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Use of allelopathy of aquatic macrophytes for algal-bloom control
- A review
Hong-Ying HU, Yu HONG
Environmental Simulation and Pollution Control State Key Joint Laboratory, Department of Environmental
Science and Engineering, Tsinghua University, Beijing 100084, PR China
Abstract In recent years, water eutrophication has been an increasing serious problem in various kinds
of waterbodies all around the world, which directly causes algal explosive growth, i.e. algal bloom. The
development of an environment-friendly, cost-effective and convenient alternative for controlling algal
bloom has gained much concern. Using the allelopathy of aquatic macrophytes as a novel and safe method
used for algal-bloom control evokes us a new prospect and will be a promising alternative. This paper
discussed the development and potential application of allelopathy of aquatic plants on algae, including the
reported aquatic macrophytes having allelopathy on algae, their allelochemicals and potential modes of
action on algae, possible ways for application and the prospect of using aquatic macrophytes to control
algal bloom.
Keywords Alleopathy, allelochemical, aquatic macrophyte, algae, algal bloom
1 Introduction
In recent years, eutrophication in worldwide waterbodies is much serious. Eutrophication causes
phytoplankton into explosive growth, i.e. algal bloom, accompanying with several negative impacts, such
as water-quality decrease not to fit standards for drinking or recreation water source and algal-toxin release
to poison animal and even human health[1]. Moreover, algal floating mats block off lights and exhaust
water dissolved oxygen to make aquatic animals and plants suffocate and reduce biological diversity[2].
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Therefore, algal-bloom control is an important issue for water environment protection.
Till now, there are three kinds of methods used for algal control, i.e. physical methods, chemical
treatment and biological manipulation. Physical methods has mechanical cleanup, carbon absorption,
ultrasonic disturbance, and ultraviolet irradiation, etc, which usually need expensive instruments, are
time-consuming and just available on small-scale waterbodies[3-7]. The algicides used for chemical
treatment usually include heavy metal compounds, pro-oxidants and organic amines, which are
cost-effective but with broad-spectrum toxicity to all the aquatic organisms[8-9]. Biological manipulation
utilizes fish, shell, zooplankton, epiphyte, actinomycete, bacteria, phycophage and etc to directly or
indirectly control the algal biomass. Microbes preferably with high mutation rates are considered not be
singly used in algal control, and moreover, algal heavy extracellular polymeric substances or other
complicated factors caused algae-grazing zooplankton and algae-lysing microbes just under
investigation[10-12]. The successful application cases are primarily on filter-feeding fish but with unstable
control effects and potential ecological risks.
In the last decades, there has been an increasing focus on the prospect of exploiting macrophytes as an
alternative strategy for controlling algal bloom. Barley straw is the most successful macrophyte in the
practical application of algal-bloom control[13]. Some reports revealed that to release inhibitory second
metabolites might be the inhibitory mode of action of barley straw, and associated epiphytes (e.g.
Pycnidiophora dispersa Clum、Zopfiella、Fusarium tricinctum) and even invertebrates might be also
involved in the process[14-19]. At present, other extensively distributed and abundant terrestrial plants are
investigated. Ridge et al. reported leaf litter had inhibition on algae, but less effective than barley straw[20].
Wang et al. observed decomposed rice straw had similar inhibitory effects with barley straw on several
kinds of cyanobacteria, e.g. Microcystis aeruginosa[21]. Terrestrial plant has some potential problems to
be applied for algal control, such as large one-off dosage, its long time before inhibitory effects occurs,
unstable effects, bad to landscape and tourism, hard to dispose its residue, its ambiguous ecological
impacts to aquatic organisms.
Considering that the low efficiency, limited applied scale, secondary pollution from the methods
mentioned above, it is still urgent to find other environment-friendly, cost-effective and convenient
methods. In aquatic ecological system, complicated interactions exist among various organisms, such as
prey relationships, the competition for nutrient, living space and light and allelopathy[22, 23]. All
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interactions are important to the structure of aquatic ecological system, especially allelopathy in much
concern, which can be a key factor to regulate the phytoplankton polulation[24, 25]. So to exploit the
allelopathy of aquatic macrophytes to control algal bloom evokes us a new prospect and will be very
promising.
2 The aquatic macrophytes having allelopathy on algae
Allelopathy was termed in 1937 by Austria scientist Molish H R[26]. Allelopathy was definited as the
inhibitory or stimulative effects of one plant (including microorganism) on another plant (including
microorganism) via releasing chemical compounds into the environment[27].
Aquatic macrophytes (including macroalgae) from the longshore, floating as well as deep-water zone in
the freshwater environment have widespread allelopathy on phytoplankton [22, 23]. Especially in lakes,
the nutritional status and lakes configuration make phytoplankton and aquatic plants at relatively higher
densities than in rivers or streams, as their slow water is more suitable for allelochemicals to release and
act on the receptors, rather than to be immediately swept or rapidly diluted [46]. Based on the field
observations and lab-scale screening, many aquatic plants (including macroalgae) have been reported to
have allelopathy on algae. The macrophytes having allelopathy on algae were showed in Table 1. As seen
in Table 1, to date, the studies on the allelopathy of aquatic macrophytes on algae are mostly done in lab
scale but few studies in field scale. However, the field-scale studies can better simulate the natural
conditions, and reflect the allelopathic process of aquatic macrophytes on algae. Therefore, it is very
important to develop the field-scale allelopathic studies in the future.
