Diverse interactions between bacteria and microalgae: A review for enhancing harmful algal bloom mitigation and biomass processing efficiency

The interactions between bacteria and microalgae play pivotal roles in resource allocation, biomass accumulation, nutrient recycling, and species succession in aquatic systems, offering ample opportunities to solve several social problems. The escalating threat of harmful algal blooms (HABs) in the aquatic environment and the lack of cheap and eco-friendly algal-biomass processing methods have been among the main problems, demanding efficient and sustainable solutions. In light of this, the application of algicidal bacteria to control HABs and enhance algal biomass processing has been promoted in the past few decades as potentially suitable mechanisms to solve those problems. Hence, this comprehensive review aims to explore the diverse interaction modes between bacteria and microalgae, ranging from synergistic to antagonistic, and presents up-to-date information and in-depth analysis of their potential biotechnological applications, particularly in controlling HABs and enhancing microalgal biomass processing. For instance, several studies revealed that algicidal bacteria can effectively inhibit the growth of Microcystis aeruginosa, a notorious freshwater HAB species, with an antialgal efficiency of 24.87 %-98.8 %. The review begins with an overview of the mechanisms behind algae-bacteria interactions, including the environmental factors influencing these dynamics and their broader implications for aquatic ecosystems. It then provides a detailed analysis of the role of algicidal bacteria in controlling harmful algal blooms, as well as their role in bioflocculation and the pretreatment of microalgal biomass. Additionally, the review identifies and discusses the constraints and challenges in the biotechnological application of these interactions. By exploring the strategic use of algicidal bacteria, this review not only underscores their importance in maintaining aquatic environmental health but also in enhancing biomass processing efficiency. It offers valuable insights into future research avenues and the potential scalability of these applications, both in situ and at an industrial level.

Recent Advances in the Research on the Anticyanobacterial Effects and Biodegradation Mechanisms of Microcystis aeruginosa with Microorganisms

Harmful algal blooms (HABs) have attracted great attention around the world due to the numerous negative effects such as algal organic matters and cyanobacterial toxins in drinking water treatments. As an economic and environmentally friendly technology, microorganisms have been widely used for pollution control and remediation, especially in the inhibition/biodegradation of the toxic cyanobacterium Microcystis aeruginosa in eutrophic water; moreover, some certain anticyanobacterial microorganisms can degrade microcystins at the same time. Therefore, this review aims to provide information regarding the current status of M. aeruginosa inhibition/biodegradation microorganisms and the acute toxicities of anticyanobacterial substances secreted by microorganisms. Based on the available literature, the anticyanobacterial modes and mechanisms, as well as the in situ application of anticyanobacterial microorganisms are elucidated in this review. This review aims to enhance understanding the anticyanobacterial microorganisms and provides a rational approach towards the future applications.

Biochemical and Physiological Responses of Harmful Karenia mikimotoi to Algicidal Bacterium Paracoccus homiensis O-4

Harmful algal blooms caused by Karenia mikimotoi frequently occur worldwide and severely threaten the marine environment. In this study, the biochemical and physiological responses of K. mikimotoi to the algicidal bacterium Paracoccus homiensis O-4 were investigated, and the effects on the levels of reactive oxygen species (ROS), malondialdehyde content, multiple antioxidant systems and metabolites, photosynthetic pigments, and photosynthetic index were examined. The cell-free supernatant in strain O-4 significantly inhibited K. mikimotoi cell growth. The bacterium caused the K. mikimotoi cells to activate their antioxidant defenses to mitigate ROS, and this effect was accompanied by the upregulation of intracellular antioxidant enzymes and non-enzyme systems. However, the overproduction of ROS induced lipid peroxidation and oxidative damage within K. mikimotoi cells, ultimately leading to algal death. In addition, the photosynthetic efficiency of the algal cells was significantly inhibited by O-4 and was accompanied by a reduction in photosynthetic pigments. This study indicates that O-4 inhibits K. mikimotoi through excessive oxidative stress and impaired photosynthesis. This research into the biochemical and physiological responses of K. mikimotoi to algicidal bacteria provides insights into the prophylaxis and control of harmful algal blooms via interactions between harmful algae and algicidal bacteria.

