Synergistic promoting effect of increasing aquatic ammonium and CO2 on Microcystis aeruginosa

Owing to climate change and intensive agricultural development, freshwater bodies have been affected by increases in both CO2 levels and chemically-reduced forms of N. However, little is known about how these changes affect cyanobacterial growth and blooms. This study explored a range of light conditions (30, 80, 130, or 200 mu mol photons/m(2)/s) wherein Microcystis aeruginosa, a widespread bloom-forming species, was exposed to different concentrations of CO2 (400 parts per million (ppm) and 1000 ppm) in a medium containing NH4+ or NO3-. The interactive effects of N sources and CO2 levels on the C/N metabolic balance and energy balance were examined to assess changes in the growth of M. aeruginosa. When the light intensity was 80 mu mol photons/ m(2)/s, elevated CO2 could reduce intracellular reactive oxygen species (ROS) in NH4+-grown M. aeruginosa. Meanwhile, cell density and chlorophyll a (Chl a) increased with increasing CO2 levels, and the increase in Chl a was significantly greater in NH4+-grown M. aeruginosa than in NO3--grown M. aeruginosa. Under light conditions of 200 mu mol photons/m(2)/s, elevated CO2 concentration caused NO3--grown M. aeruginosa to be affected by a large amount of ROS, and the growth of NO3--grown M. aeruginosa was finally suppressed. However, NH4+- grown M. aeruginosa had a smaller amount of ROS and showed improved growth as CO2 was elevated. This difference can be attributed to the faster metabolic pathways in the NH4+ environment, which manifested in a lower accumulation of 2-oxoglutarate and fatty acids as CO2 was elevated. These findings suggest that the simultaneous increase in ammonium and CO2 in aquatic ecosystems confers cyanobacteria with greater advantages than the combination of nitrate and CO2, which may aggravate cyanobacterial blooms.

Automated summary

This study found that the simultaneous increase in ammonium and CO2 provides cyanobacteria with greater advantages and may exacerbate cyanobacterial blooms in aquatic ecosystems. Under light conditions of 80 mu mol photons/m(2)/s, elevated CO2 can reduce intracellular reactive oxygen species, leading to increased cell density and chlorophyll a content. However, under light conditions of 200 mu mol photons/m(2)/s, elevated CO2 concentration affects NO3--grown cyanobacteria, suppressing their growth.