Table 1 Allelopathy of aquatic macrophytes (including macroalgae) on freshwater algae
Life styles Species Experimental
Scales
Algae inhibited References
Emergent Eleocharis microcarpa L/L* Haematococcus pluvialis, [29,30]
Macrophyte
Anabaena flos-aquae,
Oscillatoria tenuis
Eleocharis acicularis L cyanobacteria [47]
Phragmites communis L/L* Microcystis aeruginosa,
Chlorella pyrenoidosa,
Phormidium sp.
[45,73]
Berula erecta L/L* Nitzschia palea [49]
Acorus tatarinowii L/L* cyanobacteria,
green algae
[62,63]
Acorus calamus L* cyanobacteria,
green algae
[62]
Zantedeschia aethiopica L* Selenastrum carpricornutum [64]
Juncus effuses, L* Selenastrum carpricornutum [65-68]
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Life styles Species
Juncus acutus
Experimental
Scales
Algae inhibited References
Typha latifolia,
L/L* Anabaena flos-aquae, [69-72,91]
Typha minima,
Chlorella vulgaris,
Typha angustata,
Chlorella pyrenoidosa,
Typha domingensis
Scenedesmus obliquus,
Scirpus validus,
L* Chorella pyrenoidosa [90]
Sagittaria sagittigolia,
Saprganium toloniferum,
Zizania caduciflora,
Carex diandra,
Scirpus triqueter,
Trapa incisa,
Nymphoides peltatum,
Oenanthe javanica,
Polygonum amphibium,
Beckmannia rucaeformis
Microcystis aeruginosa
Floating Brasenia schreberi L* Chlorella pyrenoidosa, [33]
macrophyte
Anabaena flos-aquae
s Alternanthera philoxeroides, L/L* Scenedesmus sp., [35,48]
Lemma minor,
Spirodela polyrrhiza,
Azolla imbricate
Chlamydomonas reinhardtii
Stratiotes aloides L/F/L* Nitzschia palea
Synechococcus elongatus
Scenedesmus obliquus
[36,49,55,56]
Eichhornia crassipes L/L* Scenedesmus sp.,
Chlamydomonas reinhardtii,
Porphyridium aerugineum,
Anabaena azollae,
[38,75-77]
Nuphar lutea Cryptomonas sp. [39,40]
Cabomba caroliniana L/L* cyanobacteria [47]
Potamogeton malaianus, L/L* Scenedesmus obliquus, [78-81]
Potamogeton maackianus,
Potamogeton crispus,
Potamogeton natans,
Potamogeton pectinatus
Microcystis aeruginosa,
Pistia stratiotes L* cyanobacteria,
green algae,
golden algae, red algae
[73,74,91]
Nelumbo nucifera, L* Microcystis aeruginosa, [90]
Salvinia notans
Chlorella pyrenoidosa
Submerged Elodea canadensis L/F/L* Nitzschia palea,
[28,49],
macrophyte
cyanobacteria,
Erhard et al, unpublished
s
green algae
Elodea nuttallii L cyanobacteria,
green algae
Erhard et al, unpublished
Ceratophyllum demersum L/L* Nitzschia palea,
Chlorella pyrenoidosa,
a variety of diatoms,
cyanobacteria, green algae
[31,32,36,49,87,88,91]
Vallisneria spiralis,
Vallisneria denseserrulata,
L/L* Microcystis aeruginosa [37,47]
Limnophila sessiliflora,
Egeria densa
L cyanobacteria [47]
Ruppia maritima L Chlorella vulgaris,
Selenastrum carpricornutum
[59, 60]
Hydrilla erticillata L/L* Microcystis aeruginosa [61]
Myriophyllum spicatum L/L* globular cyanobacteria,
fibrous cyanobacteria
[41,42,82]
Myriophyllum alterniflorum,
Myriophyllum heterophyllum,
Myriophyllum brasiliense
L/L* cyanobacteria, green algae [84]
Myriophyllum verticillatum, L/L* cyanobacteria, green algae [85,86]
Potamogeton lucens L* cyanobacteria [36]
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Life styles Species Experimental
Scales
Algae inhibited References
Potamogeton crispus,
Potamogeton oxyphyllus
L cyanobacteria [47]
Potamogeton pusillus L* Microcystis aeruginosa,
Chlorella pyrenoidosa
[90]
Najas marina L cyanobacteria [50]
Chara globularis L/L* diatom, cyanobacteria,
green algae
[49,51,57,58,88]
Chara rudis,
Chara tomentosa,
Chara delicatula
L* cyanobacteria [51]
Chara vulgaris L/F cyanobacteria,phytoplankton[51,52]
Chara contraria L/L* cyanobacteria,green algae [51,88]
Chara aspera L* Phytoplankton [25,51,53]
Chara fragilis L* cyanobacteria [36]
Chara hispida F Phytoplankton [54]
Nitella sp. L Nitzschia palea [49]
Nitella gracilis,
Nitella opaca,
Nitellopsis obtusa
L* cyanobacteria [51]
L:Lab-scale,F:Field-scale,*:only with plant extract
2 The allelochemicals from aquatic macrophytes on algae
To date, there are a variety of allelochemicals having been reported. According to the basic organic units,
Rice summarized the potential types of allelochemicals into 14 kinds: water-soluble organic acids, simple
unsaturated lactone, long-chain fatty acids and polyalkynes, quinines and anthraquinones, simple phenols,
benzoic acids and derivatives, cinnamic acids and derivatives, coumarin derivatives, flavonoids, tannins,
terpenoids and steroids, amino acids and polypeptides, alkaloids and cyanohydrins, sulfides and mustard
oils, purines and nucleotides[27]. In terrestrial ecosystem, all the styles of allelochemicals were reported.
