Cyanobacteria are also able to synthesize aromatic compounds. Aromatic compounds from the genus Nostoc have conspicuous algicidal activity to cyanobacterial
species but moderate antibacterial and antifungal activities [42, 79]. Cyanobacterin
is another putative allelochemical, and inhibitory activities were observed toward
cyanobacteria, green algae, and a diatom (Table 1) with effects on oxygen evolution
during photosynthesis as a possible mechanism [80]. High carbon number alkanes
such as linoleic acid and polymethoxy-1-alkenes were also found to act as
allelochemicals. These metabolic products inhibited the growth of green algae and
the development of zebrafish embryos [81, 82]. A study by Jaja-Chimedza et al. [83]
indicated that polymethoxy alkenes are widely distributed in various species
of aquatic microalgae.
Many of the chemicals isolated from cyanobacteria have not yet been tested for
inhibitory effects in aquatic ecosystems (Table 1) but seem worth paying attention to
in future research.
3.2
Modes of Action
Cyanobacterial secondary metabolites exert their effects in target organisms by
several modes of action. In general, these mechanisms include changes in the
morphology (e.g., membrane destruction and cell lysis) and inhibition of processes
involved in photosynthesis and oxidative stress.
The presence of cyanobacteria often inhibits the growth of target organisms due
to multiple reasons. Observations through electron microscopes indicated damages
on the membranes of some sensitive species [108]. In other cases, cell differentiation
and heterocyst and akinete formation of filamentous algae were influenced by
extracts of cyanobacterial cells [33]. Effects on photosynthetic activity were tested
using different approaches. Cyanobacteria lowered the electron transport rate
(ETR max ) of some target species [22, 78], inhibited the activity of photosystem II
(e.g., Gross et al. [84]), or lowered their oxygen evolution [34, 80, 108, 111].
Allelopathic cyanobacteria can also cause oxidative stress inside of the affected
species cells which has been tested by either measuring the occurrence of reactive
oxygen species (ROS) or antioxidative enzymes. Mostly, the presence of
cyanobacterial allelochemicals contributed to an increase in ROS and stimulated
antioxidative enzyme systems and/or low molecular weight antioxidants [78, 112].
Certain compounds directly affected nucleotide synthesis. Doan et al. [103] found
that compounds from two cyanobacterial genera inhibited the RNA synthesis of
target bacteria and in vitro tests manifested that this resulted from effects on the RNA
polymerase. Rzymski et al. [72] suggested that the alkaline phosphatase (ALP)
activity was affected by a Cylindrospermopsis strain. Song et al. [81] were able to
show that M. aeruginosa allelopathy had an impact on multiple metabolic pathways
involved in energy generation and metabolism, including glycolysis, carbon fixation,
and fatty-acid biosynthesis using proteomics and metabolomics analyses. Due to
these different approaches to measure the effects of cyanobacteria on target species,
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Y. Pei et al.
species but moderate antibacterial and antifungal activities [42, 79]. Cyanobacterin
is another putative allelochemical, and inhibitory activities were observed toward
cyanobacteria, green algae, and a diatom (Table 1) with effects on oxygen evolution
during photosynthesis as a possible mechanism [80]. High carbon number alkanes
such as linoleic acid and polymethoxy-1-alkenes were also found to act as
allelochemicals. These metabolic products inhibited the growth of green algae and
the development of zebrafish embryos [81, 82]. A study by Jaja-Chimedza et al. [83]
indicated that polymethoxy alkenes are widely distributed in various species
of aquatic microalgae.
Many of the chemicals isolated from cyanobacteria have not yet been tested for
inhibitory effects in aquatic ecosystems (Table 1) but seem worth paying attention to
in future research.
3.2
Modes of Action
Cyanobacterial secondary metabolites exert their effects in target organisms by
several modes of action. In general, these mechanisms include changes in the
morphology (e.g., membrane destruction and cell lysis) and inhibition of processes
involved in photosynthesis and oxidative stress.
The presence of cyanobacteria often inhibits the growth of target organisms due
to multiple reasons. Observations through electron microscopes indicated damages
on the membranes of some sensitive species [108]. In other cases, cell differentiation
and heterocyst and akinete formation of filamentous algae were influenced by
extracts of cyanobacterial cells [33]. Effects on photosynthetic activity were tested
using different approaches. Cyanobacteria lowered the electron transport rate
(ETR max ) of some target species [22, 78], inhibited the activity of photosystem II
(e.g., Gross et al. [84]), or lowered their oxygen evolution [34, 80, 108, 111].
Allelopathic cyanobacteria can also cause oxidative stress inside of the affected
species cells which has been tested by either measuring the occurrence of reactive
oxygen species (ROS) or antioxidative enzymes. Mostly, the presence of
cyanobacterial allelochemicals contributed to an increase in ROS and stimulated
antioxidative enzyme systems and/or low molecular weight antioxidants [78, 112].
Certain compounds directly affected nucleotide synthesis. Doan et al. [103] found
that compounds from two cyanobacterial genera inhibited the RNA synthesis of
target bacteria and in vitro tests manifested that this resulted from effects on the RNA
polymerase. Rzymski et al. [72] suggested that the alkaline phosphatase (ALP)
activity was affected by a Cylindrospermopsis strain. Song et al. [81] were able to
show that M. aeruginosa allelopathy had an impact on multiple metabolic pathways
involved in energy generation and metabolism, including glycolysis, carbon fixation,
and fatty-acid biosynthesis using proteomics and metabolomics analyses. Due to
these different approaches to measure the effects of cyanobacteria on target species,
334
Y. Pei et al.