Quorum sensing regulates 'swim-or-stick' lifestyle in the phycosphere

Understanding phycosomal dynamics to improve industrial microalgae cultivation

Algal-bacterial interactions are ubiquitous in both natural and industrial systems, and the characterization of these interactions has been reinvigorated by potential applications in biosystem productivity. Different growth conditions can be used for operational functions, such as the use of low-quality water or high pH/alkalinity, and the altered operating conditions likely constrain microbial community structure and function in unique ways. However, research is necessary to better understand whether consortia can be designed to improve the productivity, processing, and sustainability of industrial-scale cultivations through different controls that can constrain microbial interactions for maximal light-driven outputs. The review highlights current knowledge and gaps for relevant operating conditions, as well as suggestions for near-term and longer-term improvements for large-scale cultivation and polyculture engineering.

Rhodobacteraceae are key players in microbiome assembly of the diatom Asterionellopsis glacialis

The complex interactions between bacterioplankton and phytoplankton have prompted numerous studies that investigate phytoplankton microbiomes with the aim of characterizing beneficial or opportunistic taxa and elucidating core bacterial members. Oftentimes, this knowledge is garnered through 16S rRNA gene profiling of microbiomes from phytoplankton isolated across spatial and temporal scales, yet these studies do not offer insight into microbiome assembly and structuring. In this study, we aimed to identify taxa central to structuring and establishing the microbiome of the ubiquitous diatom Asterionellopsis glacialis. We introduced a diverse environmental bacterial community to A. glacialis in nutrient-rich or nutrient-poor media in a continuous dilution culture setup and profiled the bacterial community over 7 days. 16S rRNA amplicon sequencing showed that cyanobacteria (Coleofasciculaceae) and Rhodobacteraceae dominate the microbiome early on and maintain a persistent association throughout the experiment. Differential abundance, co-abundance networks, and differential association analyses revealed that specific members of the family Rhodobacteraceae, particularly Sulfitobacter amplicon sequence variants, become integral members in microbiome assembly. In the presence of the diatom, Sulfitobacter species and other Rhodobacteraceae developed positive associations with taxa that are typically in high abundance in marine ecosystems (Pelagibacter and Synechococcus), leading to restructuring of the microbiome compared to diatom-free controls. These positive associations developed predominantly under oligotrophic conditions, highlighting the importance of investigating phytoplankton microbiomes in as close to natural conditions as possible to avoid biases that develop under routine laboratory conditions. These findings offer further insight into phytoplankton-bacteria interactions and illustrate the importance of Rhodobacteraceae, not merely as phytoplankton symbionts but as key taxa involved in microbiome assembly.

IMPORTANCE

Most, if not all, microeukaryotic organisms harbor an associated microbial community, termed the microbiome. The microscale interactions that occur between these partners have global-scale consequences, influencing marine primary productivity, carbon cycling, and harmful algal blooms to name but a few. Over the last decade, there has been a growing interest in the study of phytoplankton microbiomes, particularly within the context of bloom dynamics. However, long-standing questions remain regarding the process of phytoplankton microbiome assembly. The significance of our research is to tease apart the mechanism of microbiome assembly with a particular focus on identifying bacterial taxa, which may not merely be symbionts but architects of the phytoplankton microbiome. Our results strengthen the understanding of the ecological mechanisms that underpin phytoplankton-bacteria interactions in order to accurately predict marine ecosystem responses to environmental perturbations.

Algae communication, conspecific and interspecific: the concepts of phycosphere and algal-bacteria consortia in a photobioreactor (PBR)

Microalgae in the wild often form consortia with other species promoting their own health and resource foraging opportunities. The recent application of microalgae cultivation and deployment in commercial photobioreactors (PBR) so far has focussed on single species of algae, resulting in multi-species consortia being largely unexplored. Reviewing the current status of PBR ecological habitat, this article argues in favor of further investigation into algal communication with conspecifics and interspecifics, including other strains of microalgae and bacteria. These mutualistic species form the 'phycosphere': the microenvironment surrounding microalgal cells, potentiating the production of certain metabolites through biochemical interaction with cohabitating microorganisms. A better understanding of the phycosphere could lead to novel PBR configurations, capable of incorporating algal-microbial consortia, potentially proving more effective than single-species algal systems.

Diatom-Bacteria Interactions in the Marine Environment: Complexity, Heterogeneity, and Potential for Biotechnological Applications

Diatom-bacteria interactions evolved during more than 200 million years of coexistence in the same environment. In this time frame, they established complex and heterogeneous cohorts and consortia, creating networks of multiple cell-to-cell mutualistic or antagonistic interactions for nutrient exchanges, communication, and defence. The most diffused type of interaction between diatoms and bacteria is based on a win-win relationship in which bacteria benefit from the organic matter and nutrients released by diatoms, while these last rely on bacteria for the supply of nutrients they are not able to produce, such as vitamins and nitrogen. Despite the importance of diatom-bacteria interactions in the evolutionary history of diatoms, especially in structuring the marine food web and controlling algal blooms, the molecular mechanisms underlying them remain poorly studied. This review aims to present a comprehensive report on diatom-bacteria interactions, illustrating the different interplays described until now and the chemical cues involved in the communication and exchange between the two groups of organisms. We also discuss the potential biotechnological applications of molecules and processes involved in those fascinating marine microbial networks and provide information on novel approaches to unveiling the molecular mechanisms underlying diatom-bacteria interactions.

