Quorum sensing of microalgae associated marine Ponticoccus sp. PD-2 and its algicidal function regulation

Quorum sensing bacteria in microplastics epiphytic biofilms and their biological characteristics which potentially impact marine ecosystem

Microplastics (MPs) have been shown to be a new type of pollutant in the oceans, with complex biofilms attached to their surfaces. Bacteria with quorum sensing (QS) systems are important participants in biofilms. Such bacteria can secrete and detect signal molecules. When a signal molecule reaches its threshold level, bacteria with QS systems can perform several biological functions, such as biofilm formation and antibiotic metabolite production. However, the ecological effects of QS bacteria in biofilm as MPs distribute globally with ocean currents are not to be elucidate yet. In this study, polypropylene and polyvinyl chloride were selected for on-site enrichment to acquire microplastics with biofilms. Eight culturable QS bacteria in the resulting biofilm were isolated by using biosensor assays, and their biodiversity was analyzed. The profiles of the N-acyl-homoserine lactones (AHLs) produced by these bacteria were analyzed by using thin-layer chromatography (TLC)-bioautography and gas chromatography and mass spectrometry (GC-MS). Biofilm-forming properties and several biological characteristics, such as bacteriostasis, algal inhibition, and dimethylsulfoniopropionate (DMSP) degradation, were explored along with QS quenching. Results showed that QS bacteria were mainly affiliated with class Alphaproteobacteria, particularly Rhodobacteraceae, followed by class Gammaproteobacteria. TLC-bioautography and GC-MS analyses revealed that seven AHLs, namely, C6-HSL, C8-HSL, 3-oxo-C6-HSL, 3-oxo-C8-HSL, 3-oxo-C10-HSL, and two unidentified AHLs were produced. The QS system equipped bacteria with strong biofilm-forming capacity and may contribute to the keystone roles of Rhodobacteraceae. In addition, QS bacteria may exacerbate the adverse environmental effects of MPs, such as inducing the misfeeding of planktons on MPs. This study elucidated the diversity of QS bacteria in MP-associated biofilms and provided a new perspective of the effect of key membrane-forming bacteria on the marine ecological environment.

The Exploitation of Microbial Antagonists against Postharvest Plant Pathogens

Postharvest disease management is vital to increase the quality and productivity of crops. As part of crop disease protection, people used different agrochemicals and agricultural practices to manage postharvest diseases. However, the widespread use of agrochemicals in pest and disease control has detrimental effects on consumer health, the environment, and fruit quality. To date, different approaches are being used to manage postharvest diseases. The use of microorganisms to control postharvest disease is becoming an eco-friendly and environmentally sounds approach. There are many known and reported biocontrol agents, including bacteria, fungi, and actinomycetes. Nevertheless, despite the abundance of publications on biocontrol agents, the use of biocontrol in sustainable agriculture requires substantial research, effective adoption, and comprehension of the interactions between plants, pathogens, and the environment. To accomplish this, this review made an effort to locate and summarize earlier publications on the function of microbial biocontrol agents against postharvest crop diseases. Additionally, this review aims to investigate biocontrol mechanisms, their modes of operation, potential future applications for bioagents, as well as difficulties encountered during the commercialization process.

Diversity and functions of quorum sensing bacteria in the root environment of the Suaeda glauca and Phragmites australis coastal wetlands

The quorum sensing (QS) system plays a significant role in the bacteria-bacteria or plant-bacteria relationships through signal molecules. However, little is known about the distribution and functional diversity of QS bacteria in the root environment of Suaeda glauca and Phragmites australis in coastal wetlands. We explored the bacterial community by amplicon sequencing and isolated 1050 strains from the rhizosphere soil and root tissues of S. glauca and P. australis in northern China to investigate the bacterial community and AHL producers. AHL activity was found in 76 isolates, and 22 distinct strains were confirmed by 16S rRNA gene sequencing. A substantial number of AHL producers clustered in rhizobiales and sphingomonadale, which derived from the root tissues. AHL producers in the rhizosphere soil mostly belonged to rhodobacterales. The different taxa of AHL producers in the rhizosphere soil and root tissues resulted in a variation of AHL profiles that C6-HSL dominated the AHL profiles in root bacteria compared to the C8-HSL in rhizobacteria, implying different ecological roles for AHL producers in the rhizosphere soil and root tissues. Many AHL producers may form biofilms, and some can degrade DMSP and oil, demonstrating that QS bacteria in the root environment have a wide ecological roles. In our study, for one of the first times here, we explore the distribution and functional variety of AHL producers in the root environment of S. glauca-P. australis. This study expands current knowledge of the relationship between QS bacteria and coastal plants (S. glauca and P. australis), and vital roles of QS bacterial in maintaining the health of coastal wetlands.

