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Article

The Range of the Colonial Microcystis’ Biomass for Shift to Diatom Aggregates Under Aeration Mixing and Low Light

by
Xiaodong Wang
1,2,*,
Xuan Che
1,
Xingguo Liu
1,
Xinfeng Li
1,
Xiaolong Chen
1,
Yiming Li
3 and
Lin Zhu
1
1
Fishery Machinery and Instrument Research Institute, Chinese Academy of Fishery Sciences, Shanghai 200092, China
2
Key Laboratory of Urban Water Supply, Water Saving and Water Environment Governance in the Yangtze River Delta of Ministry of Water Resources, Shanghai 200092, China
3
East China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Shanghai 200090, China
*
Author to whom correspondence should be addressed.
Diversity 2024, 16(11), 695; https://doi.org/10.3390/d16110695
Submission received: 26 September 2024 / Revised: 24 October 2024 / Accepted: 6 November 2024 / Published: 13 November 2024
(This article belongs to the Special Issue Eutrophication, Aquaculture and Aquatic Ecosystem Restoration)
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Abstract

:
In order to investigate non-cyanobacteria dominance succession from Microcystis blooms, particularly to diatom dominance, an experiment using varying colonial Microcystis biomasses expressed as bulk concentrations of 2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0, 16.0, 18.0, 20.0, 22.0, and 24.0 mL L−1 was undertaken under continuous aeration mixing in a greenhouse during a hot summer where shading had reduced light level by 97%. The results showed that the algal shift process was affected by the initial biomass of the colonial Microcystis, and the algal community diversified. When the Microcystis bulk concentration was between 2.0 and 16.0 mL L−1, the bloom became dominated by diatom Nitzschia palea, which aggregated on the mucilage sheathes of the Microcystis colonies. The diatom density at bulk concentration biomass of 2.0 mL L−1 reached a maximum at 2.8 × 105 cells mL−1 on day 27. When the bulk concentration was at 18.0–24.0 mL L−1, no diatom dominance appeared. The shift from a Microcystis bloom to diatom dominance was affected by the initial Microcystis biomass, and the most suitable bulk concentration biomass for colonial Microcystis was at 2–12 mL L−1, in which the chlorophyll-a level was about from 285 to 1714 μg L−1. The mechanism underlying this algal shift may be that the low light and nutrient levels in the Microcystis bloom promoted diatom aggregation growth on the mucilage sheaths of Microcystis colonies under continuous aeration mixing.

1. Introduction

The worldwide eutrophication of water bodies has meant that algal blooms, particularly harmful cyanobacterial blooms, have become very common [1]. These cyanobacterial blooms can affect ecosystem structure and function, water supply, tourism, etc. [1,2]. The regulation and prevention of algal blooms are key to improving water quality, and many methods are used to control Microcystis blooms, which are one of the most common types of cyanobacterial blooms.
One of the major characteristics of Microcystis blooms is that each Microcystis colony consists of many Microcystis cells in a mucilage sheath. These cells can form aggregates up to several millimeters in size that float on the water [3]. The aggregates are buoyant and can accumulate on the water to form extremely large blooms [4]. When blooms occur, hydrodynamic disturbance, particularly artificial aeration, can be used to control the algal bloom by preventing it from floating [5,6] and by changing the algal community composition from Microcystis dominance to diatom or diatom and green algae dominance [7,8,9]. Artificial disturbance reduced the growth of Microcystis in Nieuwe Meer Lake, The Netherlands [7], and high-intensity hydrodynamic disturbance has been shown to change a Microcystis bloom to a diatom and green algae bloom [10,11].
The algal shift process from a Microcystis bloom to non-cyanobacteria dominance under hydrodynamic disturbance is affected by many factors, such as the biomass of Microcystis bloom, light, and nutrients [9,12]. In particular, light intensity is an important factor affecting the algal shift. Huisman et al. [11,13] have proposed the light competition theory, which suggests that light instead of nutrients is the limiting factor for algal growth with full mixing under highly eutrophic conditions. The important effects of low-light conditions on the shift in algal communities are emphasized by the light competition theory.
To date, the effect of hydrodynamic disturbance on Microcystis blooms has not been fully studied [14], although the effects of artificial mixing on algal biomass were reviewed by Visser et al. [9], and the algal succession when there is hydrodynamic disturbance to different-sized Microcystis blooms is not fully understood. Meanwhile, the biomass of Microcystis was an important factor affecting the algal shift in Microcystis blooms [12,15], and diatoms can succeed from the Microcystis bloom in a certain biomass of colonial Microcystis bloom [15,16], which provided some important information that the biomass of the Microcystis bloom can affect its algal shift in regulation. However, the effect of the colonial Microcystis bloom’s biomass on the algal shift from Microcystis dominance to diatom dominance needs more investigation to find the proper range of the colonial Microcystis’ biomass for algal shift.
When Microcystis blooms are under hydrodynamic disturbance, the Microcystis colonies can begin to “self-shade”, which reduces light intensity. Furthermore, diatoms are considered to prefer low-light conditions, whereas high irradiance leads to a decline in spring diatom maximum [17]. It is not currently clear whether reducing light conditions by shading, such as in a greenhouse with shading, will lead to increased diatom succession from various biomasses of Microcystis blooms under hydrodynamic disturbance or to the algal community diversification.
Then, an experiment using Microcystis blooms of varying biomasses under aeration mixing was undertaken in a greenhouse with low-light conditions in order to study the algal changes, particularly the diatom succession with varying biomasses of colonial Microcystis’ biomass. Thus, in order to find the proper range of the colonial Microcystis’ biomass for more diatom shift, which can provide some key information for the control of algal community composition in cyanobacterial bloom.

