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Abstract In both freshwater and marine ecosystems, phytoplankton are the most dominant primary producers, contributing substantially to aquatic food webs. Algicidal bacteria that can associate to microalgae from the phytoplankton have the capability to control the proliferation and even to lyse them. These bacteria thus play an important role in shaping species composition in pelagic environments. In this review, we discuss and categorise strategies used by algicidal bacteria for the attack on microalgae. We highlight the complex regulation of algicidal activity and defence responses that govern alga–bacteria interactions. We also discuss how algicidal bacteria impact algal physiology and metabolism and survey the existing algicidal metabolites and enzymes. The review illustrates that the ecological role of algicidal bacteria is not yet fully understood and critically discusses the challenges in obtaining ecologically relevant data. This review summarises current knowledge on the interaction of algicidal bacteria and microalgae with special emphasis on ecological and functional aspects.

Alexandriumpacificum is a typical toxic bloom-forming dinoflagellate, causing serious damage to aquatic ecosystems and human health. Many bacteria have been isolated, having algicidal effects on harmful algal species, while few algicidal bacteria have been found to be able to lyse A. pacificum. Herein, an algicidal bacterium, Shewanella Y1, with algicidal activity to the toxic dinoflagellate A. pacificum, was isolated from Jiaozhou Bay, China, and the physiological responses to oxidative stress in A. pacificum were further investigated to elucidate the mechanism involved in Shewanella Y1. Y1 exhibited a significant algicidal effect (86.64 ± 5.04% at 24 h) and algicidal activity in an indirect manner. The significant declines of the maximal photosynthetic efficiency (Fv/Fm), initial slope of the light limited region (alpha), and maximum relative photosynthetic electron transfer rate (rETRmax) indicated that the Y1 filtrate inhibited photosynthetic activities of A. pacificum. Impaired photosynthesis induced the overproduction of reactive oxygen species (ROS) and caused strong oxidative damage in A. pacificum, ultimately inducing cell death. These findings provide a better understanding of the biological basis of complex algicidal bacterium-harmful algae interactions, providing a potential source of bacterial agent to control harmful algal blooms.

Bacteria-diatom interactions in the ocean are diverse but usually studied in static conditions, which limits our understanding of their importance in marine ecosystems and biogeochemical cycles. Here, we explored the dynamic interactions between an ubiquitous marine bacterium Alteromonas sp. and a diatom Thalassiosira pseudonana under different nutrient conditions. In oligotrophic conditions, minor shifts in nutrients qualitatively altered the interactions from mutualism during early exponential growth to weak parasitism during the late stationary phase. Organic matter-activated Alteromonas chemotactically swam toward, attached on, and dramatically broke down T. pseudonana cells, leading to an aggressive parasitic behavior with a 95% algicidal rate. Meanwhile, inorganic matter-activated T. pseudonana showed amensalism against Alteromonas , resulting in an ephemeral decrease of bacterial abundance by 27%. Interestingly, when both organics and inorganics were sufficient, Alteromonas suppressed diatom growth by inhibiting the cell division, while the surviving T. pseudonana restored proliferation with a significantly smaller cell size inconducive to bacterial attachment, demonstrating an intense competition. The results further indicated that the algicidal effect of Alteromonas was controlled by the cell-specific protease activity and the number of attached bacteria on the diatom cell surface, both of which were related to nutrient conditions. Since the nature and intensity of bacteria-diatom interaction depend on the composition and richness of nutrients, it mechanistically explains the tripartite relationship among bacterial proliferation, nutrient viability, and algal demise during blooms. The algicidal behavior of copiotrophs also potentially enhances the contribution of a microbial carbon pump to carbon sequestration in the ocean. As the major producers and consumers, phytoplankton and bacteria play central roles in marine ecosystems and their interactions show great ecological significance. Whether mutualistic or antagonistic, the interaction between certain phytoplankton and bacterial species is usually seen as a derivative of intrinsic physiological properties and rarely changes. This study demonstrated that the interactions between the ubiquitously co-occurring bacteria and diatom, Alteromonas and Thalassiosira pseudonana , varied with nutrient conditions. They overcame hardship together in oligotrophic seawater but showed antagonistic effects against each other under nutrient amendment. The contact-dependent algicidal behavior of Alteromonas based on protease activity solved the paradox among bacterial proliferation, nutrient viability, and algal demise haunting other known non-contact-dependent algicidal processes and might actually trigger the collapse of algal blooms in situ . The chemotactic and swarming movement of Alteromonas might also contribute greatly to the breakdown of “marine snow,” which could redirect the carbon sequestration pathway in the ocean.

