Effects of nitrogen and phosphorus on Microcystis aeruginosa growth and microcystin production

In the present study, the effects of nitrogen (N) and phosphorus (P) on the growth of Microcystis aeruginosa and the production of microcystins (MCs) were investigated. The results showed that the growth of M. aeruginosa was not merely determined by N or P, but both nutrients were limiting for the species. Moreover, an excess of N and constant P in the culture medium could stimulate the growth of M. aeruginosa, whereas the growth of the species was inhibited in the culture medium containing excess of P and constant N. The optimal growth of M. aeruginosa was at an N:P ratio of 0.1 with the maximal optical density of 1.197 at 680 nm (OD680), whereas the maximal microcystin-LR (MC-LR) content of 228.2 μg·L−1 observed in the culture medium with an N:P ratio of 5. Interestingly, MC-LR production occurred under conditions of N starvation, thereby suggesting that the growth rate of M. aeruginosa was not related to MC-LR production under conditions of nutrient stress.

Widespread Distribution and Adaptive Degradation of Microcystin Degrader (mlr-Genotype) in Lake Taihu, China

Microbial degradation is an important route for removing environmental microcystins (MCs). Here, we investigated the ecological distribution of microcystin degraders (mlr-genotype), and the relationship between the substrate specificity of the microcystin degrader and the profile of microcystin congener production in the habitat. We showed that microcystin degraders were widely distributed and closely associated with Microcystis abundance in Lake Taihu, China. We characterized an indigenous degrader, Sphingopyxis N5 in the northern Lake Taihu, and it metabolized six microcystin congeners in increasing order (RR > LR > YR > LA > LF and LW). Such a substrate-specificity pattern was congruent to the order of the dominance levels of these congeners in northern Lake Taihu. Furthermore, a meta-analysis on global microcystin degraders revealed that the substrate-specificity patterns varied geographically, but generally matched the profiles of microcystin congener production in the degrader habitats, and the indigenous degrader typically metabolized well the dominant MC congeners, but not the rare congeners in the habitat. This highlighted the phenotypic congruence between microcystin production and degradation in natural environments. We theorize that such congruence resulted from the metabolic adaptation of the indigenous degrader to the local microcystin congeners. Under the nutrient microcystin selection, the degraders might have evolved to better exploit the locally dominant congeners. This study provided the novel insight into the ecological distribution and adaptive degradation of microcystin degraders.

Variability in microcystin quotas during a Microcystis bloom in a eutrophic lake

Microcystis is a bloom-forming genus of cyanobacteria with some genotypes that produce highly toxic microcystin hepatotoxins. In waterbodies where biological and physical factors are relatively homogenous, toxin quotas (the average amount of toxin per cell), at a single point in time, are expected to be relatively constant. In this study we challenged this assumption by investigating the spatial distribution of microcystin quotas at a single point in time on two separate occasions in a lake with a major Microcystis bloom. Microcystis cell concentrations varied widely across the lake on both sampling occasions (730- and 137-fold) together with microcystin quotas (148- and 362-fold). Cell concentrations and microcystin quotas were strongly positively correlated (R2 = 0.89, P < 0.001, n = 28; R2 = 0.67, P < 0.001, n = 25). Analysis of Microcystis strains using high-throughput sequencing of the 16S-23S rRNA intergenic spacer region showed no relationship between microcystin quota and the relative abundance of specific sequences. Collectively, the results of this study indicate an association between microcystin production and cell density that magnifies the potential for bloom toxicity at elevated cell concentrations.

Laboratory studies of the algaecide GreenTD: stability, algaecidal activity and reduction of microcystin production

GreenTD (Dichlorobenzyl derivative, MW 362.17) is a new dichlorobenzyl derivative algaecide. It is effective and selective against harmful algal blooms (HABs). HABs cause serious problems for public health and fishery industries. Algae that cause HABs include Microcystis spp., Anabaena spp., and Aphanizomenon spp. Blooms of toxin-producing Microcystis aeruginosa occur regularly in fresh water where is rich in nitrogen and phosphorus nutrients. Environmental fate studies are needed to investigate the degradation of GreenTD. In the present study, we studied the persistence of GreenTD (90% aqueous solution) in water and toxic effects GreenTD on M. aeruginosa and reduction of microcystin production in the culture media. GreenTD was added in the water pots and microcystis media tubes at levels of 25 g/0.1 ha (0.5 mg/kg) and 50 g/0.1 ha (1.0 mg/kg). Samples were collected after 1, 3, 5, 7 and 14 days. The residues of GreenTD and microcystins in water and the media were determined using high performance liquid chromatography-diode array detection and ultra-performance liquid chromatography-tandem mass spectrometry, respectively. The half-life of GreenTD at concentrations of 0.5 and 1.0 mg/kg was 4.5 and 3.6 days, respectively. This result presents a safety level suitable for the acceptable guideline of water residue. The average recoveries of microcystins RR, YR, LR, and LA were 106–115%, 103–110%, 96–105% and 89–113% in the microcystis media, respectively. The limit of detection of (LOD) the microcystins was 0.1 µg/kg. No microcystins in the media were detected at the LOD (0.1 µg/kg). GreenTD at concentrations of 0.2 and 0.5 µg/kg had a 100% of control efficacy of M. aeruginosa. No growth of the blue-green algae was observed after 14 days of GreenTD application.