2. Materials and Methods

2.1. Experimental Design

On 15 July 2020, a dense Microcystis bloom was obtained using a 200-mesh nylon screen from the water surface of the western part of Lake Taihu, China, where serious Microcystis blooms have occurred every year in recent decades [18,19]. The Microcystis bloom was dominated by Microcystis flos-aquae and Microcystis aeruginosa, which were mainly composed of Microcystis colonies in fine aggregates. The large colonial aggregates can be up to 3 mm in size.
A considerable amount of tap water was used to wash the dense bloom through a 200-mesh nylon screen and to dilute the dissolved nitrogen and phosphorus nutrients in the bloom to as close to the levels in tap water as possible. The total nitrogen (TN) and total phosphorus (TP) in the tap water were measured. The TN and TP content in the tap water were 1.158 and 0.024 mg L−1, respectively. Then, the dense bloom was fully mixed with some tap water and left to stand for 30 min. Finally, the dense bloom on the water surface was retained as the mother liquor of Microcystis bloom. The mother liquor was transferred to transparent borosilicate 10 L jars (23 cm in diameter and 35 cm in height) in the greenhouse, and 12 treatments in different bulk biomass concentrations were created.
The 12 treatments were made up from the mother liquor and diluted with tap water to 10 L. Their bulk concentrations were 2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0, 16.0, 18.0, 20.0, 22.0, and 24.0 mL L−1, which were named treatments B2, B4, B6, B8, B10, B12, B14, B16, B18, B20, B22, and B24, respectively. There was one replicate for each treatment (Table 1). During the experiment, each jar was continuously aerated with a bubble stone to ensure the Microcystis colonies remained suspended in the jars instead of floating on the surface. The flow rate of the air was controlled to sustain turbulent conditions during the experiment. The aeration intensity in air was 0.30 m3 h−1 in each jar during the experiment to keep the dissolved oxygen (DO) concentration above 6.0 mg L−1, and no sediment was provided. Tap water was used to replenish that lost by evaporation about every 3–5 days.
Chlorophyll-a (Chl-a) concentration was also chosen to reflect the biomass level. The initial total nitrogen (TN), total phosphorus (TP), bulk concentration, and Chl-a concentrations in the treatments are shown in Table 1.
The experiment was carried out in a 220 m2 greenhouse in Shanghai City, China, and the experimental area was covered with a black polyolefin sun-shading net to reduce the light. The light under the sun-shading net was measured to obtain its shading rate. The experiment lasted 45 days from 17 July to 31 August 2020, which was in a hot summer.
During the experiment, the positions of the experimental jars were re-assigned 3 times in all, about once every 10 days, to eliminate systematic differences in light regime. However, when we took pictures of the jars, we put the bottles following the concentration sequence in order to compare the color changes of the jars intuitively.

2.2. Shading Rate in the Greenhouse and Its Measurement

The light intensity was shown in photosynthetically available radiation (PAR), which was measured under the black sun-shading net in the greenhouse and outdoors at about 13:00 and 14:30 h on 25 July 2020, which was a sunny day. The PAR quantum (unit: μMol m–2 s−1) and PAR energy (unit: W m–2) levels were measured by a Spectrosense2 m with a four-channel sensor (Skye Instruments, Llandrindod Wells, UK). They were measured three times in three minutes when it was cloudless and sunny. Then, the average values were calculated to obtain the light transmittance values, which were the ratio of the light intensity under the black sun-shading net in the greenhouse to the outdoor light intensity. The shading rate was calculated as: shading rate (%) = 100 − Transmittance (%).