Especially in the field of agriculture, forestry and weed control, the most familiar allelochemicals included
long-chain fatty acids, phenolic acids, terpenoids, etc[27]. However, the reported allelochemicals of
aquatic macrophytes on algae (seen in Table 2) are still in deficiency compared with the allelochemicals
discovered in terrestrial ecosystem. The allelochemicals of just some macrophytes were fully investigated,
so next they were introduced in particular according to the life styles of aquatic plants(emergent, floating
and submerged plants).
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Table 2 Antialgal allelochemicals identified in different macrophytes and the algae inhibited
Life styles Species Allelochemicals Algae inhibited References
Emergent Eleocharis microcarpa 3-hydroxy-cyclopentenone Haematococcus pluvialis, [29,30]
Macrophytes
octadecenoic acids ,
Anabaena flos-aquae,
3-hydroxy-cyclopentyl
eicosapentaenoic acids
Oscillatoria tenuis
Phragmites communis ethyl 2-methylacetoacetate, Microcystis aeruginosa, [45,73]
p-coumaric acid, ferulic acid, Chlorella pyrenoidosa,
vanillic acid, mustard acid,
syringic acid, caffeic acid,
Phormidium sp.
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Life styles Species Allelochemicals
protocatechuic acid, gallic acid,
tetradecanoic acid,
palmitic acid, nonanoic acid,
stearic acid
Algae inhibited References
Berula erecta falcarinodiol 1,
falcarinodiol 2
Nitzschia palea [49]
Acorus tatarinowii (1,2-dimethoxy-4-(E-3'-methyl
oxiranyl)benzene),
1,2,4-trimethoxy-5-(E-3'-methyl
oxiranyl) benzene,
β-asarone, α-asarone, γ-asarone
cyanobacteria, green algae [62]
Acorus calamus α-asarone, β-asarone, γ-asarone,
trans-isoeugenol methylether
cyanobacteria, green algae [62]
Zantedeschia aethiopica cycloartane triterpenes, sterols,
neolignans, phenylpropanoids,
α-linolenic acid, linoleic acid,
neolignans
Selenastrum carpricornutum [64]
Juncus effusus phenylpropanes, glycerides,
dihydrophenanthrenes,
tetrahydropyrene
Selenastrum capricornutum [65,66]
Juncus acutus 9, 10-dihydrophenanthrenes,
dimeric phenanthrenoids,
9, 10-dihydrophenanthrenoid
Selenastrum capricornutum [67,68]
Floating
macrophytes
Typha sp. palmitic acid,
cholesteryl cis-9-octadecenoate,
20(S)-4α-methyl-24-methyl
enecholest-7-en-3-β-ol,
2-chlorophenol, salicylaldehyde,
linoleic acid, α-linolenic acid
Stratiotes aloides Lipophilic compounds
Eichhornia crassipes N-phenyl-1-naphthylamine,
N-phenyl-2-naphthylamine,
linoleic acid, linoleic acid,
benzoindenone,
dimeric phenalene, phenalene
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Anabaena flos-aquae,
Chlorella vulgaris,
Chlorella pyrenoidosa,
Scenedesmus obliquus,
Nitzschia palea
Synechococcus elongatus
Scenedesmus obliquus
Scenedesmus sp.,
Chlamydomonas reinhardtii,
Porphyridium aerugineum,
Anabaena azollae,
[69-72,91]
[36,49,55,56]
[38,75-77]
Nuphar lutea Resorcinol Cryptomonas sp. [39,40]
Pistia stratiotes linoleic acid, linolenic acid, cyanobacteria, green algae, [73,74,91]
hydroxy fatty acid, α-asarone,
polyphenols, steroidal ketones
golden algae, red algae
Potamogeton malaianus linolenic acid or isomers Scenedesmus obliquus ,
Microcystis aeruginosa
[78]
Potamogeton natans lactone diterpenes,
furano diterpenes
Selenastrum carpricornutum [79,80]
Potamogeton pectinatus labdane diterpenes Selenastrum carpricornutum [81]
Submerged Ceratophyllum demersum instable sulfides,
Nitzschia palea,
[31,32,36,49,87,88,91]
macrophytes
element sulfides
Chlorella pyrenoidosa,
a variety of diatoms,
cyanobacteria, green algae
Vallisneria spiralis 4-oxo-β-ionone,
dihydroactinidiolide,
2-ethyl-3-methylmaldeimide
Microcystis aeruginosa [37]
Nitella sp. Dithiolane Nitzschia palea [49]
Chara sp. 4-methylthio-1,2-dithiolane、
5-methylthio-1,2,3-trithiane
Nitzschia palea [49]
Chara globularis dithiolane, trithiane diatom, green algae,
cyanobacteria
[49,51,57,58,88]
Ruppia maritima ent-labdane diterpenes Chlorella vulgaris,
Selenastrum carpricornutum
[59,60]
Hydrilla erticillata 1,2-benzenedicarboxylic acid
diisooctyl ester, Di-n-butyl
