Algae-bacteria interactions that balance the planktonic microbiome

Advancements in bacterial chemotaxis: Utilizing the navigational intelligence of bacteria and its practical applications

The fascinating world of microscopic life unveils a captivating spectacle as bacteria effortlessly maneuver through their surroundings with astonishing accuracy, guided by the intricate mechanism of chemotaxis. This review explores the complex mechanisms behind this behavior, analyzing the flagellum as the driving force and unraveling the intricate signaling pathways that govern its movement. We delve into the hidden costs and benefits of this intricate skill, analyzing its potential to propagate antibiotic resistance gene while shedding light on its vital role in plant colonization and beneficial symbiosis. We explore the realm of human intervention, considering strategies to manipulate bacterial chemotaxis for various applications, including nutrient cycling, algal bloom and biofilm formation. This review explores the wide range of applications for bacterial capabilities, from targeted drug delivery in medicine to bioremediation and disease control in the environment. Ultimately, through unraveling the intricacies of bacterial movement, we can enhance our comprehension of the intricate web of life on our planet. This knowledge opens up avenues for progress in fields such as medicine, agriculture, and environmental conservation.

Uneven response of phytoplankton-bacteria coupling under Saharan dust pulse and ultraviolet radiation in the south-western Mediterranean Sea

The microbial carbon (C) flux in the ocean is a key functional process governed by the excretion of organic carbon by phytoplankton (EOC) and heterotrophic bacterial carbon demand (BCD). Ultraviolet radiation (UVR) levels in upper mixed layers and increasing atmospheric dust deposition from arid regions may alter the degree of coupling in the phytoplankton-bacteria relationship (measured as BCD:EOC ratio) with consequences for the C-flux through these compartments in marine oligotrophic ecosystem. Firstly, we performed a field study across the south-western (SW) Mediterranean Sea to assess the degree of coupling (BCD:EOC) and how it may be related to metabolic balance (total primary production: community respiration; PPT:CR). Secondly, we conducted a microcosm experiment in two contrasting areas (heterotrophic nearshore and autotrophic open sea) to test the impact of UVR and dust interaction on microbial C flux. In the field study, we found that BCD was not satisfied by EOC (i.e., BCD:EOC >1; uncoupled phytoplankton-bacteria relationship). BCD:EOC ratio was negatively related to PPT:CR ratio across the SW Mediterranean Sea. A spatial pattern emerged, i.e. in autotrophic open sea stations uncoupling was less severe (BCD:EOC ranged 1-2), whereas heterotrophic nearshore stations uncoupling was more severe (BCD:EOC > 2). In the experimental study, in the seawater both enriched with dust and under UVR, BCD:EOC ratio decreased by stimulating autotrophic processes (particulate primary production (PPP) and EOC) in the heterotrophic nearshore area, whereas BCD:EOC increased by stimulating heterotrophic processes [heterotrophic bacterial production (HBP), bacterial growth efficiency (BGE), bacterial respiration (BR)] in the autotrophic open sea. Our results show that this spatial pattern could be reversed under future UVR × Dust scenario. Overall, the impact of greater dust deposition and higher UVR levels will alter the phytoplankton-bacteria C-flux with consequences for the productivity of both communities, their standing stocks, and ultimately, the ecosystem's metabolic balance at the sea surface.

Sulfadiazine proliferated antibiotic resistance genes in the phycosphere of Chlorella pyrenoidosa: Insights from bacterial communities and microalgal metabolites

The phycosphere is an essential ecological niche for the proliferation of antibiotic resistance genes (ARGs). However, how ARGs' potential hosts change and the driving mechanism of metabolites under antibiotic stress in the phycosphere have seldom been researched. We investigated the response of Chlorella pyrenoidosa and the structure and abundance of free-living (FL) and particle-attached (PA) bacteria, ARGs, and metabolites under sulfadiazine by using real-time quantitative PCR, 16 S rRNA high-throughput. The linkage of key bacterial communities, ARGs, and metabolites through correlations was established. Through analysis of physiological indicators, Chlorella pyrenoidosa displayed a pattern of "low-dose promotion and high-dose inhibition" under antibiotic stress. ARGs were enriched in the PA treatment groups by 117 %. At the phylum level, Proteobacteria, Bacteroidetes, and Actinobacteria as potential hosts for ARGs. At the genus level, potential hosts included Sphingopyxis, SM1A02, Aquimonas, Vitellibacter, and Proteiniphilum. Middle and high antibiotic concentrations induced the secretion of metabolites closely related to potential hosts by algae, such as phytosphingosine, Lysophosphatidylcholine, and α-Linolenic acid. Therefore, changes in bacterial communities indirectly influenced the distribution of ARGs through alterations in metabolic products. These findings offer essential details about the mechanisms behind the spread and proliferation of ARGs in the phycosphere.