Phytoplankton interspecific interactions modified by symbiotic fungi and bacterial metabolites under environmentally relevant hydrogen peroxide concentrations stress

Hydrogen peroxide (H2O2), which accumulates in water and triggers oxidative stress for aquatic microbes, has been shown to have profound impacts on planktonic microbial community dynamics including cyanobacterial bloom formation. Yet, potential effects of H2O2 on interspecific relationships of phytoplankton-microbe symbiotic interactions remain unclear. Here, we investigated effects of environmentally relevant H2O2 concentrations on interspecific microbial relationships in algae-microbe symbiosis. Microbes play a crucial role in the competition between M. aeruginosa and Chlorella vulgaris at low H2O2 concentrations (∼400 nM), in which fungi and bacteria protect Microcystis aeruginosa from oxidative stress. Moreover, H2O2 stimulated the synthesis and release of extracellular microcystin-LR from Microcystis aeruginosa, while intracellular microcystin-LR concentrations remained at a relatively constant level. In the presence of H2O2, loss of organoheterocyclic compounds, organic acids and ketones contributed to the growth of M. aeruginosa, but the reduction of vitamins inhibited it. Regulation of interspecific relationships by H2O2 is achieved by its action on fungal species and bacterial secretory metabolites. This study explored the response of phytoplankton interspecific relationships in symbiotic phytoplankton-microbe interactions to environmentally relevant H2O2 concentrations stress, providing a theoretical basis for understanding the formation of harmful-algae blooming and impact of photochemical properties of water on aquatic ecological safety and stability.

Phycobacter azelaicus gen. nov. sp. nov., a diatom symbiont isolated from the phycosphere of Asterionellopsis glacialis

A diatom-associated bacterium, designated as strain F10T, was isolated from a pure culture of the pennate diatom Asterionellopsis glacialis A3 and has since been used to characterize molecular mechanisms of symbiosis between phytoplankton and bacteria, including interactions using diatom-derived azelaic acid. Its origin from a hypersaline environment, combined with its capacity for quorum sensing, biofilm formation, and potential for dimethylsulfoniopropionate methylation/cleavage, suggest it is within the family Roseobacteraceae. Initial phylogenetic analysis of the 16S rRNA gene sequence placed this isolate within the Phaeobacter genus, but recent genomic and phylogenomic analyses show strain F10T is a separate lineage diverging from the genus Pseudophaeobacter. The genomic DNA G+C content is 60.0 mol%. The predominant respiratory quinone is Q-10. The major fatty acids are C18 : 1 ω7c and C16 : 0. Strain F10T also contains C10 : 03-OH and the furan-containing fatty acid 10,13-epoxy-11-methyl-octadecadienoate (9-(3-methyl-5-pentylfuran-2-yl)nonanoic acid). The major polar lipids are diphosphatidylglycerol, phosphatidylethanolamine and phosphatidylglycerol. Based on genomic, phylogenomic, phenotypic and chemotaxonomic characterizations, strain F10T represents a novel genus and species with the proposed name, Phycobacter azelaicus gen. nov. sp. nov. The type strain is F10T (=NCMA B37T=NCIMB 15470T=NRIC 2002T).

Abundant Sulfitobacter marine bacteria protect Emiliania huxleyi algae from pathogenic bacteria

Emiliania huxleyi is a unicellular micro-alga that forms massive oceanic blooms and plays key roles in global biogeochemical cycles. Mounting studies demonstrate various stimulatory and inhibitory influences that bacteria have on the E. huxleyi physiology. To investigate these algal-bacterial interactions, laboratory co-cultures have been established by us and by others. Owing to these co-cultures, various mechanisms of algal-bacterial interactions have been revealed, many involving bacterial pathogenicity towards algae. However, co-cultures represent a significantly simplified system, lacking the complexity of bacterial communities. In order to investigate bacterial pathogenicity within an ecologically relevant context, it becomes imperative to enhance the microbial complexity of co-culture setups. Phaeobacter inhibens bacteria are known pathogens that cause the death of E. huxleyi algae in laboratory co-culture systems. The bacteria depend on algal exudates for growth, but when algae senesce, bacteria switch to a pathogenic state and induce algal death. Here we investigate whether P. inhibens bacteria can induce algal death in the presence of a complex bacterial community. We show that an E. huxleyi-associated bacterial community protects the alga from the pathogen, although the pathogen occurs within the community. To study how the bacterial community regulates pathogenicity, we reduced the complex bacterial community to a five-member synthetic community (syncom). The syncom is comprised of a single algal host and five isolated bacterial species, which represent major bacterial groups that are naturally associated with E. huxleyi. We discovered that a single bacterial species in the reduced community, Sulfitobacter pontiacus, protects the alga from the pathogen. We further found that algal protection from P. inhibens pathogenicity is a shared trait among several Sulfitobacter species. Algal protection by bacteria might be a common phenomenon with ecological significance, which is overlooked in reduced co-culture systems.