Algicidal Bacteria: A Review of Current Knowledge and Applications to Control Harmful Algal Blooms

Interactions between bacteria and phytoplankton in aqueous ecosystems are both complex and dynamic, with associations that range from mutualism to parasitism. This review focuses on algicidal interactions, in which bacteria are capable of controlling algal growth through physical association or the production of algicidal compounds. While there is some evidence for bacterial control of algal growth in the field, our understanding of these interactions is largely based on laboratory culture experiments. Here, the range of these algicidal interactions is discussed, including specificity of bacterial control, mechanisms for activity, and insights into the chemical and biochemical analysis of these interactions. The development of algicidal bacteria or compounds derived from bacteria for control of harmful algal blooms is reviewed with a focus on environmentally friendly or sustainable methods of application. Potential avenues for future research and further development and application of bacterial algicides for the control of algal blooms are presented.

The LuxS/AI-2 Quorum-Sensing System Regulates the Algicidal Activity of Shewanella xiamenensis Lzh-2

Cyanobacterial blooming is an increasing environmental issue all over the world. Algicidal bacteria are potential tools for the control of algal blooms. The algicidal activity in many bacteria exhibits quorum-sensing (QS) dynamics and the regulatory mechanism of this activity in these bacteria is unclear. In this study, combining genomic sequencing and genome editing, we have identified that the primary quorum-sensing system in the isolated algicidal strain Shewanella xiamenensis Lzh-2 is the LuxS/AI-2 signaling pathway. Disruption of the QS system through recombination deletion of the LuxS gene led to a loss of algicides production and algicidal activity. Restoration of the LuxS gene in the deletion mutant compensated the QS system and recovered the algicidal activity. Consequently, we proved that Lzh-2 regulates the algicidal activity through LuxS/AI-2 quorum-sensing system.

“Nature-like” Cryoimmobilization of Phototrophic Microorganisms: New Opportunities for Their Long-Term Storage and Sustainable Use

It was found that immobilization of cells in poly(vinyl alcohol) (PVA) cryogel can be successfully applied for concurrent cryoimmobilization, cryoconservation and long-term storage of the cells of various phototrophic microorganisms (green and red microalgae, diatoms and cyanobacteria). For the first time, it was shown for 12 different immobilized microalgal cells that they can be stored frozen for at least 18 months while retaining a high level of viability (90%), and can further be used as an inoculum upon defrosting for cell-free biomass accumulation. Application of cryoimmobilized Chlorella vulgaris cells as inocula allowed the loading of a high concentration of the microalgal cells into the media for free biomass accumulation, thus increasing the rate of the process. It was shown that as minimum of 5 cycles of reuse of the same immobilized cells as inocula for cell accumulation could be realized when various real wastewater samples were applied as media for simultaneous microalgae cultivation and water purification.

Quorum Sensing Bacteria in the Phycosphere of HAB Microalgae and Their Ecological Functions Related to Cross-Kingdom Interactions

It has been proven that the relationship between microalgae and bacteria affects the dynamic process of harmful algal blooms (HABs). Microalgae-associated microorganisms widely exist in the phycosphere and play an essential role in algae-bacteria cross-kingdom interactions. Among these processes, quorum sensing (QS), as a communication system of bacteria, is thought to participate in algae-bacteria interactions. However, the species of QS bacteria in the phycosphere and their ecological function are still unknown. In this study, microalgae-associated microorganisms with a QS system were screened by the biosensor method and identified based on 16S rRNA gene analysis. The types and number of acyl-L-homoserine lactone (AHL) signalling molecules produced by QS bacteria were analysed by thin layer chromatography (TLC) bioautography and gas chromatography-mass spectrometer (GC-MS). The film formation, β-dimethylmercaptopropionic (DMSP) degradation and algae growth effects of QS bacteria were investigated. The results showed that 113 QS bacteria were isolated from 842 microalgae-associated bacteria. Detection of AHL molecules in 10 different species of QS bacteria showed that most of them were N-(3-Oxodecanoyl)-L-homoserine lactone (OC10-HSL), N-Octanoyl-L-homoserine lactone (C8-HSL) and N-(3-Oxooctanoyl)-L-homoserine lactone (OC8-HSL). All 10 QS bacteria had film-forming ability, and they could degrade DMSP (except strain E26). The crude metabolic extracts of the 10 QS bacteria can inhibit or promote microalgae growth to different degrees. Our study is helpful to understand the role of microalgae-associated microorganisms with the QS system in algae-bacteria interactions and community succession of HAB microalgae.