Dissolved organic carbon spurs bacterial-algal competition and phosphorus-paucity adaptation: Boosting Microcystis' phosphorus uptake capacity

Dissolved organic carbon (DOC) can alter the availability of background nutrients by affecting the proliferation of heterotrophic bacteria, which exerts a notable influence on algal growth and metabolism. However, the mechanism of how allochthonous DOC (aDOC) precipitates shifts in bacterial-algal interactions and modulates the occurrence of cyanobacteria blooms remains inadequately elucidated. Therefore, this study investigated the relationship between bacteria and algae under aDOC stimulation. We found that excess aDOC triggered the breakdown and reestablishment of the equilibrium between Microcystis and heterotrophic bacteria. The rapid proliferation of heterotrophic bacteria led to a dramatic decrease in soluble phosphorus and thereby resulted in the inhibition of the Microcystis growth. When the available DOC was depleted, the rapid death of heterotrophic bacteria released large amounts of dissolved phosphorus, which provided sufficient nutrients for the recovery of Microcystis. Notably, Microcystis rejuvenated and showed higher cell density compared to the carbon-absent group. This phenomenon can be ascribed that Microcystis regulated the compositions of extracellular polymeric substances (EPS) and the expression of relevant proteins to adapt to a nutrient-limited environment. Using time of flight secondary ion mass spectrometry (TOF-SIM) and proteomic analysis, we observed an enhancement of the signal of organic matter and metal ions associated with P complexation in EPS. Moreover, Microcystis upregulated proteins related to organic phosphorus transformation to increase the availability of phosphorus in various forms. In summary, this study emphasized the role of DOC in algal blooms, revealing the underestimated enhancement of Microcystis nutrient utilization through DOC-induced heterotrophic competition and providing valuable insights into eutrophication management and control.

Metabolic relationships between marine red algae and algae-associated bacteria

Mutualistic interactions between marine phototrophs and associated bacteria are an important strategy for their successful survival in the ocean, but little is known about their metabolic relationships. Here, bacterial communities in the algal sphere (AS) and bulk solution (BS) of nine marine red algal cultures were analyzed, and Roseibium and Phycisphaera were identified significantly more abundantly in AS than in BS. The metabolic features of Roseibium RMAR6-6 (isolated and genome-sequenced), Phycisphaera MAG 12 (obtained by metagenomic sequencing), and a marine red alga, Porphyridium purpureum CCMP1328 (from GenBank), were analyzed bioinformatically. RMAR6-6 has the genetic capability to fix nitrogen and produce B vitamins (B1, B2, B5, B6, B9, and B12), bacterioferritin, dimethylsulfoniopropionate (DMSP), and phenylacetate that may enhance algal growth, whereas MAG 12 may have a limited metabolic capability, not producing vitamins B9 and B12, DMSP, phenylacetate, and siderophores, but with the ability to produce bacitracin, possibly modulating algal microbiome. P. purpureum CCMP1328 lacks the genetic capability to fix nitrogen and produce vitamin B12, DMSP, phenylacetate, and siderophore. It was shown that the nitrogen-fixing ability of RMAR6-6 promoted the growth of P. purpureum, and DMSP reduced the oxidative stress of P. purpureum. The metabolic interactions between strain RMAR6-6 and P. purpureum CCMP1328 were also investigated by the transcriptomic analyses of their monoculture and co-culture. Taken together, potential metabolic relationships between Roseibium and P. purpureum were proposed. This study provides a better understanding of the metabolic relationships between marine algae and algae-associated bacteria for successful growth.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-024-00227-z.

Sphingomonas lacusdianchii sp. nov., an attached bacterium inhibited by metabolites from its symbiotic cyanobacterium

An alpha-proteobacterial strain JXJ CY 53 T was isolated from the cyanosphere of Microcystis sp. FACHB-905 (MF-905) collected from Lake Dianchi, China. JXJ CY 53 T was observed to be an aerobic, Gram-stain-negative, oval shaped, and mucus-secreting bacterium. It had C18:1ω7c and C16:0 as the major cellular fatty acids, Q-10 as the predominant ubiquinone, and sphingoglycolipid, diphosphatidylglycerol, phosphatidylcholine, and phosphatidylmethylethanolamine as the polar lipids. The G + C content of DNA was 65.85%. The bacterium had 16S rRNA gene sequence identities of 98.9% and 98.7% with Sphingomonas panni DSM 15761 T and Sphingomonas hankookensis KCTC 22579 T, respectively, while less than 97.4% identities with other members of the genus. Further taxonomic analysis indicated that JXJ CY 53 T represented a new member of Sphingomonas, and the species epithet was proposed as Sphingomonas lacusdianchii sp. nov. (type strain JXJ CY 53 T = KCTC 72813 T = CGMCC 1.17657 T). JXJ CY 53 T promoted the growth of MF-905 by providing bio-available phosphorus and nitrogen, plant hormones, vitamins, and carotenoids. It could modulate the relative abundances of nonculturable bacteria associated with MF-905 and influence the interactions of MF-905 and other bacteria isolated from the cyanobacterium, in addition to microcystin production characteristics. Meanwhile, MF-905 could provide JXJ CY 53 T dissolved organic carbon for growth, and control the growth of JXJ CY 53 T by secreting specific chemicals other than microcystins. Overall, these results suggest that the interactions between Microcystis and its attached bacteria are complex and dynamic, and may influence the growth characteristics of the cyanobacterium. This study provided new ideas to understand the interactions between Microcystis and its attached bacteria. KEY POINTS: • A novel bacterium (JXJCY 53 T) was isolated from the cyanosphere of Microcystis sp. FACHB-905 (MF-905) • JXJCY 53 T modulated the growth and microcystin production of MF-905 • MF-905 could control the attached bacteria by specific chemicals other than microcystins (MCs).

