Water eutrophication aroused by the excessive discharge of nutrients and phosphorus has resulted in the overgrowth of harmful algal blooms (HABs) worldwide (Zhang et al. 2021a). M. aeruginosa, one kind of the cyanobacteria (Schmidt et al. 2020), not only influences negatively the water ecosystem (Lu et al. 2021b) but also produces toxic microcystins (MCs) (Zhang et al. 2020), which can pose toxic effect on liver, nervous and genital system (Zhang et al., 2020a). Up to now, many strategies have been applied to control M. aeruginosa, such as photocatalysis (Fan et al. 2021, Wang et al. 2020), hydrogen peroxide (Wang et al. 2019), copper sulfate (Anderson 2009), etc. Though these methods are fast and effective in the inhibition of M. aeruginosa, the potential threat to aquatic environment and the secondary pollution limit their large-scale application (Zhu et al. 2020). Currently, an enormous amount of effort has been devoted to developing a biological way to inhibiting the algae, different kinds of the bacteria, which are able to dissociate the algae, are found and applied to control M. aeruginosa. The reported algicidal bacteria species involved Bacillus sp. (Xuan et al. 2017), Acinetobacter (Yi et al. 2015), Streptomyces (Yu et al. 2019), etc. It was reported that M. aeruginosa could be inhibited by algicidal bacteria through direct feeding or secreting algicidal compounds and they can have a substantial effect on M. aeruginosa (Lu et al. 2021a).
The preparation of microbial inhibition reagents for the algae is usually conducted by gathering algicidal bacteria from water, soil and organisms, then enriching and culturing process. Thus,
most of current microbial algal inhibition reagents included algicidal bacteria and culture medium (Li et al. 2021).
Usually, microbial algal inhibition reagents are used by directly pouring in natural water, algicidal bacteria were then released and dispersed, which could inhibit HABs effectively. However, the environment and conditions for algicidal bacteria, including temperature, water velocity and natural enemies, are complex. Especially, it is hard to ensure algicidal bacteria to be the predominant bacterial population in natural micro-ecosystem, and may disappear due to intense competition between them. Giving a desirable community environment for algicidal bacteria is thus essential to keep the activity and effect of microbial inhibition reagents.
Capsule immobilization of algicidal bacteria is a potential solution (Hu et al. 2020, Ma et al. 2021, Wang &Coyne 2020), which can provide favorable environment for algicidal bacteria and improve the algicidal effect. For example, Shewanella sp. IRI-160 was immobilized into several carriers (Agarose, alginate hydrogel, cellulosic sponge, and polyester foam) and used in inhabiting harmful dinoflagellates, a higher activity against the target species compared to free-living bacteria (Wang &Coyne 2020) was obtained. It is considered that a suitable immobilized carrier material is critical for keeping the activity of immobilized bacteria (Chen et al. 2013). Sodium alginate (SA), with good biocompatibility and adhesion property (Bennacef et al. 2021), is the most used in loading material because SA cross-links with divalent cations to form the hydrogel (Khalid et al. 2018, Rybak 2021, Yerramathi et al. 2021). It is widely used in medical pharmaceutical (Zhang et al. 2021b), food production (Yan et al. 2021) and microbial composite (Yamaguchi et al. 2019). It has been shown that the high-density of M. aeruginosa is upper water (Aparicio Medrano et al. 2013), and keeping microbial inhibition reagents enriching in upper water can take fully advantage of algicidal bacteria. It is assumed that SA was employed as the loaded material for greater biocompatibility and environmental safety to combine the gathered algicidal bacteria and form the capsule. Furthermore, ethyl cellulose (EC) was insoluble in water and exhibits good chemical, physical and mechanical properties (Lin et al. 2018). EC was chosen to coat on the surface of the capsule to keep the capsule floating on the water. It is supposed that algicidal bacteria enriching in the floating capsule structure is a feasible approach to provide a desirable microbial population.
In current study, algicidal bacteria HL, was merged by the protoplast fusion, exhibited a better performance in the inhibition of M. aeruginosa. The algicidal bacteria HL was mixed with SA to prepare the capsule, and EC was coated on the surface of the capsule to keep floating. Scanning electron microscope (SEM) was used to characterize the morphology of algicidal bacteria HL in the capsule. The algicidal efficiency of prepared capsules on algae cells and chlorophyll-a was evaluated and compared with free-living bacteria. The algae inhibition process of capsules was speculated by the changes in algal fluorescence and oxidative stress biomarkers in M. aeruginosa. This study can provide an effective approach for improving the efficiency of algicidal bacteria in the microbial treatment of HABs.