Toxicological effects of hypoxanthine on Heterosigmaakashiwo: Mechanism of growth inhibition and change in hemolytic toxin content

Heterosigmaakashiwo is an algal species that causes harmful algal blooms (HABs) with strong hemolytic toxicity on coastal aquatic organisms. This study investigated the mechanism of growth inhibition and changes in hemolytic toxin contents in algal culture after exposure to hypoxanthine, a compound secreted by algicidal bacterium Bacillus sp.strain B1. An algal inhibition rate of 86% was observed with 1.0 mM hypoxanthine treatment on day 15. The levels of superoxide dismutase and catalase in algal cell culture increased while that of glutathione decreased during the treatment. In addition,the level of hemolytic toxin contents increased on day 3 under hypoxanthine treatment, and significantly decreased on days 6, 9, 12, and 15. Twelve fatty acids in H.akashiwo were detected by GC-MS, and the changes in the contents of C16, C18, C18:4ω3, and C20:5ω3 in the treatment group were consistent with the change in hemolytic toxin content. The four fatty acids were tested for hemolysis and it was observed that the hemolysis rate of 25 μg/mL C18:4ω3 and 5 μg/mL C20:5ω3 reached more than 80%, but C16 and C18 exhibited no hemolytic capability.Therefore, our results showed that hypoxanthine inhibited the growth of H. akashiwo through the changes of levels of antioxidants and hemolytic toxin content in the cultures, and fatty acids C18:4ω3 and C20:5ω3 were contributors to hemolytic toxins. The results confirmed that hypoxanthine is a potential algal inhibitor to prevent HABs.

Complete Genome of Marine Microalgae Associated Algicidal Bacterium Sulfitobacter pseudonitzschiae H46 with Quorum Sensing System

Sulfitobacter pseudonitzschiae H46, a marine bacterium associated with microalgae, exhibits algicidal activity against harmful algal bloom microalgae Chattonella marina and Alexandrium tamarense. The complete genome of S. pseudonitzschiae H46 was sequenced to investigate its algicidal mechanism further. One chromosome, eight circular plasmids and add one not circularized plasmid (Plas II) were identified, which are 4.93 Mb in length with 4871 open reading frames. The genome contains 43 transfer RNA (tRNA) genes; six ribosome RNA (rRNA) genes that can be classified into two 5S rRNA, two 16S rRNA, two 23S rRNA, and two small RNA (sRNA) genes. Genomic annotation revealed that S. pseudonitzschiae H46 has two pairs of LuxI/R-type Quorum-sensing (QS) systems. The QS systems can regulate the functions involved in host colonization and biofilm formation. Moreover, the existence of 49 Type-IV and Dddl genes in the chromosome indicated that Dimethylsulfoniopropionate (DMSP) catabolism of S. pseudonitzschiae H46 is the main interaction with marine microalgae. S. pseudonitzschiae H46 utilizes DMSP produced by microalgae as the carbon source, and this may induce algicidal activity. The complete genome sequence of S. pseudonitzschiae H46 can provide useful information on the underlying molecular mechanism between QS and algicidal activity.

How Do Quorum-Sensing Signals Mediate Algae-Bacteria Interactions?

Quorum sensing (QS) describes a process by which bacteria can sense the local cell density of their own species, thus enabling them to coordinate gene expression and physiological processes on a community-wide scale. Small molecules called autoinducers or QS signals, which act as intraspecies signals, mediate quorum sensing. As our knowledge of QS has progressed, so too has our understanding of the structural diversity of QS signals, along with the diversity of bacteria conducting QS and the range of ecosystems in which QS takes place. It is now also clear that QS signals are more than just intraspecies signals. QS signals mediate interactions between species of prokaryotes, and between prokaryotes and eukaryotes. In recent years, our understanding of QS signals as mediators of algae-bacteria interactions has advanced such that we are beginning to develop a mechanistic understanding of their effects. This review will summarize the recent efforts to understand how different classes of QS signals contribute to the interactions between planktonic microalgae and bacteria in our oceans, primarily N-acyl-homoserine lactones, their degradation products of tetramic acids, and 2-alkyl-4-quinolones. In particular, this review will discuss the ways in which QS signals alter microalgae growth and metabolism, namely as direct effectors of photosynthesis, regulators of the cell cycle, and as modulators of other algicidal mechanisms. Furthermore, the contribution of QS signals to nutrient acquisition is discussed, and finally, how microalgae can modulate these small molecules to dampen their effects.

