Susan LeBlanc Renaud

[D-Leu¹]MC-LY (1) ([M + H]⁺ m/z 1044.5673, Δ 2.0 ppm), a new microcystin, was isolated from Microcystis aeruginosa strain CPCC-464. The compound was characterized by ¹H and ¹³C NMR spectroscopy, liquid chromatography–high resolution tandem mass spectrometry (LC–HRMS/MS) and UV spectroscopy. A calibration reference material was produced after quantitation by ¹H NMR spectroscopy and LC with chemiluminescence nitrogen detection. The potency of 1 in a protein phosphatase 2A inhibition assay was essentially the same as for MC-LR (2). Related microcystins, [D-Leu¹]MC-LR (3) ([M + H]⁺ m/z 1037.6041, Δ 1.0 ppm), [D-Leu¹]MC-M(O)R (6) ([M + H]⁺ m/z 1071.5565, Δ 2.0 ppm) and [D-Leu¹]MC-MR (7) ([M + H]⁺ m/z 1055.5617, Δ 2.2 ppm), were also identified in culture extracts, along with traces of [D-Leu¹]MC-M(O₂)R (8) ([M + H]⁺ m/z 1087.5510, Δ 1.6 ppm), by a combination of chemical derivatization and LC–HRMS/MS experiments. The relative abundances of 1, 3, 6, 7 and 8 in a freshly extracted culture in the positive ionization mode LC–HRMS were ca. 84, 100, 3.0, 11 and 0.05, respectively. These and other results indicate that [D-Leu¹]-containing MCs may be more common in cyanobacterial blooms than is generally appreciated but are easily overlooked with standard targeted LC–MS/MS screening methods.

In aquatic ecosystems, the factors that regulate the dominance of toxin-producing cyanobacteria over non-toxin-producing strains of the same species are largely unknown. One possible hypothesis is that limiting resources lead to the dominance of the latter because of the metabolic costs associated with toxin production. In this study, we tested the effect of light intensity on the performance of a microcystin-producing strain of Microcystis aeruginosa (UTCC 300) when grown in mixed cultures with non-microcystin-producing strains with similar intrinsic growth rates (UTCC 632 and UTCC 633). The endpoints measured included culture growth rates, microcystin concentrations and composition, and mcyD gene copy numbers determined using quantitative PCR (Q-PCR). In contrast to the predicted results, under conditions of low light intensity (20 μmol·m ⁻² ·s ⁻¹ ), the toxigenic strain became dominant in both of the mixed cultures based on gene copy numbers and microcystin concentrations. When grown under conditions of high light intensity (80 μmol·m ⁻² ·s ⁻¹ ), the toxigenic strain still appeared to dominate over nontoxigenic strain UTCC 632 but less so over strain UTCC 633. Microcystins may not be so costly to produce that toxigenic cyanobacteria are at a disadvantage in competition for limiting resources.