Decline of a distinct coral reef holobiont community under ocean acidification

BACKGROUND

Microbes play vital roles across coral reefs both in the environment and inside and upon macrobes (holobionts), where they support critical functions such as nutrition and immune system modulation. These roles highlight the potential ecosystem-level importance of microbes, yet most knowledge of microbial functions on reefs is derived from a small set of holobionts such as corals and sponges. Declining seawater pH - an important global coral reef stressor - can cause ecosystem-level change on coral reefs, providing an opportunity to study the role of microbes at this scale. We use an in situ experimental approach to test the hypothesis that under such ocean acidification (OA), known shifts among macrobe trophic and functional groups may drive a general ecosystem-level response extending across macrobes and microbes, leading to reduced distinctness between the benthic holobiont community microbiome and the environmental microbiome.

RESULTS

We test this hypothesis using genetic and chemical data from benthic coral reef community holobionts sampled across a pH gradient from CO2 seeps in Papua New Guinea. We find support for our hypothesis; under OA, the microbiome and metabolome of the benthic holobiont community become less compositionally distinct from the sediment microbiome and metabolome, suggesting that benthic macrobe communities are colonised by environmental microbes to a higher degree under OA conditions. We also find a simplification and homogenisation of the benthic photosynthetic community, and an increased abundance of fleshy macroalgae, consistent with previously observed reef microbialisation.

CONCLUSIONS

We demonstrate a novel structural shift in coral reefs involving macrobes and microbes: that the microbiome of the benthic holobiont community becomes less distinct from the sediment microbiome under OA. Our findings suggest that microbialisation and the disruption of macrobe trophic networks are interwoven general responses to environmental stress, pointing towards a universal, undesirable, and measurable form of ecosystem changed. Video Abstract.

A mutualistic bacterium rescues a green alga from an antagonist

Photosynthetic protists, known as microalgae, are key contributors to primary production on Earth. Since early in evolution, they coexist with bacteria in nature, and their mode of interaction shapes ecosystems. We have recently shown that the bacterium Pseudomonas protegens acts algicidal on the microalga Chlamydomonas reinhardtii. It secretes a cyclic lipopeptide and a polyyne that deflagellate, blind, and lyse the algae [P. Aiyar et al., Nat. Commun. 8, 1756 (2017) and V. Hotter et al., Proc. Natl. Acad. Sci. U.S.A. 118, e2107695118 (2021)]. Here, we report about the bacterium Mycetocola lacteus, which establishes a mutualistic relationship with C. reinhardtii and acts as a helper. While M. lacteus enhances algal growth, it receives methionine as needed organic sulfur and the vitamins B1, B3, and B5 from the algae. In tripartite cultures with the alga and the antagonistic bacterium P. protegens, M. lacteus aids the algae in surviving the bacterial attack. By combining synthetic natural product chemistry with high-resolution mass spectrometry and an algal Ca2+ reporter line, we found that M. lacteus rescues the alga from the antagonistic bacterium by cleaving the ester bond of the cyclic lipopeptide involved. The resulting linearized seco acid does not trigger a cytosolic Ca2+ homeostasis imbalance that leads to algal deflagellation. Thus, the algae remain motile, can swim away from the antagonistic bacteria and survive the attack. All three involved genera cooccur in nature. Remarkably, related species of Pseudomonas and Mycetocola also act antagonistically against C. reinhardtii or as helper bacteria in tripartite cultures.

Assembly dynamics of eukaryotic plankton and bacterioplankton in the Yangtze River estuary: A hybrid community perspective

Estuaries, acting as transitional habitats receiving species introductions from both freshwater and marine sources, undergo significant impacts from global climate changes. Planktonic microorganisms contribute significantly to estuarine biodiversity and ecological stability. These microorganisms primarily fall into three groups: eukaryotic plankton, particle-associated bacteria, and free-living bacteria. Understanding the structural characteristics and interactions within these subcommunities is crucial for comprehending estuarine dynamics. We collected samples from three distinct locations (< 0.1 PSU, 6.6 PSU, and 19 PSU) within the Yangtze River estuary. Samples underwent analysis for physicochemical indicators, while microbial communities were subjected to 16S/18S rRNA amplicon sequencing. Additionally, simulated mixing experiments were conducted using samples of varying salinities. Estuary samples, combined with simulated experiments, were employed to collectively examine the structural characteristics and assembly processes of estuarine microbes. Our research highlights the considerable impact of phylogenetic classification on prokaryotic behavior in these communities. We observed a transition in assembly processes from primarily stochastic for particle-associated bacteria to a predominant influence of homogeneous selection as salinity increased. Particle-associated bacterial communities exhibited a greater influence of stochastic processes compared to free-living bacteria, showcasing higher stability in diversity. The variations in composition and structure of estuarine microbial subcommunities were influenced by diverse environmental factors. Particle-associated bacteria displayed elevated network characterization values and established closer interactions with eukaryotic plankton. Structural equation modeling (SEM) analysis revealed that free-living bacteria displayed a heightened sensitivity to environmental factors and exerted a more significant influence on assembly processes and network characteristics. Simulated mixing in these environments resulted in the loss of species with similar microbial taxonomic relationships. The functioning of bacterioplankton is influenced by salinity and the processes governing their assembly, particularly in relation to different living states. These findings significantly contribute to our understanding of the intricate interplay between prokaryotic and eukaryotic plankton microorganisms in highly dynamic environments, laying a robust foundation for further exploration into the ecological mechanisms governing microbial dynamics in estuaries.