Effects of mycosubtilin homolog algicides from a marine bacterium, Bacillus sp. SY-1, against the harmful algal bloom species Cochlodinium polykrikoides

The marine bacterium, Bacillus sp. SY-1, produced algicidal compounds that are notably active against the bloom-forming alga Cochlodinium polykrikoides. We isolated three algicidal compounds and identified these as mycosubtilins with molecular weights of 1056, 1070, and 1084 (designated MS 1056, 1070, and 1084, respectively), based on amino acid analyses and ¹H, ¹³C, and two-dimensional nuclear magnetic resonance spectroscopy, including ¹H-¹⁵N heteronuclear multiple bond correlation analysis. MS 1056 contains a β-amino acid residue with an alkyl side chain of C₁₅, which has not previously been seen in known mycosubtilin families. MS 1056, 1070, and 1084 showed algicidal activities against C. polykrikoides with 6-h LC₅₀ values of 2.3 ± 0.4, 0.8 ± 0.2, and 0.6 ± 0.1 μg/ml, respectively. These compounds also showed significant algicidal activities against other harmful algal bloom species. In contrast, MS 1084 showed no significant growth inhibitory effects against other organisms, including bacteria and microalgae, although does inhibit the growth of some fungi and yeasts. These observations imply that the algicidal bacterium Bacillus sp. SY-1 and its algicidal compounds could play an important role in regulating the onset and development of harmful algal blooms in natural environments.

Physiological response and morphological changes of Heterosigma akashiwo to an algicidal compound prodigiosin

Harmful algal blooms (HABs) occur all over the world, producing severely negative effects on human life as well as on marine ecosystems. The algicidal compound, prodigiosin, secreted by algicidal bacteria Hahella sp. KA22 can lyse the harmful alga Heterosigma akashiwo. This study is aimed to investigate the algicidal mechanism of prodigiosin against H. akashiwo by detecting physiological and morphological responses of H. akashiwo to presence of prodigiosin. The results indicated that prodigiosin showed strong algicidal effects on H. akashiwo at the concentration of 3 μg/mL. Chlorophyll a and protein levels of the microalgae decreased significantly while malonaldehyde levels increased at this concentration. Contents of ascorbic acid and activities of superoxide dismutase and peroxidase increased fast with the quick decrease of the reactive oxygen species (ROS). For the 3 μg/mL prodigiosin treatment group, transcription of genes related to photosynthesis and respiration were significantly inhibited at 12 h while respiration related genes increased at 24 h. Collectively, the results indicated that prodigiosin could kill the microalgae by inducing ROS overproduction which could destroy the cell integrity and change the antioxidant system levels and functional gene expression. Our results demonstrated that prodigiosin is an effective algicide for the control of harmful algae.

Complete Genome Sequence of a Quorum-Sensing Bacterium, Oceanicola sp. Strain D3, Isolated from a Microplastic Surface in Coastal Water of Qingdao, China

Oceanicola sp. strain D3 was isolated from the plastisphere of polyvinyl chloride (PVC) in coastal water of Qingdao, China. Here, we present the complete genome sequence of strain D3, which consists of a chromosome of 3,926,685 bp with a G+C content of 64.49% and 4,964 coding DNA sequences. This is the first report of a quorum-sensing (QS) system in an Oceanicola sp. strain.

Chemical ecology of the marine plankton

A review of chemically mediated interactions in planktonic marine environments covering new studies from January 2015 to December 2017.

Detection of Diverse N-Acyl-Homoserine Lactones in Vibrio alginolyticus and Regulation of Biofilm Formation by N-(3-Oxodecanoyl) Homoserine Lactone In vitro

Quorum sensing (QS) is a cell-to-cell communication system based on the exchange of small intercellular signal molecules, such as N-Acyl homoserine lactones (AHLs), which act as cell-density mediators of QS gene expression, and are highly variable both in types and amounts in most Gram-negative Proteobacteria. Understanding the regulation of AHLs may contribute to the elucidation of cell density-dependent phenomena, such as biofilm formation. Vibrio alginolyticus is among the most frequently observed marine opportunistic Vibrio pathogens. However, AHL production of this species and its effects on biofilm formation remain to be understood. Here, our study reported the diverse AHL profiles of 47 marine-isolated V. alginolyticus strains and the effects of exogenous 3-oxo-C₁₀-HSL on biofilm formation under different temperature conditions (16°C and 28°C). A total of 11 detected AHLs were produced by the isolates, of which 3-OH-C₄-HSL, 3-oxo-C₁₀-HSL and 3-oxo-C₁₄-HSL comprised the largest proportions. We also observed that moderate levels of exogenous 3-oxo-C₁₀-HSL (10 and 20 μM) could induce or enhance biofilm formation and alter its structure, while high levels (40 and 100 μM) did not significantly improve and even inhibited biofilm formation in V. alginolyticus. Further, regulation by exogenous 3-oxo-C₁₀-HSL was both concentration- and temperature-dependent in V. alginolyticus.