2.3. Water Quality and Chl-a Measurements

Water temperature (WT), dissolved oxygen (DO), and pH were measured in situ nine times at 14:00–15:00 h with a YSI multi-parameter (YSI professional plus, Yellow Spring Instruments, Yellow Springs, OH, USA) every 5–6 days. The WT was also measured every day in treatment B4 at 9:00–10:00 and 14:00–15:00 h by a mercury thermometer. The TN and TP measurements were taken according to Gross and Boyd [20], and the Chl-a concentration was measured by colorimetry with 90% ethanol extraction [21] after filtration with GF/F filters (0.7 μm pore size, Whatman, Maidstone, UK) every 3 or 4 days.

2.4. Diatom Identification and Counting

During the experiment, the algae in treatments B2 and B4 first turned brownish yellow on about day 14. It was found that the Microcystis colonies had decomposed, and the diatom genus Nitzschia, particularly the species Nitzschia palea, had appeared, but no other diatoms succeeded. Then, the algal samples focused on diatom counting were collected seven times from day 17 (3 August 2020) during the experiment.
The decay of Microcystis colonies affected the Microcystis cell counting, so Microcystis cell counting was halted. In the later stage of the experiment, a few green algae appeared, but they were not counted either.
To determine the diatom density, 50 mL of water samples were preserved using 1% Lugol’s solution and stored in the dark until required for analysis. For enumeration, two replicate aliquots were enclosed in 0.1 mL plankton counting chambers that were modified from the Palmer and Maloney design [22]. The cells were observed at 400× magnification by light microscopy with an Olympus CX31 (Olympus, Tokyo, Japan), and their morphologies were used to identify the cells at the species level [23].

2.5. Data Analysis

SPSS 24.0 software for Windows (Statistical Product and Service Solutions, IBM, Armonk, NY, USA) was used to compare the DO, pH, Chl-a, and diatom densities among the treatments using one-way analysis of variance (ANOVA). To improve the homogeneity of variances, the DO and pH data were square root transformed, while Chl-a data were log10 transformed, before the comparison [24]. No replicates had been set for each treatment, which affected the ANOVA results. Then, the ANOVA was used for some selective data. In the comparison of Chl-a, the first three samples (on days 0, 4, and 7) were used for the one-way ANOVA. Meanwhile, the three highest densities of each treatment were chosen for the one-way ANOVA for the comparison of diatom density. The least-significant difference test was chosen for the post hoc analysis. Linear regression between the transformed data of the first three samples of Chl-a and the three highest densities of diatom Nitzschia was performed to show the relationship between Chl-a and Nitzschia density.

3. Results

3.1. Chl-a, TN, and TP Concentrations at the Beginning of the Experiment

The Chl-a, Microcystis cells, TN, and TP concentrations in each treatment at the start are in Table 1, showing the higher the bulk concentration of the Microcystis bloom, the larger the Chl-a, Microcystis concentration, TN, and TP values. The bulk concentration is a simple way to represent the Microcystis biomass.

3.2. Light

The black sun-shading net in the greenhouse reduced the light by about 97% (Table 2), which provided a low-light condition.

3.3. WT, DO, and pH

The average WT at 09:00–10:00 and 14:00–15:00 h in treatment B4 was 30.7 and 32.9 °C, respectively (Figure 1), and DO was >6 mg L−1 (Figure 2). There was a declining trend in pH values, particularly in the high biomass treatments B22 and B24 (Figure 2).
The ANOVA results showed that the DO in treatments B2, B4, B6, B8, B10, and B12 was all significantly higher than that in treatments B14, B16, B18, B20, B22, and B24 (p < 0.05), and there were no significant differences in pH values between all the treatments (p > 0.05).

3.4. Changes in Chl-a and the Phenomenon of the Jars

The changes in Chl-a can be divided into two parts: the days before day 20 and the days afterwards, according to the trend of changes in Chl-a (Figure 3). Almost all the Chl-a values showed a downward trend before day 20, and then they gradually approached each other. The ANOVA results from Chl-a for the first three samples (on days 0, 4, and 7) showed that the Chl-a in treatment B2 was the lowest (p < 0.01) and that in treatment B24 was the highest (p < 0.01). The Chl-a values increased significantly from treatments B2 to B24 (p < 0.01), although there were no significant differences between the adjacent two concentration treatments, such as between treatments B8 and B10 (p > 0.05), or between treatments B10 and B12 (p > 0.05), or between treatments B12 and B14 (p > 0.05), and so on.
Changes in the phenomenon of the jars were very attractive (Figure 4). All the jars kept green in the first 7 days, then the low biomass treatments B2 and B4 first turned light brown from about day 14. Treatments B2 and B4 became dark brown from about days 20 to 24, while the other treatments became light brown too. On day 27, all the treatments turned light brown. A few days later, the low biomass treatments B2 and B4 first turned green again.