phthalate, N-butyl phthalate 2 -
methyl propyl, etc
Microcystis aeruginosa [61]

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Life styles Species Allelochemicals Algae inhibited References
Myriophyllum spicatum ellagic acid, eugeniin,
globular cyanobacteria, [41,42,82]
pyrogallic acid, gallic acid,
(+)-catechin,nonanoic acid,
tetradecanoic acid, palmitic acid,
octadecanoic acid
octadecenoic acid,
fibrous cyanobacteria
Myriophyllum alterniflorum,
Myriophyllum heterophyllum,
Myriophyllum brasiliense
polyphenol-like alleochemicals cyanobacteria, green algae [84]
Myriophyllum verticillatum, α-asarone, phenylpropane,
glycoside-like allolochemicals
cyanobacteria, green algae [85,86]
(1) Emergent aquatic plants
① Genus Acorus
Della Greca et al. first discovered the genus Acorus could produce a variety of phenolpropane-types of
allelochemicals to inhibit the growth of cyanobacteria [62]. The phenolpropane-types of allelochemicals in
Acrous gramineus (Acorus tatarinowii) mainly included 1,2,4-trimethoxy-5-(E-3'-methyl oxiranyl)
benzene, β-asarone and α-asarone, the latter two of which were in 7:1 of content ratio; However, α-asarone
and β-asarone were primary allelochemicals in Acorus calamus with 6:1 of the content ratio.
β-asarone had good inhibition on several kinds of algae, which has the equivalent inhibitory activity
with copper sulfate (permission dose for biological treatment, 2.5-3.2μM). Moreover, β-asarone was
biodegradable and had selective inhibition on algal growth (Table 3). He et al. also confirmed Acorus
gramineus had algicidal role, during the studies of the distribution of different parts of the plant to the
inhibition on algal growth, they determined the allelochemicals were mainly released by the plant root and
rhizome [63].
Table 3 Inhibitory effects of β-asarone on algae [62]
Algal species
β-asarone
2μM
copper sulfate
0.5μM 10μM
Ankistrodesmus braunii +++ - +++
Chlorella emersonii ++ - +++
Muriella aurantiaca ++ - ++
Stichococcus bacillaris - - +++
Euglena gracilis - - -
Pseudococcomixa simplex - - +++
Scenedesmus quadricauda - - ++
Chlorococcum hypnosporum +++ - +++
Coccomixa elongata - - +++
Chlamydomonas sphagnophila ++ - +++
Chlorella vulgaris - - +++
Nostoc commune - + +++
Synechococcus leopoliensis ++ +++ +++
Anabaena flos-aquae + +++ +++
+++:23-30mm of algicidal circle; ++:15-22mm of algicidal circle;
+:6-14mm of algicidal circle;-:continue to grow
② Genus Zantedeschia
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Invasive aquatic weed Zantedeschia aethiopica had strong inhibition on algae. It extract could inhibit
significantly the growth of Selenastrum capricornutum. Della Greca et al. isolated 25 kinds of
allelochemicals including cycloartane triterpenes, sterols, neolignans, phenylpropanoids, among which
long-chain fatty acids—α-linolenic acid and linoleic acid in a variety of other aquatic plants were also
found. Two new lignin compounds (neolignan 23 and 24) with the equivalent activity to copper sulfate
could generate strong inhibitory effects at 0.1μM (Fig. 1) [64].
③ Genus Juncus
Fig. 1 Allelopathic neolignans from Zantedeschia aethiopica [64]
Juncus effuses had an obvious inhibition on Selenastrum capricornutum,of which the inhibitory
allelochemicals included phenylpropanes, glycerides, dihydrophenanthrenes, and tetrahydropyrene, etc.
The results of activity analysis showed that tetrahydropyrene had the strongest inhibition activity among
them and achieved 90% of inhibition rate at 25μM. In the above-mentioned allelochemicals, the
glycosylation of dihydrophenanthrenes caused no effect even stimulation on algal growth [65, 66]. And
Juncus acutus contained a variety of allelochemicals to inhibit Selenastrum capricornutum, including the
strongest four kinds of 9, 10-dihydrophenanthrenes (structures seen in Fig. 2) of which the concentration
leading to medium effective concentration(EC50) were 11.1, 19.9, 16.8 and 16.2μM, respectively [67], five
types of dimeric phenanthrenoids with stronger inhibition than 9, 10-dihydrophenanthrenoid (Fig. 3) [68].
