180 Marine Macro- and Microalgae: An Overview
anti-microbial activity (Kamenarska et al. 2002). Similarly, chloroform and methanol fractions of an
ethanolic extract of the brown seaweed Spatoglossum asperum, which was rich in various fatty acid
esters, exhibited strong anti-fungal activities against Macrophomina phaseolina, whereas the n-hexane
fraction was active against Rhizoctonia solani and Fusarium solani (Ara et al. 2005). Similarly, a lipophilic
compound, primarily composed of pyrrole-2-carboxylic acid, pentadecanoic acid, and octadecanoic acid,
from the green coenocyte Asparagopsis taxiformis, was active against various pathogenic microorganisms
(Manilal et al. 2009). Sulphoglycerolipid 1-0-palmitoyl-3-0 (6′-sulpho-α-quinovopyranosyl)-glycerol,
isolated from the methanolic extract of the brown seaweed Sargassum wightii, showed activity against
Xanthomonas oryzae pv. oryzae (Xoo)—the causative agent of bacterial blight in rice (Arunkumar et al.
2005).
Sterols: Sterols are an important family of lipids that are present in eukaryotic cells (Nabil and Cassan
1996). Sterols are also known to have bioactivity in biological systems that can increase the innate
immunity of plants against pathogens (Wang et al. 2012). Biologically active sterols such as cholesterol,
fucosterol, stigmasterol, and ergosterol are predominant in certain red, green, and brown seaweeds
(Kamenarska et al. 2002; Sánchez-Machado et al. 2004). Ghazala et al. (2004) reported biologically
active sterols, the majority of which was β-Sitosterol isolated from the methanol extracts of freshwater,
stonewort algae, Chara contraria, and Nitella flexilis that were highly effective against fungal pathogens.
Sterols were demonstrated to induce morphological abnormalities in some plant pathogens, for example,
the effect of an extract of Sargassum carpophyllum on the fungus Pyricularia oryza (Blunt et al. 2007),
thus exhibiting the potential to suppress fungal pathogens.
Phenolic compounds: Macroalgae are also known sources of phenolic compounds which possess antimicrobial properties. These compounds are also known antioxidants and they can elicit anti-microbial
activities. Phenols, polyphenols, tannins, and tannic acid are abundant in certain brown, green, and red
seaweeds (see Sieburth and Conover 1965; Zhang et al. 2006; Liu and Gu 2012; Eom et al. 2012). A
particular type of polyphenol called phlorotannin, is unique to the brown algae; it has been demonstrated
to have strong anti-microbial activity and potential for applications against plant pathogens (Wang et
al. 2009; Gupta and Abu-Ghannam 2011). Seaweed extracts, rich in phenolic compounds, along with
sulphated galactans and fucans, as well as other halogenated compounds (Cardozo et al. 2007), together
may have the ability to reduce pathogenic microbes, including those causing plant diseases.
Other secondary metabolites: The strong anti-microbial activity of some seaweed extracts may be
linked with other metabolites, as found in various seaweeds. A variety of halogenated compounds,
as well as other primary and secondary metabolites may be anti-microbial, for example, acrylic acid
from seaweeds such as: Gracilaria corticata and Ulva lactuca (Katayama 1964; Bandara et al. 1988)
was demonstrated to have anti-microbial activity. Ethanol-soluble extracts from Gracilaria chilensis
and Lessonia trabeculata contained unspecified active compound(s) against the mycelial growth of
P. cinnamomi and also activated plant defense mechanisms to protect the challenged plant tissue against
B. cinerea infection (Jiménez et al. 2011). These authors suggested that the active fraction was neither
polysaccharide nor proteinaceous in nature, which indicated that unknown secondary metabolites or
some of the above-mentioned compounds, singly or in combination, could have been present in the
bioactive extracts. Venkatesh et al. (2011) reported 2,3 dihydro-7-methy-1,4-benzoxazine-3-one was one
of the bioactive components in the carragenophyte Kappaphycus alvarezii that has shown anti-microbial
potential. Apart from terpenes, various anti-microbial compounds such as neophytadiene, cartilagineol,
obtusol elatol, and the ester ethyl hexadecanoate were identified in the L. dendroidea extract (Peres et
al. 2012). The results were promising and demonstrated the ability of selected seaweed extracts against
different types of plant pathogens, including fungi and virus.