Antagonistic activity of Phaeobacter piscinae against the emerging fish pathogen Vibrio crassostreae in aquaculture feed algae

Aquaculture provides a rich resource of high-quality protein; however, the production is challenged by emerging pathogens such as Vibrio crassostreae. While probiotic bacteria have been proposed as a sustainable solution to reduce pathogen load in aquaculture, their application requires a comprehensive assessment across the aquaculture food chain. The purpose of this study was to determine the antagonistic effect of the potential probiotic bacterium Phaeobacter piscinae against the emerging fish pathogen V. crassostreae in aquaculture feed algae that can be an entry point for pathogens in fish and shellfish aquaculture. P. piscinae strain S26 produces the antibacterial compound tropodithietic acid (TDA). In a plate-based assay, P. piscinae S26 was equally to more effective than the well-studied Phaeobacter inhibens DSM17395 in its inhibition of the fish pathogens Vibrio anguillarum 90-11-286 and V. crassostreae DMC-1. When co-cultured with the microalgae Tetraselmis suecica and Isochrysis galbana, P. piscinae S26 reduced the maximum cell density of V. crassostreae DMC-1 by 2 log and 3-4 log fold, respectively. A TDA-deficient mutant of P. piscinae S26 inhibited V. crassostreae DMC-1 to a lesser extent than the wild type, suggesting that the antagonistic effect involves TDA and other factors. TDA is the prime antagonistic agent of the inhibition of V. anguillarum 90-11-286. Comparative genomics of V. anguillarum 90-11-286 and V. crassostreae DMC-1 revealed that V. crassostreae DMC-1 carries a greater arsenal of antibiotic resistance genes potentially contributing to the reduced effect of TDA. In conclusion, P. piscinae S26 is a promising new candidate for inhibition of emerging pathogens such as V. crassostreae DMC-1 in algal feed systems and could contribute to a more sustainable aquaculture industry.IMPORTANCEThe globally important production of fish and shellfish in aquaculture is challenged by disease outbreaks caused by pathogens such as Vibrio crassostreae. These outbreaks not only lead to substantial economic loss and environmental damage, but treatment with antibiotics can also lead to antibiotic resistance affecting human health. Here, we evaluated the potential of probiotic bacteria, specifically the newly identified strain Phaeobacter piscinae S26, to counteract these threats in a sustainable manner. Through a systematic assessment of the antagonistic effect of P. piscinae S26 against V. crassostreae DMC-1, particularly within the context of algal feed systems, the study demonstrates the effectiveness of P. piscinae S26 as probiotic and thereby provides a strategic pathway for addressing disease outbreaks in aquaculture. This finding has the potential of significantly contributing to the long-term stability of the industry, highlighting the potential of probiotics as an efficient and environmentally conscious approach to safeguarding aquaculture productivity against the adverse impact of pathogens.

Exchange or Eliminate: The Secrets of Algal-Bacterial Relationships

Algae and bacteria have co-occurred and coevolved in common habitats for hundreds of millions of years, fostering specific associations and interactions such as mutualism or antagonism. These interactions are shaped through exchanges of primary and secondary metabolites provided by one of the partners. Metabolites, such as N-sources or vitamins, can be beneficial to the partner and they may be assimilated through chemotaxis towards the partner producing these metabolites. Other metabolites, especially many natural products synthesized by bacteria, can act as toxins and damage or kill the partner. For instance, the green microalga Chlamydomonas reinhardtii establishes a mutualistic partnership with a Methylobacterium, in stark contrast to its antagonistic relationship with the toxin producing Pseudomonas protegens. In other cases, as with a coccolithophore haptophyte alga and a Phaeobacter bacterium, the same alga and bacterium can even be subject to both processes, depending on the secreted bacterial and algal metabolites. Some bacteria also influence algal morphology by producing specific metabolites and micronutrients, as is observed in some macroalgae. This review focuses on algal-bacterial interactions with micro- and macroalgal models from marine, freshwater, and terrestrial environments and summarizes the advances in the field. It also highlights the effects of temperature on these interactions as it is presently known.

