Physiological and Genetic Responses of Microcystis Aeruginosa Under Different Environmental Conditions

Cyanobacterial blooms are among the most harmful algal blooms (HABs) found in freshwaters such as rivers, lakes and creeks. Cyanobacterial HABs cause severe economic and environmental damages, as decay of dense cyanobacterial mats creates dead zones of oxygen depletion in which most aquatic biota cannot survive. Cyanotoxins produced by cyanobacteria, or blue-green algae are capable of contaminating surface waters to the point of lethality. Cyanotoxins have been implicated in a number of human deaths directly through drinking water, exposure during recreational activities and liver cancer. Despite the severity of the threat of cyanobacterial HABs, the current understanding of the gene functions and interactions underlying toxin production is limited. A better understanding of the growth kinetics, gene expression and toxin production under different environmental conditions will help develop tools for early-warning indicators for predicting HABs and engineering effective control strategies.

The major goal of this study is to investigate the behavior of cyanobacteria under different environmental conditions. For this purpose, lab-scale experiments were performed with Microcystis aeruginosa PCC7806 under different growth stresses. Nutrient and temperature are the two main focus of this study as they are the most impactful environmental factors regulating the cyanobacterial bloom. Five Nutrient settings of 5, 25, 76.86, 100, and 150 N:P ratios as well as three temperature of 20 ºC, 25 ºC, and 30 ºC were carefully selected to monitor the HABs behavior. The culture grew more homogeneous during the incubation at the two temperature of 25 ºC and 30 ºC, and less consistent followed by fluctuations in growth rate at 20 ºC. Microcystis aeruginosa grew faster in higher ratios of nitrogen to phosphorus (76.86, 100, and 150) compared to the lower ones (5 and 25) during the incubation at the two highest temperatures (25 ºC and 30 ºC), however, there was no significant difference between the cultures which grew in different N to P ratios at 20 ºC. Quantification of the Microcystis 16S rRNA gene represented a strong relationship with the growth rate. Among the selected N to P ratios, 100:1 was the most amenable one which was observed in both the growth rate and Microcystis 16S rRNA gene amplification.

Transcriptional responses of cyanobacteria were monitored using reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR) assays. Although most samples indicated no gene expression, there was some stimulation in the expression of microcystin associated genes (mcyA, mcyD, and mcyE) which were then subjected to microcystin analysis. The culture incubated at 20 °C showed more gene expression compared to 25 °C. Most of the gene expression was observed in early to mid-exponential and late stationary phase of growth at 20 °C. No extracellular microcystin was detected for the few selected samples. The intracellular microcystin, however, was detected for most of the samples which were subjected to the analysis. At least one positive gene expression was observed for the samples which showed positive microcystin. This work provide valuable insight into the mechanistic interaction of toxin production in cyanobacteria with different environmental factors, and provide fundamental physiological and transcriptional information to further explore and predict the behavior and impacts of cyanobacterial blooms in freshwater systems. The results of this study offer an opportunity to develop approaches for the prediction and monitoring of HABs as well provide insight to the molecular level approaches for mitigation of HABs.