Harmful cyanobacterial blooms occur worldwide and pose a great threat to aquatic ecosystems and public health. The application of algicidal bacteria represents an eco-friendly strategy for controlling harmful cyanobacterial blooms; thus, searching for a high efficiency of algicidal bacteria has been becoming an important and continuous task in science. Herein, we identified a bacterial strain coded Streptomyces sp. HY with a highly algicidal activity, and investigated its algicidal efficiency and mechanism against Microcystis aeruginosa. The strain HY displayed high algicidal activity toward Microcystis aeruginosa cells, with a removal rate of 93.04% within 2 days via indirect attack. Streptomyces sp. HY also showed the ability to lyse several genera of cyanobacterial strains, including Dolichospermum, Pseudanabaena, Anabaena, and Synechocystis, whereas it showed a minor impact on the green alga Scenedesmus obliquus, demonstrating its selectivity specially for targeting cyanobacteria. Its algicidal mechanism involved damages to the photosynthesis system, morphological injury of algal cells, oxidative stress, and dysfunction of the DNA repair system. Furthermore, HY treatment reduced the expression levels of genes (mcyB and mcyD) related to microcystin biosynthesis and decreased the total content of microcystin-leucine-arginine by 79.18%. Collectively, these findings suggested that the algicidal bacteria HY is a promising candidate for harmful cyanobacterial bloom control.

Algicidal bacteria are important in the control of toxic dinoflagellate blooms, but studies on the environmental behavior of related algal toxins are still lacking. In this study, Bacillus subtilis S3 (S3) showed the highest algicidal activity against Alexandrium pacificum (Group IV) out of six Bacillus strains. When treated with 0.5% (v/v) S3 bacterial culture and sterile supernatant, the algicidal rates were 69.74% and 70.22% at 12 h, respectively, and algicidal substances secreted by S3 were considered the mechanism of algicidal effect. During the algicidal process, the rapid proliferation of Alteromonas sp. in the phycosphere of A. pacificum may have accelerated the algal death. Moreover, the algicidal development of S3 released large amounts of intracellular paralytic shellfish toxins (PSTs) into the water, as the extracellular PSTs increased by 187.88% and 231.47% at 12 h, compared with the treatment of bacterial culture and sterile supernatant at 0 h, respectively. Although the total amount of PSTs increased slightly, the total toxicity of the algal sample decreased as GTX1/4 was transformed by S3 into GTX2/3 and GTX5. These results more comprehensively reveal the complex relationship between algicidal bacteria and microalgae, providing a potential source of biological control for harmful algal blooms and toxins.

Eutrophication has become an increasingly serious environmental issue and has contributed towards an explosion in harmful algal blooms (HABs) affecting local development. HABs can cause serious threats to ecosystems and human health. A newly isolated algicidal strain, Enterobacter hormaechei F2, showed high algicidal activity against the typical HAB species Microcystis aeruginosa. Potential algicides were detected through liquid chromatograph–mass spectrometer analysis, revealing that prodigiosin is an algicide and PQS is a quorum sensing molecule. RNA-seq was used to understand the algicidal mechanisms and the related pathways. We concluded that the metabolism of prodigiosin and PQS are active at the transcriptional level. The findings indicate that E. hormaechei F2 can be used as a potential biological agent to control harmful algal blooms to prevent the deterioration of the ecological and economic value of water bodies.