The β-Lactamase Activity at the Community Level Confers β-Lactam Resistance to Bloom-Forming Microcystis aeruginosa Cells

Many freshwater cyanobacteria, including Microcystis aeruginosa, lack several known antibiotic resistance genes; however, both axenic and xenic M. aeruginosa strains exhibited high antibiotic resistance against many antibiotics under our tested concentrations, including colistin, trimethoprim, and kanamycin. Interestingly, axenic PCC7806, although not the xenic NIBR18 and NIBR452 strains, displayed susceptibility to ampicillin and amoxicillin, indicating that the associated bacteria in the phycosphere could confer such antibiotic resistance to xenic strains. Fluorescence and scanning electron microscopic observations revealed their tight association, leading to possible community-level β-lactamase activity. Combinatory treatment of ampicillin with a β-lactamase inhibitor, sulbactam, abolished the ampicillin resistance in the xenic stains. The nitrocefin-based assay confirmed the presence of significant community-level β-lactamase activity. Our tested low ampicillin concentration and high β-lactamase activity could potentially balance the competitive advantage of these dominant species and provide opportunities for the less competitive species, thereby resulting in higher bacterial diversity under ampicillin treatment conditions. Non-PCR-based metagenome data from xenic NIBR18 cultures revealed the dominance of blaOXA-related antibiotic resistance genes followed by other class A β-lactamase genes (AST-1 and FAR-1). Alleviation of ampicillin toxicity could be observed only in axenic PCC7806, which had been cocultured with β-lactamase from other freshwater bacteria. Our study suggested M. aeruginosa develops resistance to old-class β-lactam antibiotics through altruism, where associated bacteria protect axenic M. aeruginosa cells.

Characterization of a bloom-associated alphaproteobacterial lineage, 'Candidatus Phycosocius': insights into freshwater algal-bacterial interactions

Marine bacterial lineages associated with algal blooms, such as the Roseobacter clade, have been well characterized in ecological and genomic contexts, yet such lineages have rarely been explored in freshwater blooms. This study performed phenotypic and genomic analyses of an alphaproteobacterial lineage 'Candidatus Phycosocius' (denoted the CaP clade), one of the few lineages ubiquitously associated with freshwater algal blooms, and described a novel species: 'Ca. Phycosocius spiralis.' Phylogenomic analyses indicated that the CaP clade is a deeply branching lineage in the Caulobacterales. Pangenome analyses revealed characteristic features of the CaP clade: aerobic anoxygenic photosynthesis and essential vitamin B auxotrophy. Genome size varies widely among members of the CaP clade (2.5-3.7 Mb), likely a result of independent genome reductions at each lineage. This includes a loss of tight adherence pilus genes (tad) in 'Ca. P. spiralis' that may reflect its adoption of a unique spiral cell shape and corkscrew-like burrowing activity at the algal surface. Notably, quorum sensing (QS) proteins showed incongruent phylogenies, suggesting that horizontal transfers of QS genes and QS-involved interactions with specific algal partners might drive CaP clade diversification. This study elucidates the ecophysiology and evolution of proteobacteria associated with freshwater algal blooms.

The coral microbiome: towards an understanding of the molecular mechanisms of coral-microbiota interactions

Corals live in a complex, multipartite symbiosis with diverse microbes across kingdoms, some of which are implicated in vital functions, such as those related to resilience against climate change. However, knowledge gaps and technical challenges limit our understanding of the nature and functional significance of complex symbiotic relationships within corals. Here, we provide an overview of the complexity of the coral microbiome focusing on taxonomic diversity and functions of well-studied and cryptic microbes. Mining the coral literature indicate that while corals collectively harbour a third of all marine bacterial phyla, known bacterial symbionts and antagonists of corals represent a minute fraction of this diversity and that these taxa cluster into select genera, suggesting selective evolutionary mechanisms enabled these bacteria to gain a niche within the holobiont. Recent advances in coral microbiome research aimed at leveraging microbiome manipulation to increase coral's fitness to help mitigate heat stress-related mortality are discussed. Then, insights into the potential mechanisms through which microbiota can communicate with and modify host responses are examined by describing known recognition patterns, potential microbially derived coral epigenome effector proteins and coral gene regulation. Finally, the power of omics tools used to study corals are highlighted with emphasis on an integrated host-microbiota multiomics framework to understand the underlying mechanisms during symbiosis and climate change-driven dysbiosis.

Impact of Marine Chemical Ecology Research on the Discovery and Development of New Pharmaceuticals

Diverse ecologically important metabolites, such as allelochemicals, infochemicals and volatile organic chemicals, are involved in marine organismal interactions. Chemically mediated interactions between intra- and interspecific organisms can have a significant impact on community organization, population structure and ecosystem functioning. Advances in analytical techniques, microscopy and genomics are providing insights on the chemistry and functional roles of the metabolites involved in such interactions. This review highlights the targeted translational value of several marine chemical ecology-driven research studies and their impact on the sustainable discovery of novel therapeutic agents. These chemical ecology-based approaches include activated defense, allelochemicals arising from organismal interactions, spatio-temporal variations of allelochemicals and phylogeny-based approaches. In addition, innovative analytical techniques used in the mapping of surface metabolites as well as in metabolite translocation within marine holobionts are summarized. Chemical information related to the maintenance of the marine symbioses and biosyntheses of specialized compounds can be harnessed for biomedical applications, particularly in microbial fermentation and compound production. Furthermore, the impact of climate change on the chemical ecology of marine organisms—especially on the production, functionality and perception of allelochemicals—and its implications on drug discovery efforts will be presented.

