Light intensities affect the growth of cyanobacteria, and it has been demonstrated
that under different light intensities, the inhibitory effect of Cylindrospermopsis sp.
varied considerably with higher light intensities enhancing the effect [125]. This
property is highly species-specific, and strains with a broader absorption spectrum
had a competitive advantage under light limitation [126]. Chia et al. [119] found that
Microcystis sp. under the influence of cylindrospermopsin increased their microcystin content when light was limiting and decreased it when light intensity was
optimum. Not only the concentration of secondary metabolites but also their varieties are changing with changing light conditions. Walsh et al. [96] documented that
volatile organic compounds (VOCs) of Microcystis sp. showed multiple patterns
under various light and iron levels. In their study, light and nutrient level seemed to
trigger the production of certain VOCs, but the functions of these compounds have
not been thoroughly deciphered yet.
Preussel et al. [71] investigated the combined effect of light and temperature.
Lower temperature increased the release of cylindrospermopsin with increasing light
intensity, whereas at 25
C, cylindrospermopsin release decreased with higher light
intensities.
The pH of water has been shown to positively affect the secretion of an algicide
by filamentous cyanobacterium Oscillatoria sp. [127], and nutrient depletion could
also enhance the allelopathic activity of cyanobacteria. Under phosphorus limitation,
Cylindrospermopsis sp. exhibited an enhanced inhibitory activity toward
Chlorophyta [125]. In Oscillatoria sp. the release of algicides was increased by a
depletion of magnesium and phosphorus [127]. Apart from the total amount of
allelochemicals, their composition was influenced by nutrient limitation [34]. As a
consequence, high nutrient levels facilitated toxic cyanobacterial strains [124].
5
Conclusions
Allelochemicals produced and released by cyanobacteria potentially provide them
with a competitive advantage due to their inhibiting effects on other members of the
phytoplankton community and due to indirect effects on organisms of different
higher trophic levels that control phytoplankton via a top-down cascade.
A huge variety of chemicals are produced and released from cyanobacterial cells.
Some of these substances, especially those that are toxic to humans and occur during
bloom events such as microcystins and saxitoxins, have been studied intensively.
However, many more potential allelochemicals such as peptides, terpenoids, alkaloids, phenyls, and others are produced by cyanobacteria. These play yet unknown
roles in the succession of natural phytoplankton communities. The most commonly
detected modes of action of these substances are photosynthesis inhibition and the
formation of reactive oxygen species, while recent proteomics and metabolomics
approaches also indicated an influence on several metabolic pathways. Several
studies have manifested that the allelopathic effects of cyanobacteria are inducible.
The presence of competitors or signaling molecules triggered the release of
allelochemicals in cyanobacteria. All these processes are regulated by various biotic
14 Effects of Cyanobacterial Secondary Metabolites on Phytoplankton Community. . . 337
that under different light intensities, the inhibitory effect of Cylindrospermopsis sp.
varied considerably with higher light intensities enhancing the effect [125]. This
property is highly species-specific, and strains with a broader absorption spectrum
had a competitive advantage under light limitation [126]. Chia et al. [119] found that
Microcystis sp. under the influence of cylindrospermopsin increased their microcystin content when light was limiting and decreased it when light intensity was
optimum. Not only the concentration of secondary metabolites but also their varieties are changing with changing light conditions. Walsh et al. [96] documented that
volatile organic compounds (VOCs) of Microcystis sp. showed multiple patterns
under various light and iron levels. In their study, light and nutrient level seemed to
trigger the production of certain VOCs, but the functions of these compounds have
not been thoroughly deciphered yet.
Preussel et al. [71] investigated the combined effect of light and temperature.
Lower temperature increased the release of cylindrospermopsin with increasing light
intensity, whereas at 25
C, cylindrospermopsin release decreased with higher light
intensities.
The pH of water has been shown to positively affect the secretion of an algicide
by filamentous cyanobacterium Oscillatoria sp. [127], and nutrient depletion could
also enhance the allelopathic activity of cyanobacteria. Under phosphorus limitation,
Cylindrospermopsis sp. exhibited an enhanced inhibitory activity toward
Chlorophyta [125]. In Oscillatoria sp. the release of algicides was increased by a
depletion of magnesium and phosphorus [127]. Apart from the total amount of
allelochemicals, their composition was influenced by nutrient limitation [34]. As a
consequence, high nutrient levels facilitated toxic cyanobacterial strains [124].
5
Conclusions
Allelochemicals produced and released by cyanobacteria potentially provide them
with a competitive advantage due to their inhibiting effects on other members of the
phytoplankton community and due to indirect effects on organisms of different
higher trophic levels that control phytoplankton via a top-down cascade.
A huge variety of chemicals are produced and released from cyanobacterial cells.
Some of these substances, especially those that are toxic to humans and occur during
bloom events such as microcystins and saxitoxins, have been studied intensively.
However, many more potential allelochemicals such as peptides, terpenoids, alkaloids, phenyls, and others are produced by cyanobacteria. These play yet unknown
roles in the succession of natural phytoplankton communities. The most commonly
detected modes of action of these substances are photosynthesis inhibition and the
formation of reactive oxygen species, while recent proteomics and metabolomics
approaches also indicated an influence on several metabolic pathways. Several
studies have manifested that the allelopathic effects of cyanobacteria are inducible.
The presence of competitors or signaling molecules triggered the release of
allelochemicals in cyanobacteria. All these processes are regulated by various biotic
14 Effects of Cyanobacterial Secondary Metabolites on Phytoplankton Community. . . 337