Fig. 2 Allelopathic 9, 10-dihydrophenanthrene from Juncus acutus [67]
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④ Genus Typha
Fig. 3 Allelopathic dimeric phenanthrenoids from Juncus acutus [68]
Dai et al indicated the ethyl acetate, ethyl ether and acetone extract of the pollen mixture of Typha
latifolia, Typha minima, Typha angustata had good inhibitory effects on Scenedesmus obliquus, Chlorella
vulgaris, among which the ethyl acetate extract had the strongest effect equivalent with that of copper
sulfate. Further analysis found that the ethyl acetate extract contained many kinds of allelochemicals,
including palmitic acid, cholesteryl cis-9-octadecenoate, etc [69]. Individual leachate or aquatic extract of
Typha latifolia could inhibit the growth of Anabaena flos-aquae, Chlorella vulgaris, Chlorella pyrenoidosa,
in which 20(S)-4α-methyl-24-methylenecholest-7-en-3-β-ol was identified as the inhibitory allelochemical
[70,71,91]. Typha domingensis released allelochemicals like 2-chlorophenol, salicylaldehyde, linoleic acid
and α-linolenic acid to inhibit algal growth[72].
⑤ Genus Phragmites
The aquatic extract of Phragmites communis had significant inhibition on Microcystis aeruginosa and
Chlorella pyrenoidosa, with 7.5g/L of EC50 value on Microcystis aeruginosa, much lower than those of the
other 20 species of aquatic plants. Through ethanol extraction, liquid-liquid separation and column
chromatography analysis, Li et al. finally obtained the efficient inhibitory fraction. The GC-MS analysis of
the fraction showed that it only contained four substances. NMR analysis on particular structures
confirmed the 78% of it was ethyl 2-methylacetoacetate (EMA). With the activity of pure substances, the
fraction was confirmed to have EMA as the only active component for inhibitory activity[45].
Nakai et al. carried out in-depth researches about Phragmites communis by comparing its alleolpathic
activities in April, October, and December. Their experimental results showed the October reed had the
best antialgal effect. Based on the best season of reed, the study found that the extract, which mainly
included phenolic acids—p-coumaric acid, ferulic acid, vanillic acid, mustard acid and syringic acid,
caffeic acid, protocatechuic acid, gallic acid, and fatty acids—tetradecanoic acid, palmitic acid, nonanoic
acid, stearic acid, total of 12 kinds of acidic substances; To assay those activities, Phormidium sp. and
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Microcystis aeruginosa were found to be alternatively or both affected by them, in addition to ferulic acid,
stearic acid, palmitic acid with no effects[73].
(2) Floating aquatic plants
① Genus Pistia
The ether extract of Pistia stratiotes had allelopathic inhibition on a variety of cyanobacteria, green
algae, golden algae, red algae, but the chloroform and methanol extract did not have. Through further
separation of the ether extract, linoleic acid, linolenic acid, hydroxy fatty acid, α-asarone, polyphenols,
steroidal ketones algicidal substances were found[74]. The alleochemical α-asarone was also isolated from
genus Acorus, the inhibitory effect of which increased with the increase of concentration and extended the
lag phase of algal cells. The respiration rate of Selenastrum capricornutum was stressed with initial
decrease and then increase, the number of mitochondria was increased and the intracellular electron dense
bodies were accumulated by α-asarone[73]. Li reported the aquatic extract of Pistia stratiotes inhibited the
growth of Chlorella pyrenoidosa with 20g/L of EC50[91].
② Genus Eichhornia
Sun et al. indicated that the root exudates of Eichhornia crassipes could significantly inhibit the growth
of Scenedesmus sp. and Chlamydomonas reinhardtii. To collect the exudates for analysis (see Fig. 4),
N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine, linoleic acid, and linoleic acid were found to have
good inhibition effects and considered as the main allelochemicals of Eichhornia crassipes[38, 75]. Della
Greca also conducted a similar study and reported that Eichhornia crassipes obviously inhibited
Porphyridium aerugineum and Anabaena azollae, for which phenalene-like allelochemicals such as
benzoindenone, dimeric phenalene and phenalene played the major roles[76, 77].
③ Genus Potamogeton
Fig. 4 Allelopathic phenylnaphthylamines from Eichhornia crassipes [75]
Wu studied the effects of Potamogeton malaianus, Potamogeton maackianus, Potamogeton crispus on
Scenedesmus obliquus and Microcystis aeruginosa, and observed that the alleopathic strength was relevant
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with the initial algal density. When the initial algal density was too low, the alleopathy was weak even
disappeared. Wu et al. also pointed out the initial algal density could affect the alleopathic effect of
Potamogeton malaianus[78].