3.5. Diatom Density

The Nitzschia density was measured from day 17 on, when the water in treatments B2 and B4 turned brownish. The jars in treatments B6, B8, B10, and B12 gradually turned brownish yellow in a similar manner to treatments B2 and B4. Later on in the experiment, the water in treatments B14, B16, B18, B20, B22, and B24 also turned brownish yellow. However, a microscopic examination showed that the diatom cell densities in treatments B14, B16, B18, B20, B22, and B24 were relatively low (Figure 5), showing that the Microcystis abundance affected the algal shift process.
Figure 5 shows that there was a clear peak period of Nitzschia density for each treatment from B2 to B16, which was somewhat normally distributed from day 17 to 41. It reflects the growth process of Nitzschia.
The ANOVA results for the three highest Nitzschia densities of each treatment showed that the diatom cell density in treatment B2 was the highest (p < 0.01) (Figure 5). The density values in treatments B4, B8, B10, and B12 were not significantly different from each other (p > 0.05), and they were all higher than those in treatments B14, B16, B18, B20, B22, and B24 (p < 0.01). In treatments B18, B20, B22, and B24 density values were the lowest (p < 0.01), while the difference among them was not significant (p > 0.05). No significant difference occurred between treatments B14 and B16 (p > 0.05), which were all lower than treatment B6 (p < 0.01). In treatment B6 density value was lower than the values in treatments B4, B8, B10, and B12 (p < 0.01). The Nitzschia density changes in each treatment can be reflected in the phenomenon changes of the jars (Figure 4).
Then, with the increase in the biomass of Microcystis bloom from B2 to B16, the peak density of Nitzschia cells reduced and the density’s peak time delayed (Figure 5). The succession time in different treatments varied: from treatment B2 to B16, the lower the Microcystis bloom biomass, the easier the succession to diatom dominance occurred at an earlier time. The succession time in the different treatments varied, and the succession to diatom dominance occurred earlier as the Microcystis bloom biomass decreased from B16 to B2. Overall, the lower the concentration of the Microcystis bloom, the earlier the appearance color changed. Then, the initial biomass of colonial Microcystis was the main factor affecting the algal shift in the present study.

3.6. Linear Regression Between Chl-a and Nitzschia Density

Linear regression between the square root transformed data of the first three samples of Chl-a and the three highest densities of diatom Nitzschia was performed to show the relationship between Chl-a and Nitzschia density (Figure 6). From the determination coefficient of linear correlation, Chl-a had the explanatory power as high as 84.0%, indicating that Chl-a can explain changes in Nitzschia density well.

3.7. Process Underlying the Algal Shift from Microcystis Colonies to Nitzschia Dominance

Photographs showing the changes in live cells during the experiment are shown in Figure 7, Figure 8 and Figure 9. During the experiment, a large number of Nitzschia cells appeared on the Microcystis colonies (Figure 7 and Figure 8). The photographs of the algae taken under a microscope at different time stages show that the Microcystis colonies gradually decayed under aeration mixing at a biomass level of 10 mL L−1 (Figure 8). At the same time, Nitzschia diatom numbers increased in the decomposing Microcystis colonies, and the colonies became increasingly dense. Furthermore, the photographs showing diatom dominance in treatment B2 on day 17 (Figure 7) indicate that the diatom aggregates were very rich. However, the algal shift from Microcystis colonies to bacteria dominance without Nitzschia aggregation in treatment B18 (18 mL L−1 bulk concentration) (Figure 9) to B24.
The process underlying the algal shift was affected by the biomass of the Microcystis bloom. The peak diatom concentration occurred on about day 28 in treatments B2, B4, and B6, whereas it was about day 32 in treatments B8, B10, and B12 and a little later in treatments B14 and B16 (Figure 5). During the later stage of the experiment, a few green algae appeared among the diatoms (Figure 7), but these were not counted. However, the algal community diversified.