Bacterial lifestyle switch in response to algal metabolites

Unicellular algae, termed phytoplankton, greatly impact the marine environment by serving as the basis of marine food webs and by playing central roles in the biogeochemical cycling of elements. The interactions between phytoplankton and heterotrophic bacteria affect the fitness of both partners. It is becoming increasingly recognized that metabolic exchange determines the nature of such interactions, but the underlying molecular mechanisms remain underexplored. Here, we investigated the molecular and metabolic basis for the bacterial lifestyle switch, from coexistence to pathogenicity, in Sulfitobacter D7 during its interaction with Emiliania huxleyi, a cosmopolitan bloom-forming phytoplankter. To unravel the bacterial lifestyle switch, we analyzed bacterial transcriptomes in response to exudates derived from algae in exponential growth and stationary phase, which supported the Sulfitobacter D7 coexistence and pathogenicity lifestyles, respectively. In pathogenic mode, Sulfitobacter D7 upregulated flagellar motility and diverse transport systems, presumably to maximize assimilation of E. huxleyi-derived metabolites released by algal cells upon cell death. Algal dimethylsulfoniopropionate (DMSP) was a pivotal signaling molecule that mediated the transition between the lifestyles, supporting our previous findings. However, the coexisting and pathogenic lifestyles were evident only in the presence of additional algal metabolites. Specifically, we discovered that algae-produced benzoate promoted the growth of Sulfitobacter D7 and hindered the DMSP-induced lifestyle switch to pathogenicity, demonstrating that benzoate is important for maintaining the coexistence of algae and bacteria. We propose that bacteria can sense the physiological state of the algal host through changes in the metabolic composition, which will determine the bacterial lifestyle during interaction.

Mini-Review of Biofilm Interactions with Surface Materials in Industrial Piping System

The growth of biofilm, which is caused by microorganism accumulation and growth on wetted surfaces, may damage industrial piping systems, increase maintenance and cleaning costs for the system sterilization, and even divulge the immune system into high risk. This article systematically analyzes the biofilm interactions with piping surface materials from the perspectives of physical convection, and biological and chemical adhesion. The thermodynamics of the flow, bacterial surface sensing, and bio-communication are the most critical factors for biofilm attachment. Furthermore, experimental analysis methods as well as biofilm control and removal approaches, are also included in this study. Finally, the resistance and growth of biofilm, as well as the practical and advanced methodology to control the biofilm and challenges associated with technology, are also discussed. Moreover, this paper may also offer a significant reference for the practice and strategic applications to address the biofilm resistance issues in industrial piping.

IMPACTS OF SUSPENSION TRAINING ON THE PHYSICAL FITNESS OF SWIMMERS

ABSTRACT Introduction Suspension training develops the physical fitness of a swimmer and improves his or her abilities. The specific fitness of a swimmer is the body's ability to adapt to load in swimming. This is also a comprehensive reflection of their physical function, conditioning, athletic ability, and overall health. Objective This study aimed to analyze the effect of suspension training on swimmers’ balance, abdominal center strength, and athletic performance. Methods This paper selects several swimmers as research volunteers. They were randomly divided into experimental and control groups. Both of them underwent physical training for three months. The experimental group adopted the suspension training method. The control group used traditional training methods. Mathematical statistics performed data analysis in both groups. Results The physical fitness of the two groups of swimmers was improved substantially after the experiment (P<0.05). The strength balance ability of the experimental group showed better performance(P<0.05). There was a significant difference between the experimental and control groups in the results of fitness index tests (P<0.01). Conclusion Suspension training has a prominent effect on the physical development of swimmers. This modality proved a better efficacy on swimmers’ performance. Level of evidence II; Therapeutic studies - investigation of treatment outcomes.

Obtaining Bioproducts from the Studies of Signals and Interactions between Microalgae and Bacteria

The applications of microalgae biomass have been widely studied worldwide. The classical processes used in outdoor cultivations of microalgae, in closed or open photobioreactors, occur in the presence of bacteria. Understanding how communication between cells occurs through quorum sensing and evaluating co-cultures allows the production of microalgae and cyanobacteria to be positively impacted by bacteria, in order to guarantee safety and profitability in the production process. In addition, the definition of the effects that occur during an interaction, promotes insights to improve the production of biomolecules, and to develop innovative products. This review presents the interactions between microalgae and bacteria, including compounds exchanges and communication, and addresses the development of new pharmaceutical, cosmetic and food bioproducts from microalgae based on these evaluations, such as prebiotics, vegan skincare products, antimicrobial compounds, and culture media with animal free protein for producing vaccines and other biopharmaceutical products. The use of microalgae as raw biomass or in biotechnological platforms is in line with the fulfillment of the 2030 Agenda related to the Sustainable Development Goals (SDGs).

