Cyanobacterial blooms exist in many freshwater bodies worldwide (Schindler, 1974). Microcystis spp., which can form colonies ranging in size from a few microns to a few millimeters, are the most common and ubiquitous toxic blooms (Paerl et al., 2014). Under warming and eutrophic conditions, Microcystis cells have the propensity to aggregate and float upward, giving rise to harmful Microcystis blooms (Luerling et al., 2017; Zhu et al., 2023). The extensive proliferation of harmful Microcystis has various negative effects on human health and environmental safety (Fiehn et al., 1998; Susanna et al., 2020). This phenomenon results in the depletion of dissolved oxygen in the water, causing disruptions to aquatic ecosystems (Paerl et al., 2011; Guo et al., 2022), in which the decay of biomass ultimately leads to oxygen depletion, causing a complete alteration of the aquatic environment (Hallegraeff, 1993). Microcystins are toxins produced by a variety of bloom-forming cyanobacteria that can cause hepatotoxicity in humans and animals (Zhou et al., 2021).

The formation of Microcystis blooms is affected by various biotic and abiotic factors, including nutrients, light, temperature (Andrew, 1991; Soranno, 1997; Hans and Valerie, 2012; Min et al., 2012), hydrodynamic conditions (Medrano et al., 2013; Chao et al., 2017), predation (Wang et al., 2010), and buoyancy of Microcystis colonies (Jacco and Luuc, 1984; Medrano et al., 2016). Due to the buoyancy regulation, Microcystis is capable of forming surface scum in eutrophic lakes (Chorus, 1999; Rainer et al., 2003). Many former studies suggest that buoyancy provides several competitive advantages for Microcystis over phytoplankton, including the capability to acquire light and carbon dioxide at the uppermost layer, and grazing avoidance (Reynolds and Walsby, 1975; Lovelock et al., 2008). Microcystis had a diurnal migration pattern, and migration causes were affected by multiple factors (Wu et al., 2019). This development will be rapid, probably on a characteristic timescale of approximately a day (Timothy et al., 2009). The sudden increases in biomass at the surface layer may lead to long-term proliferative cell interactions and mass migration of biomass. In many cases, this migration ends up forming thick scum on the surface of the water (Anne et al., 2016).

There is currently research evidence demonstrating that the regulation of gas buoyancy on vesicles and carbohydrate ballast is a crucial factor in the migration of Microcystis colony (Colin et al., 2002). The hypothesis posited suggests that the irreversible buoyancy of cyanobacterial colonies is induced by the growth of gas bubbles on or within the mucilage of the colonies (Medrano et al., 2016). They hypothesized that the irreversible buoyancy of cyanobacterial colonies is induced by the growth of bubbles on or inside the colony mucilage. These bubbles grow under conditions of oxygen supersaturation. Meanwhile, many abiotic factors can affect the buoyancy of Microcystis through these mechanisms (Hans and Jef, 2008; Paerl and Otten, 2013). For instance, light has been found to regulate buoyancy through the carbohydrate ballast mechanism. Specifically, Microcystis loses buoyancy under high light conditions, while it regains buoyancy under low light conditions. This allows Microcystis to exhibit a diel migration pattern, where it floats upward to the surface at night and sinks during the daytime. Although this migration pattern has been confirmed by many lake studies (Ma et al., 2015), there are exceptions where Microcystis forms scum on a shorter timescale of hours and can persist at the water surface even under high light conditions during the daytime. This may imply the existence of additional mechanisms for the rapid Microcystis flotation and the surface scum formation under strong light conditions.

To fill the knowledge gaps, the influence of high light on the floatation and surface scum formation of Microcystis was investigated in this study. We used two different strains of Microcystis species (Microcystis wesenbergii and Microcystis aeruginosa), which are neutrally buoyant, to study the non-buoyancy-driven floatation of Microcystis with laboratory experiments. We hypothesize that the bubbles generated from photosynthesis during high light exposure can felicitate the rapid floatation of Microcystis. We aim to study the mechanism of rapid floatation of Microcystis driven by bubble formation. This study is expected to provide new implications for the mechanism for the formation of Microcystis blooms as well as cyanobacterial species succession.

