,105,106,107,108,109,110,111,112,113,114,115]. The bioactive compounds that were identified in the extracts, as expected, cover a broad range of chemical structures, including polysaccharides, phenolics, proteins, terpenes, lipids, and halogenated compounds [116]. For example, two halogenated monoterpenes (mertensene and violacene) (Figure 3) that were extracted from the red alga Plocamium cartilagineum, and the mertensene derivatives, dibromomertensene, and dihydromertensene, showed strong insecticidal activity against the tomato moth Tuta absoluta and the cereal aphid, Schizaphis graminum [104] (Table 5).

Water and ethanol extracts from the cyanobacterium Arthrospira platensis (syn. Spirulina platensis) and the brown alga Sargassum vulgar contained phenols, tannins, and alkaloids. Further analysis of phenols in A. platensis revealed the presence of the phenolic compounds: quercetin, kaempferol, and resorcinol (Figure 2), which could be related to the insecticidal action of the extracts [27]. An evaluation of the ethanolic extracts of the macroalgal species (Caulerpa sertularioides, Laurencia johnstonii, and Sargassum horridum) revealed insecticidal and repellent activities against Asian citrus psyllid adults (Diaphorina citri). The chemical composition of the three extracts showed phenols, alkaloids, terpenes, tannins, flavonoids, saponins, and anthraquinones, which are associated with insecticidal and repellent activity [109]. In certain cases, algal extracts exhibit different insecticidal effects, depending on the insect species and on the developmental stages of the target insect. For instance, the acetone extract of Ulva lactuca was the most potent extract against Culex pipiens. In the case of the cotton leafworm, Spodoptera littoralis, ethanolic and chloroform extracts acted as larvicides, methanolic and ethanolic extracts resulted in the highest pupation inhibition, whereas etheric and methanolic extracts strongly inhibited larval growth and adult emergence [117]. In terms of agricultural pests, more research has focused on the insecticidal activity of algal extracts against the cotton insect pests of the genera Dysdercus and Spodoptera. Algal extracts of various species, including Caulerpa scalpelliformis [118], Padina pavonica [115], Sargassum tenerrimum [114], Sargassum vulgar [27], Ulva fasciata, and U. lactuca [105] were effective against these agricultural pests by causing nymphal mortality, adult mortality, abnormal development, or reducing adult lifespan, fecundity, and hatchability [14,106]. As observed throughout the text, algal extracts could be used against more than one target type (antimicrobial, insect, mite, etc.). Therefore, apart from the antifungal action, the methanolic extracts of the cyanobacterium Nostoc strain ATCC 53,789 also killed larvae of the moth Helicoverpa armigera [20].

The volatile oils from Actinotrichia fragilis, Liagora ceranoides, and Colpomenia sinuosa were hydrodistilled and contained different aliphatic alcohols and long-chain hydrocarbons as the major components. They caused 55–90% and 60–80% mortality to pests of stored products Oryzaephilus mercator and Tribolium castaneum, respectively, at a dose of 12 μL/L air after 48 h of fumigation exposure [119].

The volatile oils of A. fragilis caused 80–90% mortality in both T. castaneum and O. mercator and consisted of 49% aliphatic alcohols, mainly 1-dodecanol (39.6%) [119]. The insecticidal activity of 1-dodecanol was suggested to be related to the developing cuticle, producing a disruption in the cuticular tanning process [120].