Transcriptional and Physiological Responses to Nutrient Loading on Toxin Formation and Photosynthesis in Microcystis Aeruginosa FACHB-905

Abstract

An important goal of understanding harmful algae blooms is to determine how environmental factors affect the growth and toxin formation of toxin-producing species. In this study, we investigated the transcriptional responses of toxin formation gene (mcyB) and key photosynthesis genes (psaB, psbD and rbcL) of Microcystis aeruginosa FACHB-905 in different nutrient loading conditions using real-time reverse transcription quantitative polymerase chain reaction (RT-qPCR). Three physio-biochemical parameters (malondialdehyde (MDA), superoxide dismutase (SOD) and glutathione (GSH)) were also evaluated to provide insight into the physiological responses of Microcystis cells. We observed an upregulation of mcyB gene in nutrient-deficient conditions, especially in nitrogen (N) limitation condition, and the transcript abundance declined after the nutrient were resupplied. Differently, high transcription levels were seen in phosphorus (P) deficient treatments for key photosynthesis genes throughout the culture period, while those in N-deficient cells varied with time, suggesting an adaptive regulation of Microsystis cells to nutrient stress. Increased contents of antioxidant enzymes (SOD and GSH) were seen in both N and P-deficient conditions, suggesting the presence of excess amount of free radical generation caused by nutrient stress. The amount of SOD and GSH continued to increase even after the nutrient was reintroduced and a strong correlation was seen between the MDA and enzyme activities, indicating the robust effort of rebalancing the redox system in Microcystis cells. Based on these transcriptional and physiological responses of M. aeruginosa to nutrient loading, these results could provide more insight into Microcystis blooms management and toxin formation regulation.

Keywords: antioxidant system; mcy; microcystin; nitrogen; phosphorus.

Figure 1

Cell density of M. aeruginosa at each treatment: nitrogen (A); and phosphorus (B). Different colors indicate different treatments: control (black), nutrient-deficient (red) and nutrient-added treatments (blue).

Figure 2

Relative transcription levels of target genes in N treatments. Red colors indicate the significant results compared to the control (p < 0.05). Significant differences are noted between N-deficient and N-added treatments (p < 0.001 ***, p < 0.01 **, p < 0.05 *).

Figure 3

Relative transcription levels of target genes in P treatments. Red colors indicate the significant results compared to the control (p < 0.05). Significant differences are noted between P-deficient and P-added treatments (p < 0.001 ***, p < 0.01 **, p < 0.05 *).

Figure 4

The concentration of Malondialdehyde (MDA), Glutathione (GSH) and Superoxide dismutase (SOD) of M. aeruginosa at each treatment. Different colors represent sampling days: Day 3 (black), Day 5 (red), Day 7 (blue) and Day 9 (green). Same letters indicate observations that are statistically indistinguishable (ANOVA, p < 0.05).