Abstract
Microcystis aeruginosa is a single-celled cyanobacterium, forming large colonies on the surface of freshwater ecosystems during summer, and producing a toxin (microcystin) that in high concentration can be harmful to humans and animals. These toxic effects can be governed by abiotic environmental conditions including water temperature, light, nutrient abundance, and fluid motion. We investigated the effect of small-scale turbulence on the growth and metabolism of Microcystis aeruginosa using field measurements and laboratory bioreactor investigations. The laboratory setup included two underwater speakers, generating a quasi-homogeneous turbulent flow with turbulent kinetic energy dissipation rates up to 10-6 m2/s3, comparable to field values in the lacustrine photic zone. The role of turbulence is quantified by comparing cell number, dissolved oxygen production/uptake, and inorganic carbon uptake in stagnant condition and two sets of experiments with turbulent conditions, quantified by the Taylor micro-scale Reynolds number at Reλ = 15 and Reλ = 33. The results suggest that turbulence mediates the metabolism of Microcystis aeruginosa measured by the net oxygen production, oxygen uptake, and inorganic carbon uptake. Furthermore, small-scale turbulence marginally influenced Microcystis growth rate estimated from cell population concentration (-5% and 11% for Reλ = 33 and Reλ = 15, respectively, as compared to stagnant conditions).
Highlights
Cyanobacteria blooms are a ubiquitous nuisance in freshwater ecosystems throughout the world [1]
The estimates indicate that the highest turbulence level, Reλ = 33, yielded the lowest growth rate by 5% compared to the stagnant condition, while the Reλ = 15 condition increased kg by 11%
The population growth appears to be only marginally influenced by small-scale turbulent conditions, our study highlights an appreciable mediation of the photosynthetic metabolism by turbulence, independent of the influence of CO2 entrainment into the water, dissolved oxygen (DO) or nutrients abundance, and changes in photosynthetically active radiation (PAR) availability
Summary
Cyanobacteria blooms are a ubiquitous nuisance in freshwater ecosystems throughout the world [1]. Cyanobacteria blooms are of such interest because of the induced foul taste, odor and turbidity in lake water and due to their contribution to hypoxia and subsequent fishery collapse [4] Certain cyanobacteria, such as the Microcystis aeruginosa, produce a deadly liver toxin called microcystin. Studies do point to the use of microcystin in competition with other organisms for dominance within aquatic system, for example: 1) aiding in intracellular inorganic carbon (Ci) regulation under low environmental Ci conditions to sustain photosynthetic processes [6]; 2) inhibiting the metabolism of other microorganisms [7], and 3) maintaining colonies through promotion of polysaccharide production [8]
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