Our results show that high concentrations of exudates and extracts of M. aeruginosa can allelopathically inhibit both seed ger- mination and the early growth and photosynthesis of P. malaianus seedlings. As we did not work with axenic seeds or seedlings, we assume our experiment was subject to the interference of bacte- ria, via the degradation of exudates and extracts (Babica et al.,2005). These interactions, however, would also be expected to occur under natural conditions. The chemical nature of the sub- stances involved in the observed effects has yet to be resolved. Microcystin-LR, one of the main cyanobacterial secondary metabo- lites, was shown to have negative effects on the growth and photosynthetic oxygen production of some submerged macro- phyte species (Pflugmacher, 2002). However, Sukenik et al. (2002) excluded the possibility that retardation of photosynthesis and growth of the dinoflagellate Peridinium gatunense by Microcystis sp. were due to the presence of microcystins, and instead suggested a different, thermally stable, relatively hydrophobic component with a molecular weight <5 kDa. In our study, exudates were more active than extracts, making the involvement of microcystin rather unlikely. Also, no free microcystin could be detected in the waterof Lake Dianchi during most of 2011, except for May (0.081 g L−1 )and December (0.195 g L−1 ), whereas intracellular microcystinconcentrations were high between April and December, with max- imum concentrations of 7.19 g L−1 in June (Bao, 2012). Extracts
and exudates were applied at in situ concentrations. Li et al. (2005)
and Wan et al. (2008) reported Microcystis concentrations of up to
1.0 × 106 cells mL−1 in Lake Dianchi, which is equivalent to our low-
est concentration T1. Cyanobacteria blooms, however, can show
Microcystis densities up to 1.1 × 107 and 1.3 × 107 cells mL−1 in June
in Lake Dianchi (Luo, 2002). Jonsson et al. (2009) suggested that
local concentrations (i.e., near the producing cells) of allelochemi- cals may be orders of magnitude higher than those in bulk cell-free filtrates. Low concentrations of extracts and exudates significantly promoted seed germination and seedling growth of P. malaianus. These results are supported by a study of Li et al. (2009), which shows that the plant length and fresh weight of the submerged macrophyte Ceratophyllum oryzetorum are significantly promoted by low Microcystis cell densities grown in co-cultures
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