More than just hitchhikers: a survey of bacterial communities associated with diatoms originating from sea turtles

Diatoms and bacteria are known for being the first colonizers of submerged surfaces including the skin of marine reptiles. Sea turtle carapace and skin harbor diverse prokaryotic and eukaryotic microbes, including several epizoic diatoms. However, the importance of diatom-bacteria associations is hardly investigated in biofilms associated with animal hosts. This study provides an inventory of diatoms, bacteria and diatom-associated bacteria originating from loggerhead sea turtles using both metabarcoding and culturing approaches. Amplicon sequencing of the carapace and skin samples chloroplast gene rbcL and 16S rRNA gene detected, in total, 634 diatom amplicon sequence variants (ASVs) and 3661 bacterial ASVs, indicating high diversity. Cultures of putative epizoic and non-epizoic diatoms contained 458 bacterial ASVs and their bacterial assemblages reflected those of their host. Diatom strains allowed for enrichment and isolation of bacterial families rarely observed on turtles, such as Marinobacteraceae, Alteromonadaceae and Alcanivoracaceae. When accounting for phylogenetic relationships between bacterial ASVs, we observed that related diatom genera might retain similar microbial taxa in culture, regardless of the turtle's skin or carapace source. These data provide deeper insights into the sea turtle-associated microbial communities, and reveal the potential of epizoic biofilms as a source of novel microbes and possibly important diatom-bacteria associations.

Inherent tendency of Synechococcus and heterotrophic bacteria for mutualism on long-term coexistence despite environmental interference

Mutualism between Synechococcus and heterotrophic bacteria has been found to support their prolonged survival in nutrient-depleted conditions. However, environmental interference on the fate of their mutualism is not understood. Here, we show that exogenous nutrients disrupt their established mutualism. Once the exogenous nutrients were exhausted, Synechococcus and heterotrophic bacteria gradually reestablished their metabolic mutualism during 450 days of culture, which revived unhealthy Synechococcus cells. Using metagenomics, metatranscriptomics, and the ¹⁵N tracer method, we reveal that the associated bacterial nitrogen fixation triggered the reestablishment of the mutualism and revival of Synechococcus health. During this process, bacterial community structure and functions underwent tremendous adjustments to achieve the driving effect, and a cogeneration of nitrogen, phosphorus, iron, and vitamin by the heterotrophic bacteria sustained Synechococcus's prolonged healthy growth. Our findings suggest that Synechococcus and heterotrophic bacteria may have an inherent tendency toward mutualism despite environmental interference. This may exhibit their coevolutionary adaptations in nutrient-deficient environments.

Dynamic patterns of quorum sensing signals in phycospheric microbes during a marine algal bloom

In the marine environment, the interactions among various species based on chemical signals play critical roles in influencing microbial structure and function. Quorum sensing (QS), the well-known signal-dependent communication autoinducer, is an important regulator in complex microbial communities. Here, we explored the QS gene profiles of phycosphere bacteria during a microcosmic phytoplankton bloom using metagenomic sequence data. More than fifteen subtypes of QS systems and 211,980 non-redundant amino acid sequences were collected and classified for constructing a hierarchical quorum-sensing database. The abundance of the various QS subtypes varied at different bloom stages and showed a strong correlation with phycosphere microorganisms. This suggested that QS is involved in regulating the phycosphere microbial succession during an algal bloom. A neutral community model revealed that the QS functional gene community assemblies were driven by stochastic processes. Co-occurrence model analysis showed that the QS gene networks of phycospheric microbes had similar topological structure and functional composition, which is a potential cornerstone for maintaining signal communication and population stabilization among microorganisms. Overall, QS systems have a strong relationship with the development of algal blooms and participate in regulating algal-associated microbial communities as chemical signals. This research reveals the chemical and ecological behavior of algal symbiotic bacteria and expands the current understanding of microbial dynamics in marine algal blooms.

Metaproteomics reveals unique metabolic niches of dominant bacterial groups in response to rapid regime shifts during a mixed dinoflagellate bloom

The dynamics of bacterial composition and metabolic activity during a distinct phytoplankton bloom have been reported. However, there is limited information on the bacterial community response to drastic environmental changes caused by species succession during a mixed-species bloom. This study investigated active bacterial groups and metabolic activity during a mixed bloom formed by dinoflagellates Prorocentrum obtusidens and Karenia mikimotoi using a metaproteomic approach. Bacterial community structure and dominant bacterial groups varied rapidly with the bloom regime shifts caused by species succession. Pseudoalteromonas and Vibrio dominated the bacterial community in the P. obtusidens-dominated regime, while Alteromonas, Cytophaga-Flavobacteria-Bacteroides (CFB) group, and marine Roseobacter clade (MRC) were the major contributors in other regimes, with the most abundant taxa being Alteromonas in the K. mikimotoi-dominated regime and the CFB group in the dissipation regime. Specific metabolic niches and unique substrate specificity of different bacterial groups enabled them to dominate and thrive in different bloom regimes. High metabolic plasticity in signal response, substrate utilization, motility, and adhesion are essential for bacteria to respond to drastic bloom regime shift, and the predominance of specific bacteria under unique bloom regimes may be the result of long-term coevolution between bacteria and bloom-forming phytoplankton species.