Micrococcus lacusdianchii sp. nov., an attached bacterium inhibited by metabolites from its symbiotic algae

A novel actinobacterial strain, designated as JXJ CY 30 T, was isolated from the phycosphere of Microcystis aeruginosa FACHB-905 (Maf) collected from Lake Dianchi, China. The strain was a Gram-stain-positive, aerobic and coccus-shaped actinobacterium. It had alanine, glutamic acid, aspartic acid, and lysine in the peptidoglycan, and mannose, ribose and arabinose in its cell wall sugars, anteiso-C15:0 and iso-C15:0 as the main cellular fatty acids, MK-7 and MK-8 as the major respiratory quinones, and phosphatidylglycerol, diphosphatidylglycerol, phosphatidylinositol, glycolipid, and an unidentified phospholipid as the polar lipids. The DNA G + C content was 73.08%. Its 16 S rRNA gene sequence shared 99.14%, and 98.75% similarities with Micrococcus flavus DSM 19079 T and M. porci KD337-16T, respectively, and ≤98.41% similarities with other type strains of the genus Micrococcus. It formed independent clade with M. flavus DSM 19079 T on the phylogenetic trees. The digital DNA-DNA hybridization and average nucleotide identity values between strain JXJ CY 30 T and M. flavus DSM 19079 T and M. porci KD337-16T were 48.0% and 92.1%, 25.5% and 83.2%, respectively. These data above indicated that strain JXJ CY 30 T represented a new species of the genus Micrococcus, and the species epithet is proposed as Micrococcus lacusdianchii sp. nov. (type strain JXJ CY 30 T = KCTC 49378 T = CGMCC 1.17508 T). Strain JXJ CY 30 T can potentially provide Maf with various nutrients such as available phosphorus and nitrogen, plant hormones, various vitamins and carotenoids for growth, while it was inhibited by metabolites from its symbiotic algae Maf.

Algal blooms in the ocean: hot spots for chemically mediated microbial interactions

The cycling of major nutrients in the ocean is affected by large-scale phytoplankton blooms, which are hot spots of microbial life. Diverse microbial interactions regulate bloom dynamics. At the single-cell level, interactions between microorganisms are mediated by small molecules in the chemical crosstalk that determines the type of interaction, ranging from mutualism to pathogenicity. Algae interact with viruses, bacteria, parasites, grazers and other algae to modulate algal cell fate, and these interactions are dependent on the environmental context. Recent advances in mass spectrometry and single-cell technologies have led to the discovery of a growing number of infochemicals - metabolites that convey information - revealing the ability of algal cells to govern biotic interactions in the ocean. The diversity of infochemicals seems to account for the specificity in cellular response during microbial communication. Given the immense impact of algal blooms on biogeochemical cycles and climate regulation, a major challenge is to elucidate how microscale interactions control the fate of carbon and the recycling of major elements in the ocean. In this Review, we discuss microbial interactions and the role of infochemicals in algal blooms. We further explore factors that can impact microbial interactions and the available tools to decipher them in the natural environment.

Communication mediated interaction between bacteria and microalgae advances photogranulation

Recently, photogranules composed of bacteria and microalgae for carbon-negative nitrogen removal receive extensive attention worldwide, yet which type of bacteria is helpful for rapid formation of photogranules and whether they depend on signaling communication remain elusive. Varied signaling communication was analyzed using metagenomic method among bacteria and microalgae in via of two types of experimentally verified signaling molecule from bacteria to microalgae, which include indole-3-acetic acid (IAA) and N-acyl homoserine lactones (AHLs) during the operation of photo-bioreactors. Signaling communication is helpful for the adaptability of bacteria to survive with algae. Compared with non-signaling bacteria, signaling bacteria more easily adapt to the varied conditions, evidenced by the increased abundance in the operated reactors. Signaling bacteria are easier to enter the phycosphere, and they dominate the interactions between bacteria and algae rather than non-signaling bacteria. The co-abundance groups (CAGs) with signaling bacteria possess higher abundance than that without signaling bacteria (22.27 % and 6.67 %). Importantly, signaling bacteria accessibly interact with microalgae, which possess higher degree centralities and 32.50 % of them are keystone nodes in the network, in contrast to only 18.66 % of non-signaling bacteria. Thauera carrying both IAA and AHLs synthase genes are highly enriched and positively correlated with nitrogen removal rate. Our work not only highlights the essential roles of signaling communication between microalgae and bacteria in the development of photogranules, but also enriches our understanding of microbial sociobiology.

Modulation of growth, microcystin production, and algal-bacterial interactions of the bloom-forming algae Microcystis aeruginosa by a novel bacterium recovered from its phycosphere

Harmful algal blooms (HABs) in natural waters are of escalating global concern due to their detrimental impact on environmental health. Emerging evidence indicates that algae-bacteria symbionts can affect HAB features, though much about this interplay remains largely unexplored. The current study isolated a new species of Mucilaginibacter (type strain JXJ CY 39T) from culture biomass of the bloom-causing Microcystis aeruginosa FACHB-905 (Maf) from Lake Dianchi, China. Strain JXJ CY 39T was an aerobic, Gram-stain-negative rod bacterium that grew at 5-38°C, pH 4.0-11.0, and 0-3.0% NaCl. Taxonomic evaluation proposed a new species, with Mucilaginibacter lacusdianchii sp. nov., as the species epithet. Experimental results revealed that strain JXJ CY 39T spurred the growth of Maf by supplying soluble phosphorus and nitrogen during cultivation, despite the unavailability of soluble phosphorus and nitrogen. Additionally, by producing the plant hormone indole-3-acetate, strain JXJ CY 39T possibly impacted Maf's functionality. Results from co-culture experiments with other strains from Maf biomass showed possible effects of strain JXJ CY 39T on the relationship between Maf and other cohabiting bacteria, as well as microcystin toxin production characteristics. Although Maf could foster the growth of strain JXJ CY 39T by supplying organic carbon, the strain's growth could be regulated via specific chemical compounds based on antibiotic assays. Community composition analysis disclosed that this Mucilaginibacter strain positively affected Maf's growth and modified densities and types of bacteria linked to Maf. Overall, these results suggest that the interactions between important HAB-causing organisms and their attached bacteria are complex, dynamic, and may influence the growth characteristics of algae.

