The Influence of Micronutrient Trace Metals on Microcystis aeruginosa Growth and Toxin Production

Abstract

Microcystis aeruginosa is a widespread cyanobacteria capable of producing hepatotoxic microcystins. Understanding the environmental factors that influence its growth and toxin production is essential to managing the negative effects on freshwater systems. Some micronutrients are important cofactors in cyanobacterial proteins and can influence cyanobacterial growth when availability is limited. However, micronutrient requirements are often species specific, and can be influenced by substitution between metals or by luxury uptake. In this study, M. aeruginosa was grown in modified growth media that individually excluded some micronutrients (cobalt, copper, iron, manganese, molybdenum) to assess the effect on growth, toxin production, cell morphology and iron accumulation. M. aeruginosa growth was limited when iron, cobalt and manganese were excluded from the growth media, whereas the exclusion of copper and molybdenum had no effect on growth. Intracellular microcystin-LR concentrations were variable and were at times elevated in treatments undergoing growth limitation by cobalt. Intracellular iron was notably higher in treatments grown in cobalt-deplete media compared to other treatments possibly due to inhibition or competition for transporters, or due to irons role in detoxifying reactive oxygen species (ROS).

Keywords: cyanobacteria; growth limitation; microcystin.

Figure 1

Microcystis aeruginosa growth through time under variable micronutrient conditions. (A) Transfer 1 and (B) Transfer 2. Error bars are standard error of the mean.

Figure 2

Specific growth rate in treatments exposed to depletion of different micronutrients across two transfers. Asterisk (*) denotes significant difference relative to the control of the same transfer (One-way ANOVA: p-value < 0.05).

Figure 3

Scatterplot of cell volume relative to cells in the control treatment. Cell volume was measured once the treatment exhibited a growth limitation and compared to the control cell volume at the same time point (C1—control day 20; C2—control day 60). Data for the FeEDTA treatment was not obtained due to the rapid decrease in cell density. Asterisk (*) denotes significant difference relative to the control. Error bars are ± standard error of the mean.

Figure 4

Changes in intracellular microcystin-LR cell quotas throughout the experiment. Samples from the FeEDTA treatment had insufficient sample mass for analysis so are excluded. Error bars are standard error of the mean. Asterisks (*) denote significant difference to control at same time point (PERMANOVA: p-value < 0.05).

Figure 5

Differences in the intracellular quota of iron in treatments depleted of different micronutrients after 31 days. Samples from the FeEDTA treatment had insufficient sample mass for analysis so are excluded. Error bars are standard error of the mean. Asterisk (*) denotes significant difference compared to the control (One-way ANOVA: p-value < 0.05). A log10 transformation was performed to satisfy the assumptions of parametric statistical analyses.