Our study reveals a novel mechanism for the rapid flotation of Microcystis under light conditions, which is species-specific. Under such conditions, M. wesenbergii was capable of forming surface scum, while M. aeruginosa could not form visible scum throughout the experiments. This timescale is shorter than the observed timescale for scum formation in conventional bloom formation process. The timescale for the formation of cyanobacterial blooms in the wild is influenced by various factors, including the nutrient status of the water body, temperature, and light intensity. Generally, the formation of cyanobacterial blooms can range from several days to several weeks. In highly eutrophic waters, under warm and stable climatic conditions, blooms may develop rapidly (Christopher et al., 2011). Considering the difference in DO and EPS content between the treatments in our study, we speculated two possible reasons for the observed phenomenon: 1) exopolysaccharides may play an important role in algal cell adhesion and surface bloom formed, and 2) the rising process of O₂ bubbles accompanied by algal migration to the surface improves the biological amount leading to the formation of surface blooms. Under strong light conditions, the formation of bubbles can drive the formation of surface blooms.

For the first hypothesis, we analyzed the extracellular substances secreted by two Microcystis species. Light could affect the extracellular substances secreted by Microcystis and produce varying amounts of polysaccharides, thus affecting the aggregate formation. The SEM results confirmed the presence of significant secretions in the M. wesenbergii samples. These secretions can promote aggregate formation and upward migration to the water surface with the help of the micro-bubbles ( Figure 1D ). The S-EPS released from M. wesenbergii solution played an important role in the aggregation process ( Figures 3A, B ).

Studies have shown that proteins and polysaccharides are indispensable for maintaining the cross-linked structure of extracellular substances (Yang et al., 2008; Gan et al., 2012; Huiqun et al., 2018). The effect of EPS on the microbial aggregation process is mainly reflected in the bridging effect. However, the alteration of different types of EPS or the proportion of certain components can also affect this process. Meanwhile, the protein can promote the stability of maintaining the polymer structure (Badiaa et al., 2010). The protein in EPS contains more negatively charged amino acids ( Figure 2 ). Compared with polysaccharides, the electrostatic bridging between EPS and divalent and polyvalent cations is more obvious. The existence of EPS and algal aggregates also contributes to better preservation of the integrity of colony algal cells. Furthermore, B-EPS, which tightly binds to the cell membrane of the algae, can serve as a binding molecule between algae cells, promoting the combination of Microcystis cells into a colony structure. CLSM results also confirmed that the large algal floc that rises to the surface was formed by large amounts of proteins and polysaccharides that adhere together tightly to form a “macroalgal collective” ( Figure 1B ). Indeed, EPS with multifunctional groups can promote the adsorption and chelation with various organic and inorganic matter to form aggregates in aqueous systems (Bai et al., 2016; Zhang et al., 2019).

The FT-IR spectroscopy of the Microcystis floating and migrating to the surface formed the surface film ( Figure 6 ). This provides physical and chemical evidence for its formation and helps understand the occurrence of possible biomolecular groups or chemical changes. During the formation of surface blooms, the main functional groups in the extracellular substances secreted by Microcystis, such as hydroxyl and carboxyl groups, could have acted as binding sites and bridges with the high-molecular-weight adsorption sites of Microcystis S-EPS to form a network structure, which tightly packed single-celled Microcystis, and bubbles rise with photosynthesis ( Figure 1 ). FT-IR analysis results showed that all samples contained a large number of hydrogen bonds, as evidenced by the broad peak of 3,500–3,400 cm⁻¹ and 3,431–3,408 cm⁻¹. There were noticeable changes in glycosidic bonds. The EPS was generally supported by a hydrogen bond system, and a higher ratio of hydrogen bonds resulted in stronger intermolecular interaction (Zhu et al., 2014). These findings were consistent with those of previous studies (Yang et al., 2016; Li et al., 2021).