4. Discussion

4.1. Algal Shifts and Diversity Under Hydrodynamic Mixing

The results showed that the Microcystis biomass affected the algal shift from Microcystis dominance to diatom. Similarly, the algal shift under hydrodynamic disturbance at deep Hartbeespoort Dam in South Africa was affected by the abundance of Microcystis [12]. At high Microcystis abundances, the algae diversified, in which the diatoms Cyclotella meneghiniana and Melosira (syn. Aulacoseira) granulate, and the cryptophytes Chroomonas sp. and Cryptomonas sp. occurred more frequently. In contrast, some green algae tended to increase across a broad spectrum of temperature and nutrient conditions when Microcystis was rare [12]. Furthermore, the bacterial community composition is also affected by the biomass of the Microcystis bloom under anoxic conditions [25,26,27].
The algal shift from cyanobacteria, particularly Microcystis blooms, to diatom or/and green algae dominance under hydrodynamic mixing has been reported by a number of previous studies [7,9,10,11,15,16,28], in which the algal species increased and algae diversified. Mixing also shifted the Microcystis blooms from Lake Taihu, China, to dominance by diatoms and green algae [29]. Deep mixing favored diatoms, such as Asterionella, Fragilaria, and Staurastrum, and hampered the cyanobacteria (Microcystis and Anabaena) [30]. Artificial disturbance shifted a cyanobacterial bloom to a diatom bloom in Ford Lake, MI, USA [8]; continuous hydrodynamic mixing has been shown to weaken the dominance of Microcystis, which was beneficial to other algae [31] and increased the algal diversity; hydrodynamic mixing changed the dominant algae from Microcystis to green algae [32], which increased the number of algal species and the improved algal diversity; and artificial aeration led to the dominant bloom-forming cyanobacteria being replaced by diatoms in a small tropical reservoir [5].

4.2. Reasons for the Algal Shift Under Hydrodynamic Mixing

Some researchers have summarized the reasons why hydrodynamic disturbance favors dominance by diatoms and green algae rather than buoyant cyanobacteria [9,29]. These include buoyancy regulation, competition for nutrients and light, and sedimentation losses. The most important reason seems to be that Microcystis colonies lose the ability to float on water subjected to artificial turbulence, while the negatively buoyant green algae and diatoms benefit from disturbed conditions with fluctuating irradiance [9,11,12].
In this experiment, every treatment was disturbed to the same degree by aeration mixing, which prevented the fine particle Microcystis colonies from becoming suspended in the water. Instead, the colonies kept rolling in the water all the time. Meanwhile, the fine-particle Microcystis colonies began “self-shading” with continuous aeration mixing, and the results showed that the higher the Microcystis biomass, the lower the light intensity in the jars. The low light produced by self-shading due to aeration mixing and shading from the black sun-shading net in the greenhouse (Table 2) are very important factors effecting the algal shift from Microcystis dominance to diatom N. palea dominance in treatments B2–B16.
The results from this study supported the light competition theory established by Huisman et al. [11], which emphasized the important role played by low-light conditions in the competition between buoyant and sinking phytoplankton species in eutrophic waters with hydrodynamic disturbance. Diatoms and green algae are considered to be better adapted to fluctuating light conditions [11]. Furthermore, low-light conditions are considered to be better for diatoms, whereas high irradiance decreases spring diatom dominance [17]. Research on a reservoir in the Czech Republic showed that the growth rates of the two most abundant diatom species were positively correlated with daily light exposure instead of nutrient concentration during the spring diatom bloom [33]. The results from this study suggest that the shift from Microcystis dominance to N. palea dominance in treatments B2–B16 is related to the low-light conditions, which were more appropriate for diatoms and subsequently led to them becoming dominant.
The various Microcystis biomass treatments also had different TN and TP concentrations (Table 1), although dissolved nutrients were not measured during this experiment. Table 1 shows that the greater the biomass, the higher the nutrient levels, which suggests that Microcystis biomass can affect nutrient levels as well as light intensity in the jars. Linear regression between the square root transformed data of the first three samples of Chl-a and the three highest density of diatom Nitzschia showed that Chl-a can explain changes in Nitzschia density well (Figure 6). Moreover, the changes in the Microcystis colonies (Figure 7, Figure 8 and Figure 9) showed that the Microcystis colonies decayed to different extents in the various biomass treatments. When the Microcystis biomass was at the levels shown in treatments B18–B24, the Microcystis colonies gradually decayed, but there were few diatom cells (Figure 9). It is obvious that the nutrient contents at bulk concentration of 18–24 mL L−1 were abundant enough for algal growth, which was in contrast to treatments B2–B16. However, diatoms did not propagate when the bulk concentrations was 18–24 mL L−1, which suggests that some other factors were affecting the process. It is probable that the nutrient and light levels were not appropriate for diatom succession in treatments B18–B24, whereas the conditions were appropriate for diatom dominance in treatments B2–B16.
The shift from Microcystis blooms to diatom dominance can be summarized as follows: the Microcystis bloom gradually decays under aeration mixing and low-light conditions, which provides nutrients for the algal shift. The low light and nutrient levels are appropriate for diatom growth when the bulk concentration of the Microcystis bloom is between 2 and 16 mL L−1 and the bloom is suspended in the water and physically lifted by aeration mixing, and the most suitable bulk concentration biomass for colonial Microcystis was in 2–12 mL L−1, in which the chlorophyll-a level was about from 285 to 1714 μg L−1.

