Rapid industrialization has resulted in severe environmental pollution, and blue-green algal blooms have become a global phenomenon with increasingly remarkable intensity and frequency throughout the world in recent decades. Microcystis aeruginosa
is the most typical bloom-forming freshwater cyanobacteria in freshwater all over the world [1
blooms can initiate severe environmental and ecological events, causing blockage of drinking water supply systems, producing unpleasant odors, reducing water clarity, removing dissolved oxygen during decomposition, and so on [2
]. Some species of Microcystis aeruginosa
are potentially toxic and can produce microcystins, which may pose severe health risks to humans and other mammals [3
]. Many studies have been conducted to explore the influence mechanism to solve algal blooms [4
]. It is generally believed that significant influence factors of water eutrophication are climate [6
] (illumination, temperature, etc.), nutrients [7
], and hydrodynamic conditions [9
]. The effect of hydrodynamics on algae is reflected in two aspects: hydraulic flush and the effect of small-scale turbulence on algal growth. Some researchers have argued about the influence of hydrodynamic force on algae’s physiological and ecological characteristics, such as cell division, cell volume, cell morphology, and photosynthetic characteristics [12
The turbulent intensity is characterized by many parameters, such as energy dissipation rate ε
], Reynolds number Re [17
], shear stress τ [18
], and so on, among which the most common parameter is turbulent energy dissipation rate. It was reported that the turbulent energy dissipation rate was an important variable in studying the effects of small-scale fluid flow on microbial physiology at the cellular level [19
], which reflected the energy transfer in the turbulent environment, from large-scale to small-scale and ultimate dissipation due to molecular adhesion. In addition, hydrodynamic influences would be covered up by other factors due to complicated influence factors in natural water bodies. Therefore, a single-factor method was adopted in the laboratory experiments in most research studies. Sullivan et al. reported that the division rate of Lingulodinium polyedrum
cells increased with the increase of ε from 10−8
]. Missaghi found that the cell biomass of Microcystis aeruginosa
reached the maximum at ε~8.0 × 10−5
Apart from field observation and laboratory experiments, many researchers are devoted to the development of mathematical models. The flow velocity influence functions of algal growth were proposed by analyzing algal data of different areas [21
]. However, the research results showed poor comparability due to different parameters selected to characterize the turbulent intensity in laboratory experiments and mathematical models.
Though many scholars have investigated the impact of small-scale hydrodynamic- or wind-induced turbulence on algal growth to unveil the natural phenomena through observations and tests, the influence mechanism still remains unclear. In addition, there are few mathematical functions available to describe the effect of flow turbulence on Microcystis aeruginosa growth. In this paper, laboratory experiments were conducted to simulate the growth of Microcystis aeruginosa under different turbulent conditions. The influence of hydrodynamics characterized by turbulent intensity on the growth of Microcystis aeruginosa was analyzed, and a mathematical function to quantify the impact of flow turbulence on Microcystis aeruginosa growth was proposed.
2. Materials and Methods
FACHB 905 was purchased from the Freshwater Algae Culture Collection of the Institute of Hydrobiology (FACHB), Chinese Academy of Sciences, Wuhan, China. Before the experiment, algae cells were cultured in BG11 (Table 1
) medium under light intensity 4000 lx and temperature 26 °C for one week until the logarithmic growth phase.
2.2. Experimental Apparatus and Methods
In the experiment, 2000 mL beakers were placed in an artificial climate chamber with a magnetic stirrer to control the turbulent intensity by adjusting the rotor speed. Turbulent intensity was set in the range of 100–200 RPM due to the magnetic stirrer’s limitation, and was conducted at 0, 100, 150, and 200 RPM. The experiments were carried out in an artificial climate chamber under constant temperature (25.8 °C), light intensity (4000 lx), and light/dark ratio (12 h/12 h). A certain amount of medium and algae in the logarithmic growth phase were added to the 2000 mL beakers. From the day of inoculation, samples were taken simultaneously every day, and then the medium was supplemented to 1800 mL. Cell density, chlorophyll-a concentration, and the rate of photosynthetic oxygen evolution were measured every day at the same time. The test process continued until the stable phase. The triplicate experiments were conducted at constant temperature and light. The medium and vessels were sterilized with an autoclave before the investigation. In addition, the experiment was conducted under axenic conditions.
The cell density (cells/mL) was measured with a hemocytometer and optical microscope (CX21, Olympus America Inc., Melville, NY, USA), while the chlorophyll-a concentration (μg/L) and the chlorophyll fluorescence intensity (Chla-f) were measured using PHYTO-PAM (Heinz Walz GmbH, Eichenring, Germany). A liquid oxygen electrode (CHLOROLAB 2, Hansatech, Norfolk, UK) was used to determine the rate of photosynthetic oxygen evolution (μmol O2/(mg Chl-a × h)).