Seaweed extracts against plant pathogens
In their natural environment, some seaweeds maybe exposed to a number of seemingly stressful conditions
including microbial attack. Thus, it is natural that a variety of compounds produced during interactions
with the environment might be anti-microbial in nature. In order to understand the real potential of
anti-microbial activity (Kamenarska et al. 2002). Similarly, chloroform and methanol fractions of an
ethanolic extract of the brown seaweed Spatoglossum asperum, which was rich in various fatty acid
esters, exhibited strong anti-fungal activities against Macrophomina phaseolina, whereas the n-hexane
fraction was active against Rhizoctonia solani and Fusarium solani (Ara et al. 2005). Similarly, a lipophilic
compound, primarily composed of pyrrole-2-carboxylic acid, pentadecanoic acid, and octadecanoic acid,
from the green coenocyte Asparagopsis taxiformis, was active against various pathogenic microorganisms
(Manilal et al. 2009). Sulphoglycerolipid 1-0-palmitoyl-3-0 (6′-sulpho-α-quinovopyranosyl)-glycerol,
isolated from the methanolic extract of the brown seaweed Sargassum wightii, showed activity against
Xanthomonas oryzae pv. oryzae (Xoo)—the causative agent of bacterial blight in rice (Arunkumar et al.
2005).
Sterols: Sterols are an important family of lipids that are present in eukaryotic cells (Nabil and Cassan
1996). Sterols are also known to have bioactivity in biological systems that can increase the innate
immunity of plants against pathogens (Wang et al. 2012). Biologically active sterols such as cholesterol,
fucosterol, stigmasterol, and ergosterol are predominant in certain red, green, and brown seaweeds
(Kamenarska et al. 2002; Sánchez-Machado et al. 2004). Ghazala et al. (2004) reported biologically
active sterols, the majority of which was β-Sitosterol isolated from the methanol extracts of freshwater,
stonewort algae, Chara contraria, and Nitella flexilis that were highly effective against fungal pathogens.
Sterols were demonstrated to induce morphological abnormalities in some plant pathogens, for example,
the effect of an extract of Sargassum carpophyllum on the fungus Pyricularia oryza (Blunt et al. 2007),
thus exhibiting the potential to suppress fungal pathogens.
Phenolic compounds: Macroalgae are also known sources of phenolic compounds which possess antimicrobial properties. These compounds are also known antioxidants and they can elicit anti-microbial
activities. Phenols, polyphenols, tannins, and tannic acid are abundant in certain brown, green, and red
seaweeds (see Sieburth and Conover 1965; Zhang et al. 2006; Liu and Gu 2012; Eom et al. 2012). A
particular type of polyphenol called phlorotannin, is unique to the brown algae; it has been demonstrated
to have strong anti-microbial activity and potential for applications against plant pathogens (Wang et
al. 2009; Gupta and Abu-Ghannam 2011). Seaweed extracts, rich in phenolic compounds, along with
sulphated galactans and fucans, as well as other halogenated compounds (Cardozo et al. 2007), together
may have the ability to reduce pathogenic microbes, including those causing plant diseases.
Other secondary metabolites: The strong anti-microbial activity of some seaweed extracts may be
linked with other metabolites, as found in various seaweeds. A variety of halogenated compounds,
as well as other primary and secondary metabolites may be anti-microbial, for example, acrylic acid
from seaweeds such as: Gracilaria corticata and Ulva lactuca (Katayama 1964; Bandara et al. 1988)
was demonstrated to have anti-microbial activity. Ethanol-soluble extracts from Gracilaria chilensis
and Lessonia trabeculata contained unspecified active compound(s) against the mycelial growth of
P. cinnamomi and also activated plant defense mechanisms to protect the challenged plant tissue against
B. cinerea infection (Jiménez et al. 2011). These authors suggested that the active fraction was neither
polysaccharide nor proteinaceous in nature, which indicated that unknown secondary metabolites or
some of the above-mentioned compounds, singly or in combination, could have been present in the
bioactive extracts. Venkatesh et al. (2011) reported 2,3 dihydro-7-methy-1,4-benzoxazine-3-one was one
of the bioactive components in the carragenophyte Kappaphycus alvarezii that has shown anti-microbial
potential. Apart from terpenes, various anti-microbial compounds such as neophytadiene, cartilagineol,
obtusol elatol, and the ester ethyl hexadecanoate were identified in the L. dendroidea extract (Peres et
al. 2012). The results were promising and demonstrated the ability of selected seaweed extracts against
different types of plant pathogens, including fungi and virus.
Seaweed extracts against plant pathogens
In their natural environment, some seaweeds maybe exposed to a number of seemingly stressful conditions
including microbial attack. Thus, it is natural that a variety of compounds produced during interactions
with the environment might be anti-microbial in nature. In order to understand the real potential of