4.3. Reasons for Diatom Aggregation Formation Under Hydrodynamic Mixing

The diatom N. palea cells were attached to the Microcystis colonies, leading to diatom aggregations (Figure 7 and Figure 8). The succession process from Microcystis colonies to diatom aggregation in treatments B2–B10 (Figure 7 and Figure 8) involved N. palea cells becoming attached to Microcystis colonies or N. palea cells forming their own colonies. The suspended colonies provided an attachment matrix for Nitzschia aggregation in this experiment.
Both diatoms and the filamentous cyanobacteria Pseudoanabaena sp. can grow on Microcystis colonies because the mucilaginous sheath acts as a physical matrix for diatom cell aggregation [34]. Some cryptophytes and diatoms can also coexist with Microcystis [12]. Nitzschia is a common genus that forms aggregates in marine snow [35]. It and some other diatoms can also grow on senescent Phaeocystis globosa colonies [36].
The decay of Microcystis blooms is very common in natural bloom water bodies, and Nitzschia is well-known to aggregate on Microcystis colonies [12,15,16,34]. Sometimes, green algae would become dominant after Microcystis bloom was regulated with hydrodynamic mixing [31,32]. On the other hand, some studies have focused on the decomposition of Microcystis blooms, particularly black bloom formations with large Microcystis biomasses under anaerobic conditions [25,37,38], in which the low DO levels play an important role. When the Microcystis bloom biomass is high and without hydrodynamic disturbance, the Microcystis bloom decomposed sharply over the first 4 days [26], and the DO level decreased to 1.15 ± 0.15 mg L−1 on day 6 [26]. Although these studies do not show any microscope pictures of the decomposition process, the decomposition time under aeration mixing in the current study was longer than 6 days, which helped maintain the mucilage sheath of the colonies and allowed them to remain as aggregates.
This experiment was carried out in a greenhouse during a hot summer with high temperatures (Figure 1), but diatom blooms can occur during hot summers in some natural water bodies [39]. Aeration improved the DO level, which was mainly maintained at >6 mg L−1 (Figure 2) and produced aerobic conditions. It is probable that the aerobic conditions caused by the aeration mixing under the low-light conditions slowed down the decomposition of Microcystis colonies in hot summer compared to anoxic or anaerobic conditions. The decomposition of Microcystis colonies was slow in this study, and the mucilage sheath of the Microcystis colonies did not completely disintegrate, which meant that it was able to provide a physical attachment matrix for diatom cell aggregation.
The diatoms in marine snow are aggregated by extracellular polysaccharides (EPS), which are also the main components of the mucilage sheaths surrounding Microcystis colonies [40]. A study on the composition of EPS in Microcystis colonies [41] showed a carbohydrate composition similar to that of the adhesive EPS produced by diatoms, where rhamnose, fucose, and xylose are common components [42]. Our results showed that diatom aggregation benefited from the mucilage sheaths of Microcystis colonies. The low light and aeration mixing also increased diatom growth and aggregation in treatments B2–B16.
In this experiment, the treatments with proper biomass of colonial Microcystis and aeration mixing promoted the biomass of Nitzschia cells, and the roles of non-aerated treatments were not investigated. Then, if non-aerated treatments were set to compare with the mixed ones in this experiment, it would be clearer to show the factors affecting the aggregation of Nitzschia. This experiment showed that there is a range of the colonial Microcystis’ biomass for the algal shift under aeration mixing and low light. The range of the colonial Microcystis’ biomass for the shift to diatom aggregates is a useful part of the series of studies for explaining the Nitzschia aggregates.