Effect of acetochlor on the symbiotic relationship between microalgae and bacteria

In this study, two strains of symbiotic bacteria (SOB-1 and SOB-2) were isolated from Scenedesmus obliquus, and various algal-bacterial mutualistic systems were established under acetochlor (ACT) stress conditions. Following exposure to varying ACT concentrations from 2.0 to 25.0 μg/L, the capacity for co-cultured bacteria to degrade ACT was enhanced in 7 days by up to 226.9% (SOB-1) and 193.0% (SOB-2), compared with axenic algae, although bacteria exposed to higher ACT concentrations exacerbated algal metabolic stress, oxidative states, apoptosis and cellular lysis. ACT reduced carbohydrates in the phycosphere by up to 31.5%; compensatory nutrient plunder and structural damage by bacteria were the potential exploitation pathways determined based on the inhibition of bacterial infection using a glucanase inhibitor. The ACT-induced reduction in algal antimicrobial substances, including fatty acids and phenolics (by up to 58.1% and 56.6%, respectively), also facilitated bacterial exploitation of algae. ACT-dependent interspecific interaction coefficients between algae and bacteria generated from long-term symbiosis cultures implied that bacteria moved from mutualism (0 and 2.0 μg/L ACT) to exploitation (7.9 and 25.0 μg/L ACT). The population dynamic model under incremental ACT-concentration scenarios inferred that theoretical systematic extinction may occur in algal-bacterial systems earlier than in axenic algae. These outcomes provide interspecific insights into the distortion of algal-bacterial reciprocity due to the ecotoxicological effects of ACT.

Dynamics of the benthic and planktic microbiomes in a Planktothrix-dominated toxic cyanobacterial bloom in Australia

Cyanobacterial blooms are a concerning issue that threaten ecosystems, ecology and animal health. Bloom frequency has increased tremendously in recent times due to pollution, eutrophication of waterways, climate change, and changes in microbial community dynamics within the aquatic environment. Information about the spatiotemporal variation in microbial communities that drive a cyanobacterial bloom is very limited. Here, we analysed the spatiotemporal diversity and composition of bacterial communities, with a focus on cyanobacteria, during the bloom phase in a natural reservoir in Eastern Australia using high throughput amplicon sequencing. Sampling points and season had no influence on the richness and evenness of microbial communities during the bloom period, however some compositional differences were apparent across the seasons. Cyanobacteria were highly abundant during summer and autumn compared to winter and spring. The dominant cyanobacterial taxa were Planktothrix, Cyanobium and Microcystis and were found to be significantly abundant during summer and autumn. While cyanobacterial abundance soared in summer (25.4 %), dominated by Planktothrix (12.2 %) and Cyanobium (8.0 %), the diversity was highest in autumn (24.9 %) and consisted of Planktothrix (7.8 %), Nodularia (5.3 %), Planktothricoides (4.6 %), Microcystis (3.5 %), and Cyanobium (2.3 %). The strongly correlated non-photosynthetic Gastranaerophilales found in the sediment and water, suggested vertical transmission from the animal gut through faeces. To our knowledge, this is the first report of Planktothrix-driven toxic cyanobacterial bloom in Australia. Our study expands current understanding of the spatiotemporal variation in bacterial communities during a cyanobacterial bloom and sheds light on setting future management strategies for its control.

Light-dark fluctuated metabolic features of diazotrophic and non-diazotrophic cyanobacteria and their coexisting bacteria

Cyanobacteria, the most abundant photosynthetic organisms in oceans, are tightly associated with diverse microbiota. However, the relationships between heterotrophic bacteria and cyanobacteria, particularly the diazotrophic group, are not fully understood. Here, we compared diel gene expressions of N2 fixing cyanobacteria Crocosphaera watsonii WH0003 and non-diazotrophic Synechococcus sp. RS9902 and their associated bacteria using metatranscriptomics approach. WH0003 showed significant up-regulation of O2 restriction and oxidative phosphorylation related genes at nighttime due to large carbon and energy investments for active N2 fixation. In contrast, RS9902 had higher expression for those genes at daytime. The two cyanobacteria hosted distinct bacterial communities with clear separate substrate utilization niches to reduce competition. Light-dark partitioning of nutrient acquisition among the dominant bacterial groups likely contributed to the dynamic balance for community coexistence. Moreover, particle-attached (PA) bacteria in RS9902 largely expressed glycoside hydrolases to hydrolyze complex carbohydrate compounds, while free-living (FL) bacteria priorly assimilated soluble, diffusible molecules. Spatial partitioning of nutrient acquisition between PA and FL bacteria implied that location initially influenced metabolic features of host associated bacteria. Our results advance knowledge on light-dark regulated metabolic activities of diazotrophic and non-diazotrophic cyanobacteria, and provide new insights into the coexisting strategies of different bacterial groups.