2.3. Statistical Analysis and Software
The data were presented as mean ± standard deviation, with one-way ANOVA to analyze the differences in the growth rate among all groups, with p < 0.05 as the level of significance. Origin (OriginLab, Northampton, MA, USA) and MATLAB (MATLAB, R2018a, MathWorks) were adopted for data analyses. The commercial computational fluid dynamics (CFD) simulation software FLUENT (Ansys Fluent Inc., Lebanon, NH, USA) was used to compute the turbulent energy dissipation rate ε.
The results suggest different effects on Microcystis aeruginosa
growth corresponding to various turbulence conditions, in particular the scenario of slight turbulence, which is consistent with Hondzo’s findings of a nearly 2-fold increase in the growth rate of Selenastrum capricornutum
achieved for an energy dissipation rate 10−7
]. Such promotion is probably due to two mechanisms: the augmentation of both nutrient absorption and algal photosynthetic efficiency.
Karp-Boss defined an area around algal cells as the diffusion boundary layer, where the nutrient concentration is less than 90% of the surrounding concentration. Uniform flow and shear flow could distort the diffusion boundary layer and make the concentration gradient steeper [29
]. Moreover, Hondzo reported that flow turbulence could cause algal extracellular diffusion layer thinning and could facilitate the transport of nutrients to algal cells [16
]. In addition, it was reported that turbulence affected algal growth through algal photosynthesis efficiency. Mitsuhashi found that turbulence shear effectively reduced Chlorella’s
photosynthesis efficiency [18
]. Li reported that different turbulent intensities promoted the photosynthetic activity of algal cells. In this study, the rate of photosynthetic oxygen evolution was larger under turbulence conditions than the control group (0 RPM) in both the lag growth and logarithmic growth phases. However, there was little difference in algal cell photosystem II activity with the increase of turbulent intensity. It is therefore indicated that strong turbulence may have no adverse effect on the photosynthetic activity of Microcystis aeruginosa
, which means the decrease of Microcystis aeruginosa
’s growth could not be explained from the view of photosynthesis.
Some studies have also suggested turbulence has an effect on the division and proliferation of algal cells. Sullivan discovered that the cell division rate of Lingulodinium polyedrum
tended to present as a straight climb when ε increased from 10−8
. When ε reached 10−3
, the rate decreased noticeably [12
]. Bolli held that strong turbulence (ε = 2.7 × 10−3
) had an inhibitory effect on the cyst proliferation of Alexandrium minimum
and A. catenella
]. Microcystis aeruginosa
cells were unicellular cells that undergo cell division to produce daughter cells [31
]. In the experiment, the average growth rate of Microcystis aeruginosa
first increased and then decreased with the increase of turbulent intensity. The peak value occurred when the turbulent intensity was 6.44 × 10−2
(150 RPM), over which the increasing rate was curbed instead. It is therefore inferred that the turbulence may slow down the growth of Microcystis aeruginosa
by inhibiting cell division.
Laboratory simulations and microcosms can provide a more controlled analysis and evaluation of phenomena in the natural environment that may not be possible in situ. However, lab simulations may introduce conditions and circumstances with an inevitable discrepancy from the natural environment. For example, the vortices in a fluid-filled beaker generated by a stirring bar may not be equivalent to more straight-line currents in a larger volume in the natural environment. Some laboratory-generated vortices may partially simulate natural spiral eddies that are set up in a natural aquatic environment. However, other experimental lab apparatuses (such as the impact forces of the impeller stir bar on the suspended algae) would not necessarily be present in the natural environment. Some of these aspects of the laboratory experiment should be considered in the future. The current results are based on the experiments of Microcystis aeruginosa, and the effect of flow turbulence on other algae species remains to be explored.
Laboratory cultures of Microcystis aeruginosa in beakers were carried out under different turbulent conditions to identify the quantitative relationship between the algal growth rate and the turbulent intensity. The results indicated that flow turbulence could promote the growth of Microcystis aeruginosa and algal photosynthetic activity. Both the chlorophyll-a concentration and algal biomass increased under the designed turbulent condition (energy dissipation rate of 7.40 × 10−3, 6.44 × 10−2, and 0.19 m2/s3) compared with the control group. However, the peak growth rate of Microcystis aeruginosa occurred when ε was 6.44 × 10−2 m2/s3, over which the rate declined, probably due to unfavorable impacts of strong turbulence. In comparison, the maximum rate of photosynthetic oxygen evolution occurred when ε was 0.19 m2/s3. Based on the results, an exponential function was proposed to represent the effect of flow turbulence on Microcystis aeruginosa growth. Three parameters were included in the function to define the algal growth pattern in the turbulent environment. The parameter values for defining the growth pattern of Microcystis aeruginosa were further calibrated with the experiment results in the study.