Can a shift in dominant species of Microcystis alter growth and reproduction of waterfleas?

The bloom-forming species Microcystis wesenbergii and M. aeruginosa occur in many lakes globally, and may exhibit alternating blooms both spatially and temporally. As environmental changes increase, cyanobacteria bloom in more and more lakes and are often dominated by M. wesenbergii. The adverse impact of M. aeruginosa on co-existing organisms including zooplanktonic species has been well-studied, whereas studies of M. wesenbergii are limited. To compare effects of these two species on zooplankton, we explored effects of exudates from different strains of microcystin-producing M. aeruginosa (Ma905 and Ma526) and non-microcystin-producing M. wesenbergii (Mw908 and Mw929), on reproduction by the model zooplankter Daphnia magna in both chronic and acute exposure experiments. Specifically, we tested physiological, biochemical, molecular and transcriptomic characteristics of D. magna exposed to Microcystis exudates. We observed that body length and egg and offspring number of the daphnid increased in all treatments. Among the four strains tested, Ma526 enhanced the size of the first brood, as well as total egg and offspring number. Microcystis exudates stimulated expression of specific genes that induced ecdysone, juvenile hormone, triacylglycerol and vitellogenin biosynthesis, which, in turn, enhanced egg and offspring production of D. magna. Even though all strains of Microcystis affected growth and reproduction, large numbers of downregulated genes involving many essential pathways indicated that the Ma905 strain might contemporaneously induce damage in D. magna. Our study highlights the necessity of including M. wesenbergii into the ecological risk evaluation of cyanobacteria blooms, and emphasizes that consequences to zooplankton may not be clear-cut when assessments are based upon production of microcystins alone.