Growth inhibitory effect of Microcystis on Aphanizomenon flos-aquae isolated from cyanobacteria bloom in Lake Dianchi, China

Abstract

Severe eutrophication of freshwater ecosystem promotes frequent and long-lasting cyanobacteria blooms. In many lakes, the population dynamics of the dominant cyanobacteria species follows seasonal shifts. In Lake Dianchi, a plateau lake in China, blooms involve the alternating seasonal succession of two dominant cyanobacteria species, Aphanizomenon and Microcystis. In this study, we investigated the inhibitory effects of five Microcystis strains on a strain of Aphanizomenon flos-aquae, all isolated during blooms in Lake Dianchi. We used dialysis tubing involved batch and semi-continuous co-culture bioassays to investigate the interaction between these two genera. Our results revealed that different Microcystis inhibited the growth of A. flos-aquae to various degrees, with the strains Microcystis ichthyoblabe kutz 1313 and Microcystis aeruginosa 905 (Ma905) exhibiting the strongest inhibitory effects. The axenic Ma905 did not differ from a non-axenic strain in the inhibitory effect, demonstrating that the growth inhibitory effects we observed were due to Microcystis rather than bacteria. Filtrate of most Microcystis monoculture in different growth phases had a negligible effect on A. flos-aquae. However, the filtrate of Microcystis viridis 1337 (Mv1337) and Ma905 co-cultured with A. flos-aquae from day 21 and day 28, respectively, exhibited stronger inhibitory effects than did their corresponding monoculture filtrates, suggesting that the allelopathic effect of some Microcystis strains is inducible. Results of experiments using solid phase extraction (SPE) fractions of Microcystis monoculture were similar to those from the dialysis tubing bioassays. The microcystin content of the SPE fractions of Ma905 and Mv1337 from day 28 contained 380μgL−1 and 290μgL−1 microcystin-LR (MC-LR), respectively. However, a further bioassay with pure MC-LR with concentrations of 250μgL−1 and 500μgL−1 did not inhibit A. flos-aquae, and further excluded MC-LR as a candidate allelochemical. In summary, our research demonstrates that some Microcystis strains are able to suppress the growth of A. flos-aquae. This finding indicates that allelopathic effects likely play a role in driving the alternating seasonal succession of these dominant cyanobacteria blooms in Lake Dianchi.