Nutritional status regulates algicidal activity of Aeromonas sp. L23 against cyanobacteria and green algae

Algicidal bacteria have received broad acceptance as an ecofriendly tool for controlling harmful algal blooms. However, their practical application is still limited to the lab-scale tests due to the complex alga–bacterium interactions in different nutrient statuses. In this study, the Aeromonas sp. L23 that exhibit relatively wide-spectrum in algicidal activity was isolated from a eutrophic agricultural lake. The physiological response of cyanobacteria and green to the algicidal activity under varied nutritional status were studied in an alga-bacterial co-culture. The algicidal activities of L23 against Microcystis aeruginosa UTEX LB 2385, Microcystis aeruginosa NHSB, Anabaena variabilis AG10064, Scenedesmus quadricauda AG10003, and Chlorella vulgaris AG10034 were 88 ± 1.2%, 94 ± 2.6%, 93 ± 0.5%, 82 ± 1.1%, and 47 ± 0.9%, respectively. The L23 cells had low algicidal activity in cell pellet (3%–9%) compared with the cell-free supernatant (78%–93%), indicating that the activity is induced by extracellular substances. Adding glucose, NaNO₃, NH₄Cl, and KH₂PO₄ to the co-culture raised the algicidal activity of the L23 against green algae by 5%–50%. Conversely, a 10%–20% decrease in activity occurred against the target cyanobacteria except M. aeruginosa UTEX LB 2385. These results indicated that the interspecific algicidal activity changes according to the nutritional status, which means that the alga-bacterium interaction will be more complex in the field where the nutritional status changes from time to time.

Combined algicidal effect of urocanic acid, N-acetylhistamine and l-histidine to harmful alga Phaeocystis globosa

The algicidal compounds produced by Bacillus sp. strain B1 against Phaeocystis globosa, one of the main red-tide algae, were isolated and identified in a previous study as urocanic acid (uro), l-histidine (his) and N-acetylhistamine (ace). The 96 h median effective concentration EC50 values indicated the algicidal effect order of uro (8 μg mL-1) > ace (16 μg mL-1) > his (23 μg mL-1). The interaction between uro and ace had a synergistic effect on Phaeocystis globosa, accelerated the increase in its intracellular reactive oxygen species (ROS) levels, and further decreased the activities of antioxidases after 96 h, causing destruction of cell membrane integrity and nuclear structure. However, the other two binary mixtures uro + his and ace + his were both antagonistic to Phaeocystis globosa. The increase in the level of ROS indicated that the algal cells suffered from oxidative damage. The surplus ROS induced the increase in malondialdehyde (MDA) content and activities of antioxidant enzymes including superoxide dismutase (SOD) and catalase (CAT), all of which reached maxima after 72 h treatment. Transmission electron microscopy (TEM) analysis revealed that these nitrogen-containing compounds caused destruction of cell membrane integrity, chloroplasts and nuclear structure. The present study will provide useful information for the combined effect of algicidal compounds on the harmful alga Phaeocystis globosa. This is the first report to explore single and combined algicidal effects of three nitrogen-containing compounds against the harmful alga Phaeocystis globosa.

Anticyanobacterial effect ofl-lysine onMicrocystis aeruginosa

Cyanobacterial blooms can cause serious environmental problems and threaten aquatic organisms and human health. It is therefore essential to effectively control cyanobacterial blooms in aquatic ecosystems. In the present study, the anticyanobacterial effect of l-lysine on Microcystis aeruginosa was examined. The results showed that the growth of M. aeruginosa (>90%) was effectively inhibited by l-lysine at dosages of 5.0, 6.5, and 8.0 mg L⁻¹ after 3 d treatment. The content of superoxide anion radicals, MDA content and SOD activity in M. aeruginosa cells increased after 1 d of treatment with l-lysine (3.0, 5.0, 6.5, and 8.0 mg L⁻¹), revealing that l-lysine induced oxidative stress in the cyanobacterial cells. The chlorophyll-a and protein contents in M. aeruginosa treated with l-lysine (3.0, 5.0, 6.5, and 8.0 mg L⁻¹) decreased after 2 d, indicating damage of the photosynthetic system by l-lysine treatment. Additionally, the production of exopolysaccharide by M. aeruginosa also increased and the expression of polysaccharide synthesis genes was upregulated by 3.0 mg L⁻¹l-lysine after 3 d of treatment. In response to the algicidal effects of l-Lysine, M. aeruginosa upregulated exopolysaccharide synthesis. Electron microscopic observations demonstrated that the cell membrane of M. aeruginosa was broken down during treatment with l-lysine (≥3.0 mg L⁻¹). Our results revealed that the effects of l-lysine on M. aeruginosa cells were comprehensive, and l-lysine is therefore an efficient anticyanobacterial reagent.

l-lysine had an anticyanobacterial effect on Microcystis aeruginosa, which involved growth inhibition, physiological and metabolic disturbance, and cell membrane damage.