The analysis of the non-polar extract of Potamogeton malaianus showed that its major active substance
for the inhibition was unsaturated fatty acids, or might be linolenic acid or isomers with the molecular
weight of 278 in the formula of C18H30O2; This substance had only 25% of inhibition rate at 24.2mg/L on
Selenastrum carpricornutum, which was less effective compared with many other aquatic plant
allelochemicals. Failure to get highly efficient allelochemicals might be due to select the inefficient
extraction.
Cangiano et al. found the petroleum ether, ethyl acetate and methanol extracts of Potamogeton natans all
inhibited Selenastrum carpricornutum to a certain extent. Isolation and identification of allelochemicals
from the extracts showed that all had ineffective lactone diterpenes, of which EC50s were more than 40μM,
and even some of which promoted algal growth at low concentration[79]. Failure to get highly effective
allelochemicals might be due to the separation methods.
Della Greca et al. separated the petroleum ether and ethyl acetate extracts of Potamogeton natans and
identified furano diterpenes, in which EC50 of substances 4 and 5 were 4.40 and 2.84μM, respectively (see
Fig. 5)[80]. Waridel et al. found the polar extract of Potamogeton pectinatus could significantly inhibit the
growth of Selenastrum carpricornutum. Further analysis indicated the extract had several abdane
diterpenes, in which the EC50s of labdane diterpene 5, 4, 1, and 6 were 6.1, 17.2, 18.2, and 47.1μM,
respectively (see Fig. 6)[81].
Fig. 5 Allelopathic furano diterpenes from Potamogeton natans [80]
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(3)Submerged aquatic plants
① Genus Myriophyllum
Fig. 6 Allelopathic abdane diterpenes from Potamogeton pectinatus [81]
Gross et al. reported that the acetone extract of Myriophyllum spicatum had remarkable inhibition on
many globular and fibrous cyanobacteria but was ineffective on many green algae and diatoms. The extract
analysis showed that hydrolytic tannins, ellagic acid (EA), and many polyphenols occupied the great
content, in which eugeniin as the main allelochemical was 1.5% of the dry weight of the extract[82].
Nakai et al. studied the leachate of Myriophyllum spicatum with similar results, especially discovered
the water-soluble inhibitory allelochemicals in the secretions, of which the molecular weights were less
than 1000 Dalton. Based on the analysis of HPLC and APCI-MS, pyrogallic acid (PA), gallic acid (GA),
(+)-catechin (CATECH) were identified[41]. At the earilier year, Planas et al. also pointed out the PA, GA,
and EA existed but CATECH was firstly discovered by Nakai in Myriophyllum spicatum[83]. Subsequently,
Nakai et al. investigated the left organic fraction obtained as above and nonanoic acid, tetradecanoic acid,
palmitic acid, octadecanoic acid and octadecenoic acid, a variety of fatty-acid-like allelochemicals were
identified in it, of which nonanoic acid, cis-6-octadecenoic, and cis-9-octadecenoic acids had the strongest
inhibitory activities[42]. Majority of the phenolic and fatty acids discovered above are by far the strongest
allelochemicals, of which EC50s were seen in Table 4.
Gross reported the plants of genus Myriophyllum, Myriophyllum alterniflorum and Myriophyllum
heterophyllum could excrete polyphenol-like alleochemicals like Myriophyllum verticillatum,
Myriophyllum brasiliense, and Myriophyllum spicatum to inhibit the growth of green algae and
cyanobacteria[84], which might be due to the close genetic relationship among the plant species in the
same genus (Gross et al, unpublished). In addition to polyphenols, α-asarone, phenylpropane
glycoside-like allolochemicals were identified from Myriophyllum verticillatum[85, 86].
Table 4 EC 50 values of allelochemicals from Myriophyllum spicatum on Microcystis aeruginosa
Allelochemicals EC 50(mg/L) References
gallic acid 1.0
pyrogallic acid 0.65
ellagic acid 5.1
(+)-catechin 5.5
nonanoic acid 0.5±0.3
cis-9-octadecenoic acids 1.6±0.4
cis-6-octadecenoic acids 3.3±0.4
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② Genus Ceratophyllum
Ceratophyllum demersum could produce instable sulfides to inhibit the growth of phytoplankton.
Follow-up studies showed the instable sulfides were oxidized to produce element sulfur and also inhibited
Nitzschia palea, Chlorella pyrenoidosa, and a variety of diatoms, green algae and cyanobacteria. The
modes of action of the instable sulfides or element sulfur might affect the activity of photosystem II,
carbon fixation process thereby to decrease assimilation of algal cells [31, 32, 36, 49, 87, 88, 91].
③ Genus Vallisneria
Xian et al. found the leaves of Vallisneria spiralis had effective inhibition on Microcystis aeruginosa.