Potential applications of microalgae-bacteria consortia in wastewater treatment and biorefinery

The use of microalgae-bacteria consortia (MBC) for wastewater treatment has garnered attention as their interactions impart greater environmental adaptability and stability compared with that obtained by only microalgae or bacteria use, thereby improving the efficiency of pollutant removal and bio-product productivity. Additionally, the value-added bio-products produced via biorefineries can improve economic competitiveness and environmental sustainability. Therefore, this review focuses on the interaction between microalgae and bacteria that leads to nutrient exchange, gene transfer and signal transduction to comprehensively understand the interaction mechanisms underlying their strong adaptability. In addition, it includes recent research in which MBC has been efficiently used to treat various wastewater. Moreover, the review summarizes the use of MBC-produced biomass in a biorefining context to produce biofuel, biomaterial, high-value bio-products and bio-fertilizer. Overall, more effort is needed to identify the symbiotic mechanism in MBC to provide a foundation for circular bio-economy and environmentally friendly development programmes.

Noctiluca scintillans bloom alters the composition and carbohydrate utilization of associated bacterial community and enriches potential pathogenic bacterium Vibrio anguillarum

Noctiluca scintillans (red) is a widely distributed heterotrophic dinoflagellate and a prominent red tide forming species. This study investigated the effects of Noctiluca blooms on marine microbial diversity and functionality using multi-omics approaches. Our findings revealed significant differences in the community composition of Noctiluca-associated bacteria compared to those associated with autotrophic plankton and free-living bacteria in the surrounding seawater. The dominant bacterial groups within the Noctiluca-associated community shifted at various bloom stages, which could be attributed to changes in prey composition of Noctiluca. During the non-bloom stage, Burkholderiaceae, Carnobacteriaceae, and Pseudomonadaceae dominated the community, while Vibrionaceae became dominant during the bloom stage, and Saprospiraceae, Crocinitomicaceae, and Pirellulaceae thrived during the post-bloom stage. Compared to the non-bloom stage, Noctiluca-associated bacterial community at the bloom stage exhibited significant down-regulation of genes related to complex carbohydrate metabolism, while up-regulation of genes related to glucose transportation and utilization. Furthermore, we identified Vibrio anguillarum, a potential pathogenic bacterium to marine fish, as a major component of the Vibrionaceae family during the bloom stage. The occurrence of V. anguillarum associated with Noctiluca blooms may be attributed to the increased availability of its preferred carbon sources and its high capabilities in glucose transportation, motility and chemotaxis. Moreover, the presence of Vibrio infection genes (hap, hlyA, rtxA) encoding vibriolysin, hemolysin, and RTX (Repeats-in-toxin) toxin in the V. anguillarum genome, with the hap gene showing high expression levels during Noctiluca blooms, indicates an elevated risk of infection. This study underscores the unique composition of the bacterial community associated with red tide forming heterotrophic dinoflagellates and suggests that Noctiluca cells may serve as reservoirs and vectors for pathogenic bacteria, potentially posing a threat to fish-farming and the health of other marine organisms.

Treatment of industrial wastewaters by algae-bacterial consortium with Bio-H2 production: Recent updates, challenges and future prospects

Currently, scarcity/security of clean water and energy resources are the most serious problems worldwide. Industries use large volume of ground water and a variety of chemicals to manufacture the products and discharge large volume of wastewater into environment, which causes severe impacts on environment and public health. Fossil fuels are considered as major energy resources for electricity and transportation sectors, which release large amount of CO2 and micro/macro pollutants, leading to cause the global warming and public health hazards. Therefore, algae-bacterial consortium (A-BC) may be eco-friendly, cost-effective and sustainable alternative way to treat the industrial wastewaters (IWWs) with Bio-H2 production. A-BC has potential to reduce the global warming and eutrophication. It also protects environment and public health as it converts toxic IWWs into non or less toxic (biomass). It also reduces 94%, 90% and 50% input costs of nutrients, freshwater and energy, respectively during IWWs treatment and Bio-H2 production. Most importantly, it produce sustainable alternative (Bio-H2) to replace use of fossil fuels and fill the world's energy demand in eco-friendly manner. Thus, this review paper provides a detailed knowledge on industrial wastewaters, their pollutants and toxic effects on water/soil/plant/humans and animals. It also provides an overview on A-BC, IWWs treatment, Bio-H2 production, fermentation process and its enhancement methods. Further, various molecular and analytical techniques are also discussed to characterize the A-BC structure, interactions, metabolites and Bio-H2 yield. The significance of A-BC, recent update, challenges and future prospects are also discussed.