5. Conclusions

The algal shift process from Microcystis bloom to diatom dominance was affected by the initial biomass of the colonial Microcystis. In this experiment, when the shading rate was about 97%, the most suitable bulk concentration biomass range for colonial Microcystis to shift to diatom aggregation in the colonies was in 2–12 mL L−1, in which the chlorophyll-a level was about from 285 to 1714 μg L−1. The algal shift may be related to the low light and eutrophic conditions, which encouraged diatom growth. The Microcystis colonies aggregates provided a physical substrate for the attachment and growth of diatoms on Microcystis colonies under the aeration mixing. Aeration mixing under low-light conditions is a method to control Microcystis blooms, which shift the Microcystis dominance to diatom dominance. Moreover, the bulk concentration biomass of colonial Microcystis is a shortcut way to show the biomass, as the Microcystis colonies can float and accumulate on the water surface.

Author Contributions

Conceptualization, X.W. and X.C. (Xuan Che); methodology, X.W. and X.L. (Xingguo Liu); investigation, X.W. and X.L. (Xinfeng Li); statistical analysis and taxonomic determination, X.W. and L.Z.; writing—original draft preparation, X.W. and X.C. (Xiaolong Chen); writing—review and editing, X.C. (Xuan Che), X.L. (Xingguo Liu) and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program of China, grant number: 2019YFD0900305, the National Freshwater Genetic Resource Center of China, grant number: FGRC:18537, and the Young Elite Scientists Sponsorship Program by CAST, grant number: 2022QNRC001.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from X.W. upon reasonable request.

Acknowledgments

The authors are deeply grateful to all those involved in the implementation of this study for their assistance, guidance, and support, as well as the two anonymous reviewers for their thorough work with the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the writing of the manuscript or in the decision to publish.