The chloroform extract had 91% of inhibition rate on algal growth, which was stronger than petroleum
ether, ethyl acetate extract, and n-butanol extract even with only 15% of inhibition rate. By silica gel
column chromatography to separate the chloroform extract, seven purified fractions were obtained, among
which the activity of fraction A was strongest; Three fractions were collected after further purification by
column chromatography on fraction A, among which fraction A1 had the best algicidal activity. Using
high-resolution GC-MS to analyze the chemical composition of fraction A1, 4-oxo-β-ionone(4%),
dihydroactinidiolide(18.9%) and 2-ethyl-3-methylmaldeimide(77.1%) were identified.
2-ethyl-3-methylmaldeimide was chlorophyll-oxidation products, a kind of alkaloid, the inhibitory activity
of which was the strongest in the above-mentioned substances[37].
3 Modes of action of allelochemicals from aquatic macrophytes on algae
Einhellig suggested that the allelochemical separation and identification be a bottleneck to block the study
of modes of action of allelochemicals [43]. The field of allelopathy of aquatic macrophytes on algae also
validates this point. The reports on the modes of action of allelochemicals from aquatic macrophytes (e.g.
Myriophyllum spicatum, Phragmites communis, Acorus tatarinowii, Eichhornia crassipes, Ceratophyllum
demersum) on algae are still limited without systematical and coherent outcome [44, 45]. From reports,
there are four aspects of possible modes of action.
(1) To destroy the cell structure of algae
Allelochemicals from Phragmites communis could destroy the membrane structure of Chlorella
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pyrenoidosa and Microcystis aeruginosa. By analyzing the types and relative content of phospholipid fatty
acids in algal cell membrane, Li et al. found that unsaturated fatty acid contents of C18:3 and C18:2 in
Chlorella pyrenoidosa were increased from 23.31%, 11.46% to 37.68%, 25.91%, respectively. And the
unsaturated fatty acid contents of C18:1 and C18:2 of Microcystis aeruginosa were increased from 30.26%,
18.85% to 42.88%, and 28.46%, respectively. The proportion increase of unsaturated fatty acids in the
algal cell membrane directly caused the increase of fluidity of cell membrane, the decrease of substance
selectivity of cell membrane, the decrease of cell stability and the leakage of intracellular contents (such as
K + ,Mg 2+ ,Ca 2+ ). In addition to the damage of cell membrane, the cellular thylakoid lamellar structure
disappeared; the nucleolus area became irregular, starch grains increased, and the volume of vacuole
increased. The allelochemicals of Phragmites communis did not inhibit Chlorella vulgaris, accordingly, in
which the relative content of fatty acids of the cell membrane did not change and the cell structure was
intact, aside from a slight abruption between cell wall and membrane and the volume increase of starch
grains[91].
Men et al. found the algae-algae conjointed structure reduced significantly, nucleus disappeared and cell
organs like mitochondria disintegrated after inspecting the influence of allelochemicals from Phragmites
communis on Scenedesmus obliquus[92]. A lot of irregular fragments of broken cells formed when the root
exudates of Pistia stratiotes inhibited Chlamydomonas reinhardtii [35]. Microscopic observations on the
effects of the exudates of Hydrilla verticillata on Microcystis aeruginosa showed that the cell membrane
apparently separated from the cell wall and thylakoid lamellae became loose and cluttered and the damage
effects aggravated with the extension of time [93].
(2) To affect the photosynthesis of algal cells
Damage on the photosynthesis of algal cells can be divided into damage on light reaction (thylakoid
reaction) and damage on dark reaction (carbon fixation reaction). The root exudates of Eichhornia
crassipes were able to disintegrate chloroplast of Scenedesmus. The content of chlorophyll a decreased
significantly and the content of its degradation products (chlorophyll a esters without magnesium)
increased. The photosynthetic rate dropped sharply, resulting in serous damage on thylakoid reaction[34].
When Hydrilla verticillata and Microcystis aeruginosa were co-cultured, the chlorophyll a content of
Microcystis aeruginosa cells was significantly lower than that of the control (after 9d of cultivation, only
12.5% of the control)[93]. The exudates of Ceratophyllum demersum and Myriophyllum spicatum could
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suppress the growth of several species of algae by inhibiting their photosystem II[88]. Leu et al. indicated
that eugeniin inhibited the electron-transfer process between the acceptor and donor in the photosystem II
when investigating the mode of action of polyphenol-like allelochemicals from Myriophyllum spicatum
[44]. Wium-Andersen et al. reported that the unstable sulfide or sulfur exudates of Chara globularis could
inhibit the dark reaction process(carbon fixation process) in the photosynthesis of diatoms and other
phytoplankton[58,87].
At present many kinds of aquatic plants has been discovered to be able to suppress the algal
photosynthetic process. For example, Stratiotes aloides affected the photosynthesis of Scenedesmus
obliquus[49,56], Berula erecta and Nitella sp. inhibited the photosynthesis of Nitzschia palea[49], Chara
could inhibited the photosynthesis of several algal species[51, 89].
