it is difficult to define unified criteria to compare the damage caused by
cyanobacteria among different target organisms.
3.3
Impact of Signaling Molecules
Recent research indicates that signaling molecules might be important in regulating
the production of secondary metabolites in cyanobacteria and that their allelopathic
activity can be enhanced in the presence of the target species. Ma et al. [113] found
that the allelopathic effect of Microcystis sp. was inducible by observing that cocultured Microcystis strains have stronger inhibitory effects on Aphanizomenon.
Detecting the signaling molecules, however, is difficult; only a study deciphered
both signal molecules and allelochemicals in a dynamic system. Song et al. [81]
found that nitric oxide (NO) acted as a signal molecule released from the target green
alga Chlorella vulgaris. This molecule triggered the release of the allelochemical
linoleic acid in M. aeruginosa cells.
4
Interfering Factors
Environmental factors can significantly change the growth conditions for
cyanobacteria and thus their production and release of allelochemicals. In addition,
allelopathic activities of cyanobacteria can vary depending on their growth phase
and initial density.
4.1
Biotic Factors
Zhang et al. [114] found evidence for a strong impact of initial cell density and thus
biomass on the allelopathic effect of Microcystis on macrophytes. The initial biomass ratio determined the outcome of competition tests between the two both bloomforming cyanobacteria Microcystis sp. and Anabaena sp. [68].
Different growth phases of cyanobacteria populations resulted in the release of
different secondary metabolites, and cells exhibited the strongest allelopathic potential in specific developmental periods. Volk [101] isolated three potential
allelochemicals from two different growth phases of a Nostoc strain. The one
secreted during the linear phase is non-toxic to eukaryotic cells, but the two
chemicals isolated from the stationary phase possessed toxicity to a human amniotic
epithelial cell line [100]. Other studies seem to confirm that allelopathic effects from
the exponential growth phase of cyanobacteria were the strongest compared to other
developmental stages independent of the target species [28, 29, 36, 115]. Interestingly, several studies indicated that some cyanobacterial strains stimulated the target
organisms to some extent during the cyanobacteria decline phase [28, 115]. These
results support earlier findings of Keating (1977) who found that different
14 Effects of Cyanobacterial Secondary Metabolites on Phytoplankton Community. . . 335
cyanobacteria among different target organisms.
3.3
Impact of Signaling Molecules
Recent research indicates that signaling molecules might be important in regulating
the production of secondary metabolites in cyanobacteria and that their allelopathic
activity can be enhanced in the presence of the target species. Ma et al. [113] found
that the allelopathic effect of Microcystis sp. was inducible by observing that cocultured Microcystis strains have stronger inhibitory effects on Aphanizomenon.
Detecting the signaling molecules, however, is difficult; only a study deciphered
both signal molecules and allelochemicals in a dynamic system. Song et al. [81]
found that nitric oxide (NO) acted as a signal molecule released from the target green
alga Chlorella vulgaris. This molecule triggered the release of the allelochemical
linoleic acid in M. aeruginosa cells.
4
Interfering Factors
Environmental factors can significantly change the growth conditions for
cyanobacteria and thus their production and release of allelochemicals. In addition,
allelopathic activities of cyanobacteria can vary depending on their growth phase
and initial density.
4.1
Biotic Factors
Zhang et al. [114] found evidence for a strong impact of initial cell density and thus
biomass on the allelopathic effect of Microcystis on macrophytes. The initial biomass ratio determined the outcome of competition tests between the two both bloomforming cyanobacteria Microcystis sp. and Anabaena sp. [68].
Different growth phases of cyanobacteria populations resulted in the release of
different secondary metabolites, and cells exhibited the strongest allelopathic potential in specific developmental periods. Volk [101] isolated three potential
allelochemicals from two different growth phases of a Nostoc strain. The one
secreted during the linear phase is non-toxic to eukaryotic cells, but the two
chemicals isolated from the stationary phase possessed toxicity to a human amniotic
epithelial cell line [100]. Other studies seem to confirm that allelopathic effects from
the exponential growth phase of cyanobacteria were the strongest compared to other
developmental stages independent of the target species [28, 29, 36, 115]. Interestingly, several studies indicated that some cyanobacterial strains stimulated the target
organisms to some extent during the cyanobacteria decline phase [28, 115]. These
results support earlier findings of Keating (1977) who found that different
14 Effects of Cyanobacterial Secondary Metabolites on Phytoplankton Community. . . 335