The effective suppression of cyanobacteria by submerged macrophytes is a key mechanism underlying the successful restoration of aquatic vegetation in some eutrophic water bodies. However, the responses and functional roles of epiphytic microorganisms in this process remained largely unclear, limiting a clear understanding of how macrophytes inhibit cyanobacterial growth. In this study we investigated the temporal dynamics of the epiphytic microbiome on Vallisneria natans before, during and after exposure to toxic cyanobacterium Microcystis , corresponding to three distinct physiological stages of the plant: pre-stress, stress, and recovery. It was observed that the diversity of epiphytic bacteria and eukaryotic algae increased during the stress stage, while that of other eukaryotes, particularly fungi and protozoa, decreased. The complexity and stability of the epiphytic microbiome were enhanced, with bacteria emerging as central hubs in the co-occurrence network in response to Microcystis stress. More importantly, a selective enrichment and recruitment of potential algicidal bacteria, particularly Streptomyces , Pseudomonas and Chryseobacterium , occurred on macrophyte surfaces during the stress phase. Their abundance peaked under Microcystis stress and returned to baseline levels during the plant recovery phase. Our findings demonstrate that V. natans did not function alone, but rather actively recruited and sustained a beneficial microbiome to enhance its suppressive effects on Microcystis . This study revealed a previously neglected macrophyte-epiphytic microbiome synergy, providing novel mechanistic insights into how submerged vegetation effectively suppresses harmful cyanobacteria.

Harmful algal blooms have deleterious effects on aquatic ecosystems and human health. The application of algicidal bacteria is a promising and environmentally friendly method of preventing and eradicating harmful algal blooms. In this study, a screen for algicidal agents against harmful algal blooms was used to identify an algicidal bacterial strain (strain O-4) isolated from a Karenia mikimotoi culture. Strain O-4 exhibited a strong inhibitory effect on harmful K. mikimotoi and was identified as Paracoccus homiensis via 16S rRNA gene sequence analysis. This strain killed K. mikimotoi by secreting active algicidal compounds, which were stable at temperatures of −80–121 °C but were sensitive to strongly acidic conditions (pH = 2). The algicidal properties of strain O-4 against K. mikimotoi were cell density- and time-dependent. No significant changes or negative effects were noted for two other Chlorophyta species, which highlighted the specificity of the studied algicidal substance. Finally, single-factor experiments revealed the optimum growth conditions of strain O-4 under different pH and temperature conditions. Therefore, strain O-4 has the potential to be used as a bio-agent for reducing the biomass of harmful K. mikimotoi blooms.

Blooms of harmful cyanobacteria Microcystis aeruginosa lead to an adverse effect on freshwater ecosystems, and thus extensive studies on the control of this cyanobacteria’s blooms have been conducted. Throughout this study, we have found that the two bacteria Aeromonas bestiarum HYD0802-MK36 and Pseudomonas syringae KACC10292T are capable of killing M. aeruginosa. Interestingly, these two bacteria showed different algicidal modes. Based on an algicidal range test using 15 algal species (target and non-target species), HYD0802-MK36 specifically attacked only target cyanobacteria M. aeruginosa, whereas the algicidal activity of KACC10292T appeared in a relatively broad algicidal range. HYD0802-MK36, as a direct attacker, killed M. aeruginosa cells when direct cell (bacterium)-to-cell (cyanobacteria) contact happens. KACC10292T, as an indirect attacker, released algicidal substance which is located in cytoplasm. Interestingly, algicidal activity of KACC10292T was enhanced according to co-cultivation with the host cyanobacteria, suggesting that quantity of algicidal substance released from this bacterium might be increased via interaction with the host cyanobacteria.

Algicidal bacteria can lyse microalgal blooms and trigger shifts within plankton communities. Resistant algal species can escape lysis, and have the opportunity to dominate the phytoplankton after a bacterial infection. Despite their important function in ecosystem regulation, little is known about mechanisms of resistance. Here, we show that the diatom Chaetoceros didymus releases eicosanoid oxylipins into the medium, and that the lytic algicidal bacterium, Kordia algicida, induces the production of several wound-activated oxylipins in this resistant diatom. Neither releases nor an induction occurs in the susceptible diatom Skeletonema costatum that is lysed by the bacterium within a few days. Among the upregulated oxylipins, hydroxylated eicosapentaenoic acids (HEPEs) dominate. However, also, resolvins, known lipid mediators in mammals, increase upon exposure of the algae to the algicidal bacteria. The prevailing hydroxylated fatty acid, 15-HEPE, significantly inhibits growth of K. algicida at a concentration of approximately 1 µM. The oxylipin production may represent an independent line of defense of the resistant alga, acting in addition to the previously reported upregulation of proteases.