Metagenomics reveals bacterioplankton community adaptation to long-term thermal pollution through the strategy of functional regulation in a subtropical bay

Thermal effluents from coastal nuclear power plants have led to undesirable pollution and subsequent ecological impacts on local marine ecosystems. However, despite the ecological importance, we know little about the impacts on functionality of bacterioplankton subjected in systems with long-term thermal pollution. We used metagenomic sequencing to study of the effect of thermal pollution on bacterioplankton community metagenomics in summer in a subtropical bay located on the northern coast of the South China Sea. Thermal pollution (>15 y), which resulted in an increase in the summer seawater temperature around 8°C and caused seawater temperature up to approximate 39°C, significantly decreased bacterioplankton metabolic potentials in photosynthesis, organic carbon synthesis, and energy production. The bacterioplankton community metagenomics underwent a significant change in its structure from Synechococcus-dominant autotrophy to Alteromonas, Vibrio, and Pseudoalteromonas-dominated heterotrophy, and significantly up-regulated genes involved in organic compound degradation and dissimilatory nitrate reduction for the matter and energy acquisition under thermal pollution. Moreover, the bacterioplankton community metagenomics showed an up-regulation with heating of genes involved in DNA repair systems, heat shock responsive chaperones and proteins, and proteins involved in other biological processes, such as biofilm formation and the biosynthesis of unsaturated fatty acids and glycan, to adapt to the thermal environment. Collectively, it indicates a functional regulation of bacterioplankton adaptation to high-temperature stress, which might advance the understanding of the molecular mechanisms of community adaptation to global extreme warming in aquatic ecosystems.

Towards understanding mechanisms and functional consequences of bacterial interactions with members of various kingdoms in complex biofilms that abound in nature

The microbial world represents a phenomenal diversity of microorganisms from different kingdoms of life which occupy an impressive set of ecological niches. Most, if not all, microorganisms once colonise a surface develop architecturally complex surface-adhered communities which we refer to as biofilms. They are embedded in polymeric structural scaffolds serve as a dynamic milieu for intercellular communication through physical and chemical signalling. Deciphering microbial ecology of biofilms in various natural or engineered settings has revealed co-existence of microorganisms from all domains of life, including Bacteria, Archaea and Eukarya. The coexistence of these dynamic microbes is not arbitrary, as a highly coordinated architectural setup and physiological complexity show ecological interdependence and myriads of underlying interactions. In this review, we describe how species from different kingdoms interact in biofilms and discuss the functional consequences of such interactions. We highlight metabolic advances of collaboration among species from different kingdoms, and advocate that these interactions are of great importance and need to be addressed in future research. Since trans-kingdom biofilms impact diverse contexts, ranging from complicated infections to efficient growth of plants, future knowledge within this field will be beneficial for medical microbiology, biotechnology, and our general understanding of microbial life in nature.

Bioactive Compounds for Quorum Sensing Signal-Response Systems in Marine Phycosphere

Quorum sensing in the phycosphere refers to a sensor system in which bacteria secrete bioactive compounds to coordinate group behavior relying on cell density. It is an important way for algae and bacteria to communicate with each other and achieve interactions. It has been determined that quorum sensing is widely presented in the marine phycosphere, which involves a variety of bioactive compounds. Focused on these compounds in marine phycosphere, this review summarizes the types and structures of the compounds, describes the methods in detection and functional evaluation, discusses the ecological functions regulated by the compounds, such as modulating microbial colonization, achieving algae–bacteria mutualism or competition and contributing to marine biogeochemical cycles. Meanwhile, the application prospects of the compounds are also proposed, including controlling harmful algal blooms and producing biofuel. Future research should focus on improving detection techniques, developing more model systems and investigating the effects of climate change on the quorum-sensing pathway to further understand the mechanism and application potential of quorum sensing compounds. This review aims to present an overview of current research carried out in order to provide the reader with perspective on bioactive compounds involved in quorum sensing.

Chemotaxis shapes the microscale organization of the ocean's microbiome

The capacity of planktonic marine microorganisms to actively seek out and exploit microscale chemical hotspots has been widely theorized to affect ocean-basin scale biogeochemistry^1-3, but has never been examined comprehensively in situ among natural microbial communities. Here, using a field-based microfluidic platform to quantify the behavioural responses of marine bacteria and archaea, we observed significant levels of chemotaxis towards microscale hotspots of phytoplankton-derived dissolved organic matter (DOM) at a coastal field site across multiple deployments, spanning several months. Microscale metagenomics revealed that a wide diversity of marine prokaryotes, spanning 27 bacterial and 2 archaeal phyla, displayed chemotaxis towards microscale patches of DOM derived from ten globally distributed phytoplankton species. The distinct DOM composition of each phytoplankton species attracted phylogenetically and functionally discrete populations of bacteria and archaea, with 54% of chemotactic prokaryotes displaying highly specific responses to the DOM derived from only one or two phytoplankton species. Prokaryotes exhibiting chemotaxis towards phytoplankton-derived compounds were significantly enriched in the capacity to transport and metabolize specific phytoplankton-derived chemicals, and displayed enrichment in functions conducive to symbiotic relationships, including genes involved in the production of siderophores, B vitamins and growth-promoting hormones. Our findings demonstrate that the swimming behaviour of natural prokaryotic assemblages is governed by specific chemical cues, which dictate important biogeochemical transformation processes and the establishment of ecological interactions that structure the base of the marine food web.