For the second hypothesis, we analyzed the changes in oxygen quantity and electric potential during the formation of surface blooms. By modulating their buoyancy using internal gas vesicles, individual Microcystis can migrate along the water column at a speed of up to 1 mm/h (Walsby and Anthony, 1994). At the same time, the bubbles formed by the increase in sample oxygen are beneficial to the formation of water blooms on the surface of Microcystis (Johnk et al., 2008). This supports our conclusion that the rising process of O₂ bubbles is accompanied by algal migration to the surface, which improves the biological amount and forms surface blooms. The O₂ supersaturation is generally caused by concentrated photosynthesis, which in turn forms bubbles. The trapped O₂ bubbles provide lift and gather most of the biomass on the surface of the water column to form a denser foam layer. Also, this rapid migration process is irreversible. The foam will remain stable for several weeks if the nutritional conditions are sufficient. According to literature reports, blooming occurs without a major increase in overall biomass, and the threshold for irreversible migration concentration is above 10⁶ cells/mL (Klemer et al., 1982). As cyanobacteria migrate toward the free surface, their effective concentration increases, and the system moves away from the critical point for blooming. The rate of oxygen production per unit biomass decreases under low light intensity or under weak photosynthetic mechanisms in harsh environments (Dervaux et al., 2015). Instead, light boosts photosynthesis and makes more bubbles. It is possible that oxygen production is the limiting factor for bloom formation.

The zeta potential is the potential of the shear plane in the double electric layer of charged particles in solution, which can reflect the stability of the colloidal system (Zhao et al., 2017). During the experimental stage ( Figure 2B ), the main force in M. wesenbergii solution is gravity rather than repulsion. The surface charge of protein molecules in the system is high, which maintains the stability of the system through electrostatic repulsion, making it difficult for protein molecules to accumulate (Bojorquez-Velazquez et al., 2016).

In addition, we performed a Fisher exact test bar plot on the species richness of M. wesenbergii before and after the experiment ( Supplementary Figure 2 ). The results indicated a significant increase in species abundance at the level of Microcystis phylum after the experiment. Subsequently, individuals were randomly sampled from the specimens, and the dilution curve was based on the number of individuals and species ( Supplementary Figure 3A ). The species richness of M. wesenbergii before and after the experiment was compared by drawing dilution curves. Under the condition of extracting the same sequence, the number of operational taxonomic units (OTUs) of M. wesenbergii was higher after the experiment, indicating that the species richness of M. wesenbergii was higher. In this dilution graph ( Supplementary Figure 3B ), eventually, the curve tended to flatten out, further indicating that the number of samples taken was reasonable. Alpha diversity refers to the diversity within a specific region or ecosystem. The statistical t-test was used to detect significant differences between each of the two sets of index values ( Supplementary Figure 3C ).

Environmental isolates of M. wesenbergii form large colonies and exhibit complex vertical migration dynamics due to their dynamic glycan ballast ability to compete with oxygen-mediated upward migration. While similar experiments on indoor samples may still be qualitative, it would be interesting to test the mechanisms identified in this study using natural samples. Although the aggregation and binding of cyanobacteria EPS to form algal blooms have been reported (Parker et al., 2000; Micheletti et al., 2008), the underlying mechanisms were still poorly understood, and more work is needed to elucidate these binding processes. In this regard, similar experiments on algae with differences in EPS production can provide valuable data for further understanding the cell aggregation process. In future studies, we should also focus on effective and special substances that facilitate the aggregation process and identify the structure and properties of these substances. The dominant niche of high temperature-adapted cyanobacteria genera will be further reinforced with global warming and elevated carbon dioxide in the future. The cyanobacterial dominance and succession are inherently attributed to the distinctive traits of cyanobacteria including colony formation, gas vesicles, toxin release, and nitrogen fixation (Wang et al., 2021; Firsova et al., 2023). In the future, we will further explore the extracellular substances released by Microcystis and the physiological and biochemical effects of Microcystis.

The present study revealed that the two different species of Microcystis (M. wesenbergii and M. aeruginosa) ascended and gathered under light and dark conditions. It focused on the impact of the EPS and bubbles released by Microcystis on the formation of surface scum during the process of floating and gathering to the surface scum. The results showed the following:

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

TY: Conceptualization, Data curation, Formal Analysis, Validation, Visualization, Writing – original draft, Writing – review & editing. JP: Data curation, Methodology, Project administration, Visualization, Writing – review & editing. HW: Conceptualization, Writing – review & editing. CT: Supervision, Project administration, Resources, Writing – review & editing. CW: Supervision, Project administration, Resources, Writing – review & editing. BX: Supervision, Project administration, Resources, Writing – review & editing. MP: Supervision, Project administration, Resources, Writing – review & editing. XW: Funding acquisition, Project administration, Resources, Software, Supervision, Writing – review & editing.