(3) To affect the respiration of algal cells
The allelochemical α-asarone of Acorus tatarinowii reduced the growth rate of Selenastrum
capricornutum. Through observation of cell ultrastructure, the increase in the number of algal
mitochondria was found, base on which the cellular respiration was speculated to be affected; the results of
assay pointed out that after 48h of cultivation, the algal respiration rate with α-asarone treatment was only
60% of the control, but the algae increased the respiratory rate to 120% of control later. Pollio et al.
considered the cell respiration was destroyed, the decrease in phosphorus/oxygen ratio(adenosine
diphosphate/oxygen, ADP/O) might cause the decoupling of oxidative phosphorylation, so even if the
respiration rate and the number of mitochondria was increased, but the efficiency of cell respiration has not
improved [86]. During the inhibition process of the allelochemicals of Phragmites communis on algae, the
damage of cell respiration was also observed. That the respiration rate was increased early at low
concentration but decreased at high concentration was observed, but with the extension of time the
respiration rate gradually decreased[91].
(4) To affect the enzymatic activity of algal cells
Different allelochemicals could affect the activities of different enzymes in algal cells. Because of the
functional differences of enzymes, allelochemicals could increase some enzymatic activities but inhibited
other enzymatic activities. Eugeniin from Myriophyllum spicatum, as well as several other phenolic
acid-like allelochemicals could effectively inhibited the alkaline phosphatase activity in algal cells, and
mixed phenolic acid-like allelochemcials showed stronger inhibition than anyone of them[41,82]. The
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allelochemicals of Phragmites communis reduced the activities of superoxide dismutase (SOD) and
peroxidase (POD), to decline the ability to remove reactive oxygen species, and disorder the cell redox
state, to cause cell death[45]. The exudates of Eichhornia crassipes stressed the SOD activity of
Scenedesmus from initial increase to gradual decrease, but increased POD activity all through[48].
4 Possible application ways for algal-bloom control using allelopathy
The potential ways for utilizing allelochemicals of aquatic macrophytes contain transplanting live plants,
delivering harvested and dried plants, extracting allelochemicals from plants and synthesizing
allelochemicals with natural structures.
Transplanting live plants is suitable for small water bodies like landscape ponds. It can satisfy the
requirement of algae inhibition and beautify the environment. The management of aquatic plants is also
easy because of the small area. However, transplanting live plants costs a lot of time and labor and the
plants are affected easily and greatly by season change. The method is effective slow and timely poor. So
the plants may not be able to suffer from algal bloom. On the other hand it is not suitable for large water
area because the plants may reproduce excessively.
In the decay process, the plants may release allelochemicals and provide habitat places for fish, shrimp,
and aquatic organisms. However, this method spoils the beauty of environment. And the release of
nutrients in the process could cause the secondary pollution and trigger other environmental problems.
Extracting allelochemicals from plants and applying them into water bodies is a promising method
because of its easy control and quick effect. Allelochemicals from macrophytes are natural algicides. Rice
claimed that allelochemicals might replace the man-made herbicides, at least could greatly reduce the
amount of herbicide usage[27]. Therefore, extraction, separation, identification and synthesis of
allelochemicals can provide new algicides for algal-bloom control. Combination with other recycle
methods, such as papermaking, methane fermentation system, etc can solve the problem of residue
disposal.
Synthetic allelochemicals can make up biomass shortage of aquatic plants. And the efficiency of
synthetic allelochemicals can be improved during the synthesizing procedure to satisfy the requirement of
algal-bloom control. Therefore, the synthetic method will become a future development direction.
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There are much more researches and successful cases of allelochemicals in the world. But most of them
focus on the control of agricultural weeds, such as cineole[94], biological toxin from rhizobium of
bean[95], strigol[96] and agrostemin[97]. However, there are no practical cases of allelochemicals from
aquatic macrophytes and the relevant studies still needs to be done.
5 The prospect of using aquatic macrophytes to control algal bloom
In recent years, the research of the allelopathy of aquatic macrophytes on freshwater algae is developing
very fast. With many findings on new allelochemicals, it will become the most promising method to
control algal bloom. However, there still exist some important problems to be solved before it can be used
in practice.
(1) The existing studies mainly focus on floating and submerged macrophytes, less studies on the
emergent macrophytes. The investigation on the allelopathic activities of the macrophytes with abundant
biomass is still extremely limited.
(2) Some allelochemicals have been identified from macrophytes, but the highly-effective
allelochemicals are still very few. So it still will be an important issue to search for highly-effective
allelochemicals from aquatic macrophytes.
(3) The investigation on the modes of action of allelochemical inhibition in the field of allelopathy of
aquatic macrophytes on algae is less sufficient than in other fields. The insufficiency of the study will
weaken and block the development on the other aspects. It could become the bottleneck of the practicality
of allelochemical inhibition on algae. The studies on the modes of action of allelochemicals will help
modify the structure of allelochemicals artificially and adjust their activities to be much more efficient.
(4) Most of the existing researches are carried out in lab scale. The lab-scale system was too simple to
reflect the actual circumstances of field. In actual waterbodies, algal species diversity in ‘microcosm’ exists,
accompanying with other aquatic organisms. So how allelochemicals affect the community structure of
aquatic organisms (including algae) as well as how they migrated and transformed in waterbodies are the
important problems for the understandings about the effects of allelochemicals on the aquatic ecosystem.
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