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Figure 1. Changes in water temperature at 9:00–10:00 and 14:00–15:00 h every day in treatment B4.
Figure 1. Changes in water temperature at 9:00–10:00 and 14:00–15:00 h every day in treatment B4.
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Figure 2. Changes in dissolved oxygen (DO) and pH value at 14:00–15:00 h in each treatment.
Figure 2. Changes in dissolved oxygen (DO) and pH value at 14:00–15:00 h in each treatment.
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Figure 3. Changes in Chl-a in the 12 treatments. Day 0 was 17 July 2020, and day 41 was 27 August 2020.
Figure 3. Changes in Chl-a in the 12 treatments. Day 0 was 17 July 2020, and day 41 was 27 August 2020.
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Figure 4. Changes in the phenomenon of all the jars in different treatments under the black polyolefin sun-shading net in the greenhouse. The 0 d, 4 d, 7 d, 10 d, 14 d, 17 d, 20 d, 24 d, 27 d, 32 d, and 36 d in the figure represent the picture on days 0, 4, 7, 10, 14, 17, 20, 24, 27, 32, and 36, respectively. The distribution of the jars in different treatments in the greenhouse is presented in 4a. The last subfigure Pla represents the placement diagram of the jars in different treatments under the black polyolefin sun-shading net in the greenhouse.
Figure 4. Changes in the phenomenon of all the jars in different treatments under the black polyolefin sun-shading net in the greenhouse. The 0 d, 4 d, 7 d, 10 d, 14 d, 17 d, 20 d, 24 d, 27 d, 32 d, and 36 d in the figure represent the picture on days 0, 4, 7, 10, 14, 17, 20, 24, 27, 32, and 36, respectively. The distribution of the jars in different treatments in the greenhouse is presented in 4a. The last subfigure Pla represents the placement diagram of the jars in different treatments under the black polyolefin sun-shading net in the greenhouse.
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Figure 5. The changes in Nitzschia cell density in each treatment from day 17 to day 41. The Nitzschia density was measured from day 17 on, when the water in treatments B2 and B4 turned brownish.
Figure 5. The changes in Nitzschia cell density in each treatment from day 17 to day 41. The Nitzschia density was measured from day 17 on, when the water in treatments B2 and B4 turned brownish.
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Figure 6. Linear regression between the square root transformed data of the first three samples of Chl-a and the three highest densities of diatom Nitzschia. The red line represents linear regression, and the black squares are scatter plots.
Figure 6. Linear regression between the square root transformed data of the first three samples of Chl-a and the three highest densities of diatom Nitzschia. The red line represents linear regression, and the black squares are scatter plots.
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Figure 7. Representative process for the algal shift from Microcystis colonies to Nitzschia dominance with aggregation. The Nitzschia dominance declines in treatment B2 (2 mL L−1 bulk concentration). The 14 d, 17 d, 26 d, and 34 d in the figure represent microscope photographs on days 14, 17, 26, and 34, respectively.
Figure 7. Representative process for the algal shift from Microcystis colonies to Nitzschia dominance with aggregation. The Nitzschia dominance declines in treatment B2 (2 mL L−1 bulk concentration). The 14 d, 17 d, 26 d, and 34 d in the figure represent microscope photographs on days 14, 17, 26, and 34, respectively.
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Figure 8. Representative process showing the algal shift from Microcystis colonies to Nitzschia aggregations in treatment B10 (10 mL L−1 bulk concentration). The 10 d, 13 d, 17 d, and 26 d in the figure represent microscope photographs on days 10, 13, 17, and 26, respectively.
Figure 8. Representative process showing the algal shift from Microcystis colonies to Nitzschia aggregations in treatment B10 (10 mL L−1 bulk concentration). The 10 d, 13 d, 17 d, and 26 d in the figure represent microscope photographs on days 10, 13, 17, and 26, respectively.
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Figure 9. Representative process showing the algal shift from Microcystis colonies to bacteria dominance without Nitzschia aggregation in treatment B18 (18 mL L−1 bulk concentration). The 3 d, 20 d, 26 d, and 34 d in the figure represent microscope photographs on days 3, 20, 26, and 34, respectively.
Figure 9. Representative process showing the algal shift from Microcystis colonies to bacteria dominance without Nitzschia aggregation in treatment B18 (18 mL L−1 bulk concentration). The 3 d, 20 d, 26 d, and 34 d in the figure represent microscope photographs on days 3, 20, 26, and 34, respectively.
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Table 1. Chl-a, Microcystis cells, TN, and TP concentrations in each treatment at the start.
Table 1. Chl-a, Microcystis cells, TN, and TP concentrations in each treatment at the start.
TreatmentThe Bulk Concentration
of the Microcystis
Bloom (mL L−1)		Chl-a
(μg L−1)		Microcystis Cells
Concentration (106 cells mL−1)		TN
(mg L−1)		TP
(mg L−1)
B22.0285.721.994.4980.314
B44.0575.544.259.0900.632
B66.0867.186.4513.5220.924
B88.01140.988.6217.9921.254
B1010.01445.6010.8122.5321.586
B1212.01732.3213.0727.1241.896
B1414.02045.0415.6331.5682.132
B1616.02298.7817.5436.0242.614
B1818.02579.5819.5640.4922.910
B2020.02867.5021.9345.0103.158
B2222.03156.9423.9149.4773.467
B2424.03448.6826.7155.1453.852
Table 2. Photosynthetically available radiation (PAR) levels outdoors and under the black sun-shading net in the greenhouse. The shading rate was calculated on 25 July 2020.
Table 2. Photosynthetically available radiation (PAR) levels outdoors and under the black sun-shading net in the greenhouse. The shading rate was calculated on 25 July 2020.
TimePARUnder the Black Sun-Shading NetOutdoors Shading Rate of the Sun-Shading Net in the Greenhouse
13:00Quantum/(μmol m−2 s−1)48.02 ± 0.221649.36 ± 8.2797.1%
Energy/(W m−2)8.52 ± 0.03320.90 ± 9.5097.3%
14:30Quantum/(μmol m−2 s−1)46.59 ± 0.381569.17 ± 8.6097.0%
Energy/(W m−2)8.08 ± 0.05257.59 ± 1.5796.9%
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Wang, X.; Che, X.; Liu, X.; Li, X.; Chen, X.; Li, Y.; Zhu, L. The Range of the Colonial Microcystis’ Biomass for Shift to Diatom Aggregates Under Aeration Mixing and Low Light. Diversity 2024, 16, 695. https://doi.org/10.3390/d16110695

AMA Style

Wang X, Che X, Liu X, Li X, Chen X, Li Y, Zhu L. The Range of the Colonial Microcystis’ Biomass for Shift to Diatom Aggregates Under Aeration Mixing and Low Light. Diversity. 2024; 16(11):695. https://doi.org/10.3390/d16110695

Chicago/Turabian Style

Wang, Xiaodong, Xuan Che, Xingguo Liu, Xinfeng Li, Xiaolong Chen, Yiming Li, and Lin Zhu. 2024. "The Range of the Colonial Microcystis’ Biomass for Shift to Diatom Aggregates Under Aeration Mixing and Low Light" Diversity 16, no. 11: 695. https://doi.org/10.3390/d16110695

APA Style

Wang, X., Che, X., Liu, X., Li, X., Chen, X., Li, Y., & Zhu, L. (2024). The Range of the Colonial Microcystis’ Biomass for Shift to Diatom Aggregates Under Aeration Mixing and Low Light. Diversity, 16(11), 695. https://doi.org/10.3390/d16110695

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