Plasmid-Mediated Stabilization of Prophages

Mobile genetic elements (MGEs) drive bacterial evolution, alter gene availability within microbial communities, and facilitate adaptation to ecological niches. In natural systems, bacteria simultaneously possess or encounter multiple MGEs, yet their combined influences on microbial communities are poorly understood. Here, we investigate interactions among MGEs in the marine bacterium Sulfitobacter pontiacus. Two related strains, CB-D and CB-A, each harbor a single prophage. These prophages share high sequence identity with one another and an integration site within the host genome, yet these strains exhibit differences in “spontaneous” prophage induction (SPI) and consequent fitness. To better understand mechanisms underlying variation in SPI between these lysogens, we closed their genomes, which revealed that in addition to harboring different prophage genotypes, CB-A lacks two of the four large, low-copy-number plasmids possessed by CB-D. To assess the relative roles of plasmid content versus prophage genotype on host physiology, a panel of derivative strains varying in MGE content were generated. Characterization of these derivatives revealed a robust link between plasmid content and SPI, regardless of prophage genotype. Strains possessing all four plasmids had undetectable phage in cell-free lysates, while strains lacking either one plasmid (pSpoCB-1) or a combination of two plasmids (pSpoCB-2 and pSpoCB-4) produced high (>10⁵ PFU/mL) phage titers. Homologous plasmid sequences were identified in related bacteria, and plasmid and phage genes were found to be widespread in Tara Oceans metagenomic data sets. This suggests that plasmid-dependent stabilization of prophages may be commonplace throughout the oceans.

IMPORTANCE The consequences of prophage induction on the physiology of microbial populations are varied and include enhanced biofilm formation, conferral of virulence, and increased opportunity for horizontal gene transfer. These traits lead to competitive advantages for lysogenized bacteria and influence bacterial lifestyles in a variety of niches. However, biological controls of “spontaneous” prophage induction, the initiation of phage replication and phage-mediated cell lysis without an overt stressor, are not well understood. In this study, we observed a novel interaction between plasmids and prophages in the marine bacterium Sulfitobacter pontiacus. We found that loss of one or more distinct plasmids—which we show carry genes ubiquitous in the world’s oceans—resulted in a marked increase in prophage induction within lysogenized strains. These results demonstrate cross talk between different mobile genetic elements and have implications for our understanding of the lysogenic-lytic switches of prophages found not only in marine environments, but throughout all ecosystems.

A novel phage indirectly regulates diatom growth by infecting diatom-associated biofilm-forming bacterium

Algae and heterotrophic bacteria have close and intricate interactions, which are regulated by multiple factors in the natural environment. Phages are the major factor determining bacterial mortality. However, their impacts on the algae-associated bacteria and thus on the algae-bacteria interactions are poorly understood. Here, we obtained a diatom-associated bacterium Stappia indica SNL01 that could form biofilm and had an inhibitory effect on the growth of diatom Thalassiosira pseudonana. Meanwhile, the phage SI01 with a double-stranded circular DNA genome (44,247 bp) infecting S. indica SNL01 was isolated. Phylogenetic analysis revealed that phage SI01 represents a novel member of the Podoviridae family. The phage contained multiple lysis genes encoding for cell wall lysing muramidase and spore cortex lysing SleB, as well as depolymerase-like tail spike protein. By lysing the host bacterium and inhibiting the formation of biofilm, this phage could indirectly promote the growth of the diatom. Our results shed new insights into how phages indirectly regulate algal growth by infecting bacteria closely associated with algae or in the phycosphere. IMPORTANCE The impact of phage infection on the algae-bacteria relationship in the ocean is poorly understood. Here, a novel phage infecting the diatom-associated bacterium Stappia indica SNL01 was isolated. This bacterium could form biofilm and had a negative effect on diatom growth. We revealed that this phage contained multiple lysis genes and could inhibit the formation of bacterial biofilm, thus indirectly promoting diatom growth. This study implicates that phages are not only important regulators of bacteria but also have substantial indirect effects on algae as well as the algae-bacteria relationship.

Tight Adherence (Tad) Pilus Genes Indicate Putative Niche Differentiation in Phytoplankton Bloom Associated Rhodobacterales

The multiple interactions of phytoplankton and bacterioplankton are central for our understanding of aquatic environments. A prominent example of those is the consistent association of diatoms with Alphaproteobacteria of the order Rhodobacterales. These photoheterotrophic bacteria have traditionally been described as generalists that scavenge dissolved organic matter. Many observations suggest that members of this clade are specialized in colonizing the microenvironment of diatom cells, known as the phycosphere. However, the molecular mechanisms that differentiate Rhodobacterales generalists and phycosphere colonizers are poorly understood. We investigated Rhodobacterales in the North Sea during the 2010–2012 spring blooms using a time series of 38 deeply sequenced metagenomes and 10 metaproteomes collected throughout these events. Rhodobacterales metagenome assembled genomes (MAGs) were recurrently abundant. They exhibited the highest gene enrichment and protein expression of small-molecule transporters, such as monosaccharides, thiamine and polyamine transporters, and anaplerotic pathways, such as ethylmalonyl and propanoyl-CoA metabolic pathways, all suggestive of a generalist lifestyle. Metaproteomes indicated that the species represented by these MAGs were the dominant suppliers of vitamin B₁₂ during the blooms, concomitant with a significant enrichment of genes related to vitamin B₁₂ biosynthesis suggestive of association with diatom phycospheres. A closer examination of putative generalists and colonizers showed that putative generalists had persistently higher relative abundance throughout the blooms and thus produced more than 80% of Rhodobacterales transport proteins, suggesting rapid growth. In contrast, putative phycosphere colonizers exhibited large fluctuation in relative abundance across the different blooms and correlated strongly with particular diatom species that were dominant during the blooms each year. The defining feature of putative phycosphere colonizers is the presence of the tight adherence (tad) gene cluster, which is responsible for the assembly of adhesive pili that presumably enable attachment to diatom hosts. In addition, putative phycosphere colonizers possessed higher prevalence of secondary metabolite biosynthetic gene clusters, particularly homoserine lactones, which can regulate bacterial attachment through quorum sensing. Altogether, these findings suggest that while many members of Rhodobacterales are competitive during diatom blooms, only a subset form close associations with diatoms by colonizing their phycospheres.

