of humans and livestock poisoned by cyanobacterial bloom have been documented
since long [11, 12, 37]. Most studies have focused on microcystins, hepatotoxic
cyclic peptides that have been restricted by WHO to 1 μg L
À1 for drinking water
[38]. However, cyanobacteria have also been observed to adversely affect other
organisms by releasing microcystins but also other secondary metabolites.
2.2.1 Bacteria
Heterotrophic bacteria can be strong competitors of phytoplankton and thus also affect
their succession, in particular in environments where phytoplankton production is
limited by the availability of mineral nutrients (e.g., Bratbak and Thingstad [39]).
Some cyanobacteria have been observed to suppress the growth of gram-negative and
positive strains [40]. Metabolites from filamentous cyanobacteria Lyngbya sp.
inhibited the model bacteria genus Bacillus, but no effects were found on other
bacteria taxa, such as Pseudomonas and Streptococcus [41]. This result indicates
that the inhibitory effects on bacteria are also target-specific. Allelochemicals from
the cyanobacterium Nostoc insulare inhibited both gram-positive and –negative species of bacteria at the level of μg/mL [42]. The bioactive metabolites from Phormidium
sp. strongly retrained the taxa of Acidobacteria subgroup 6 and Gemmatimonadetes
when both groups formed natural community assemblages. They were totally
vanished in the allelochemical treatment group while strains of Rhodospirillaceae
and members of Flectobacillus considerably increased [43].
2.2.2 Macrophytes
Aquatic macrophytes can strongly affect phytoplankton abundance and thus potentially
also their succession through several different direct and indirect mechanisms. These
interactions can result in the stabilization of clear-water conditions and the occurrence
of alternative stable states in different aquatic ecosystems [44–46]. Mohamed [47]
recently reviewed the available literature on macrophyte-cyanobacterial allelopathic
interactions and found studies on allelopathic activities of cyanotoxins affecting more
than ten different emerged and submerged macrophyte species. He concluded that
although most studies were conducted at concentrations beyond environmental relevance, there are still indications for harmful allelopathic effects of microcystins on
macrophytes under realistic in situ conditions. Cyanobacterial allelochemicals have
been found to affect seedling germination, seedling growth, and leaf photosynthesis of
macrophytes. Photosynthesis of seedling leaves seems the most sensitive to
cyanobacterial allelochemicals [48]. Some studies indicate interesting mutual allelopathic effects among cyanobacteria and macrophytes, e.g., Xu et al. [49] showing
negative effects of exudates of Microcystis sp. on seedling vitality and growth of two
variants of the submerged macrophyte species Ottelia acuminata, while the culture
water of mature macrophytes promoted the growth of cyanobacteria.
2.2.3 Zooplankton
Grazing of phytoplankton by zooplankton is one of the main factors influencing
seasonal phytoplankton succession [1]. Generalist and tolerant grazers may reduce
cyanobacterial blooms [50], while selective and tolerant grazers are expected to
14 Effects of Cyanobacterial Secondary Metabolites on Phytoplankton Community. . . 327
since long [11, 12, 37]. Most studies have focused on microcystins, hepatotoxic
cyclic peptides that have been restricted by WHO to 1 μg L
À1 for drinking water
[38]. However, cyanobacteria have also been observed to adversely affect other
organisms by releasing microcystins but also other secondary metabolites.
2.2.1 Bacteria
Heterotrophic bacteria can be strong competitors of phytoplankton and thus also affect
their succession, in particular in environments where phytoplankton production is
limited by the availability of mineral nutrients (e.g., Bratbak and Thingstad [39]).
Some cyanobacteria have been observed to suppress the growth of gram-negative and
positive strains [40]. Metabolites from filamentous cyanobacteria Lyngbya sp.
inhibited the model bacteria genus Bacillus, but no effects were found on other
bacteria taxa, such as Pseudomonas and Streptococcus [41]. This result indicates
that the inhibitory effects on bacteria are also target-specific. Allelochemicals from
the cyanobacterium Nostoc insulare inhibited both gram-positive and –negative species of bacteria at the level of μg/mL [42]. The bioactive metabolites from Phormidium
sp. strongly retrained the taxa of Acidobacteria subgroup 6 and Gemmatimonadetes
when both groups formed natural community assemblages. They were totally
vanished in the allelochemical treatment group while strains of Rhodospirillaceae
and members of Flectobacillus considerably increased [43].
2.2.2 Macrophytes
Aquatic macrophytes can strongly affect phytoplankton abundance and thus potentially
also their succession through several different direct and indirect mechanisms. These
interactions can result in the stabilization of clear-water conditions and the occurrence
of alternative stable states in different aquatic ecosystems [44–46]. Mohamed [47]
recently reviewed the available literature on macrophyte-cyanobacterial allelopathic
interactions and found studies on allelopathic activities of cyanotoxins affecting more
than ten different emerged and submerged macrophyte species. He concluded that
although most studies were conducted at concentrations beyond environmental relevance, there are still indications for harmful allelopathic effects of microcystins on
macrophytes under realistic in situ conditions. Cyanobacterial allelochemicals have
been found to affect seedling germination, seedling growth, and leaf photosynthesis of
macrophytes. Photosynthesis of seedling leaves seems the most sensitive to
cyanobacterial allelochemicals [48]. Some studies indicate interesting mutual allelopathic effects among cyanobacteria and macrophytes, e.g., Xu et al. [49] showing
negative effects of exudates of Microcystis sp. on seedling vitality and growth of two
variants of the submerged macrophyte species Ottelia acuminata, while the culture
water of mature macrophytes promoted the growth of cyanobacteria.
2.2.3 Zooplankton
Grazing of phytoplankton by zooplankton is one of the main factors influencing
seasonal phytoplankton succession [1]. Generalist and tolerant grazers may reduce
cyanobacterial blooms [50], while selective and tolerant grazers are expected to
14 Effects of Cyanobacterial Secondary Metabolites on Phytoplankton Community. . . 327