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.

Biological and Physiochemical Methods of Biofilm Adhesion Resistance Control of Medical-Context Surface

The formation of biofilms on medical-context surfaces gives the EPS embedded bacterial community protection and additional advantages that planktonic cells would not have such as increased antibiotic resistance and horizontal gene transfer. Bacterial cells tend to attach to a conditioning layer after overcoming possible electrical barriers and go through two phases of attachments: reversible and irreversible. In the first, bacterial attachment to the surface is reversible and occurs quickly whilst the latter is permanent and takes place over a longer period of time. Upon reaching a certain density in the bacterial community, quorum sensing causes phenotypical changes leading to a loss in motility and the production of EPS. This position paper seeks to address the problem of bacterial adhesion and biofilm formation for the medical surfaces by comparing inhabiting physicochemical interactions and biological mechanisms. Several physiochemical methodologies (e.g. ultrasonication, alternating magnetic field and chemical surface coating) and utilizing biological mechanisms (e.g. quorum quenching and EPS degrading enzymes) were suggested. The possible strategical applications of each category were suggested and evaluated to a balanced position to possibly eliminate the adhesion and formation of biofilms on medical-context surfaces.

Hydrocarbon-Degrading Bacteria Found Tightly Associated with the 50-70 μm Cell-Size Population of Eukaryotic Phytoplankton in Surface Waters of a Northeast Atlantic Region

The surface of marine eukaryotic phytoplankton can harbour communities of hydrocarbon-degrading bacteria; however, this algal–bacterial association has, hitherto, been only examined with non-axenic laboratory cultures of micro-algae. In this study, we isolated an operationally-defined community of phytoplankton, of cell size 50–70 μm, from a natural community in sea surface waters of a subarctic region in the northeast Atlantic. Using MiSeq 16S rRNA sequencing, we identified several recognized (Alcanivorax, Marinobacter, Oleispira, Porticoccus, Thalassospira) and putative hydrocarbon degraders (Colwelliaceae, Vibrionaceae) tightly associated with the phytoplankton population. We combined fluorescence in situ hybridisation with flow-cytometry (FISH-Flow) to examine the association of Marinobacter with this natural eukaryotic phytoplankton population. About 1.5% of the phytoplankton population contained tightly associated Marinobacter. The remaining Marinobacter population were loosely associated with either eukaryotic phytoplankton cells or non-chlorophyll particulate material. This work is the first to show the presence of obligate, generalist and putative hydrocarbonoclastic bacteria associated with natural populations of eukaryotic phytoplankton directly from sea surface water samples. It also highlights the suitability of FISH-Flow for future studies to examine the spatial and temporal structure and dynamics of these and other algal–bacterial associations in natural seawater samples.

Diatom modulation of select bacteria through use of two unique secondary metabolites

Unicellular eukaryotic phytoplankton, such as diatoms, rely on microbial communities for survival despite lacking specialized compartments to house microbiomes (e.g., animal gut). Microbial communities have been widely shown to benefit from diatom excretions that accumulate within the microenvironment surrounding phytoplankton cells, known as the phycosphere. However, mechanisms that enable diatoms and other unicellular eukaryotes to nurture specific microbiomes by fostering beneficial bacteria and repelling harmful ones are mostly unknown. We hypothesized that diatom exudates may tune microbial communities and employed an integrated multiomics approach using the ubiquitous diatom Asterionellopsis glacialis to reveal how it modulates its naturally associated bacteria. We show that A. glacialis reprograms its transcriptional and metabolic profiles in response to bacteria to secrete a suite of central metabolites and two unusual secondary metabolites, rosmarinic acid and azelaic acid. While central metabolites are utilized by potential bacterial symbionts and opportunists alike, rosmarinic acid promotes attachment of beneficial bacteria to the diatom and simultaneously suppresses the attachment of opportunists. Similarly, azelaic acid enhances growth of beneficial bacteria while simultaneously inhibiting growth of opportunistic ones. We further show that the bacterial response to azelaic acid is numerically rare but globally distributed in the world's oceans and taxonomically restricted to a handful of bacterial genera. Our results demonstrate the innate ability of an important unicellular eukaryotic group to modulate select bacteria in their microbial consortia, similar to higher eukaryotes, using unique secondary metabolites that regulate bacterial growth and behavior inversely across different bacterial populations.