Antagonistic actions of Paucibacter aquatile B51 and its lasso peptide paucinodin toward cyanobacterial bloom-forming Microcystis aeruginosa PCC7806

Superior antagonistic activity against axenic Microcystis aeruginosa PCC7806 was observed with Paucibacter sp. B51 isolated from cyanobacterial bloom samples among 43 tested freshwater bacterial species. Complete genome sequencing, analyzing average nucleotide identity and digital DNA-DNA hybridization, designated the B51 strain as Paucibacter aquatile. Electron and fluorescence microscopic image analyses revealed the presence of the B51 strain in the vicinity of M. aeruginosa cells, which might provoke direct inhibition of the photosynthetic activity of the PCC7806 cells, leading to perturbation of cellular metabolisms and consequent cell death. Our speculation was supported by the findings that growth failure of the PCC7806 cells led to low pH conditions with fewer chlorophylls and down-regulation of photosystem genes (e.g., psbD and psaB) during their 48-h co-culture condition. Interestingly, the concentrated ethyl acetate extracts obtained from B51-grown supernatant exhibited a growth-inhibitory effect on PCC7806. The physical separation of both strains by a filter system led to no inhibitory activity of the B51 cells, suggesting that contact-mediated anti-cyanobacterial compounds might also be responsible for hampering the growth of the PCC7806 cells. Bioinformatic tools identified 12 gene clusters that possibly produce secondary metabolites, including a class II lasso peptide in the B51 genome. Further chemical analysis demonstrated anti-cyanobacterial activity from fractionated samples having a rubrivinodin-like lasso peptide, named paucinodin. Taken together, both contact-mediated inhibition of photosynthesis and the lasso peptide secretion of the B51 strain are responsible for the anti-cyanobacterial activity of P. aquatile B51.

Genetic evidence for algal auxin production in Chlamydomonas and its role in algal-bacterial mutualism

Interactions between algae and bacteria are ubiquitous and play fundamental roles in nutrient cycling and biomass production. Recent studies have shown that the plant auxin indole acetic acid (IAA) can mediate chemical crosstalk between algae and bacteria, resembling its role in plant-bacterial associations. Here, we report a mechanism for algal extracellular IAA production from L-tryptophan mediated by the enzyme L-amino acid oxidase (LAO1) in the model Chlamydomonas reinhardtii. High levels of IAA inhibit algal cell multiplication and chlorophyll degradation, and these inhibitory effects can be relieved by the presence of the plant-growth-promoting bacterium (PGPB) Methylobacterium aquaticum, whose growth is mutualistically enhanced by the presence of the alga. These findings reveal a complex interplay of microbial auxin production and degradation by algal-bacterial consortia and draws attention to potential ecophysiological roles of terrestrial microalgae and PGPB in association with land plants.

Impact of airborne algicidal bacteria on marine phytoplankton blooms

Ocean microbes are involved in global processes such as nutrient and carbon cycling. Recent studies indicated diverse modes of algal-bacterial interactions, including mutualism and pathogenicity, which have a substantial impact on ecology and oceanic carbon sequestration, and hence, on climate. However, the airborne dispersal and pathogenicity of bacteria in the marine ecosystem remained elusive. Here, we isolated an airborne algicidal bacterium, Roseovarius nubinhibens, emitted to the atmosphere as primary marine aerosol (referred also as sea spray aerosols) and collected above a coccolithophore bloom in the North Atlantic Ocean. The aerosolized bacteria retained infective properties and induced lysis of Gephyrocapsa huxleyi cultures.This suggests that the transport of marine bacteria through the atmosphere can effectively spread infection agents over vast oceanic regions, highlighting its significance in regulating the cell fate in algal blooms.

Taxonomic, Phylogenomic and Bioactivity Profiling of Novel Phycosphere Bacterium from Model Cyanobacterium Synechococcus elongatus PCC 7942

Phycosphere niches host rich microbial consortia that harbor dynamic algae-bacteria interactions with fundamental significance in varied natural ecosystems. Hence, culturing the uncultured microbial majority of the phycosphere microbiota is vital for deep understanding of the intricate mechanisms governing the dynamic interactions, and also to provide novel and rich microbial resources, and to discover new natural bioactive metabolites. Synechococcus elongatus PCC 7942 is a robust model cyanobacterium widely used in environment, synthesis biology, and biotechnology research. To expand the number of novel phycosphere species that were brought into culture and to discover the natural bioactivities, we presented a new yellow-pigmented bacterium named ABI-127-1, which was recovered from the phycosphere of PCC 7942, using an optimized bacterial isolation procedure. Combined polyphasic taxonomic and phylogenomic characterization was performed to confidently identify the new isolate as a potential novel species belonging to the genus Qipengyuania. The observed bioactivity of strain ABI-127-1 with promoting potential towards the growth and CO2 fixation efficiency of the host microalgae was measured. Additionally, the bacterial production of active bioflocculant exopolysaccharides was evaluated after culture optimization. Thus, these findings revealed the potential environmental and biotechnological implications of this new microalgae growth-promoting bacterium isolated from the phycosphere microenvironment.

Devosia lacusdianchii sp. nov., an attached bacterium inhibited by metabolites from its symbiotic Microcystis

A novel alphaproteobacterial strain JXJ CY 41T was isolated from a culture mass of Microcystis, collected from Lake Dianchi, south-west, China. Strain JXJ CY 41T was gram-strain-negative, aerobic, motile, with rod-shaped cells (0.4-1.0 × 1.7-3.5 μm). It was positive for catalase and starch hydrolysis, negative for oxidase and hydrolysis of Tweens (20, 40, and 80). Growth occurred at 10-44 °C, pH 5.0-10.0, and 0-5.0% (w/v) NaCl. Major fatty acids included C16:0 (28.1%), 11-methyl C18:1 ω7c (36.7%) and C18:1 ω7c (20.8%). Q10 was the sole ubiquinone. The polar lipids were diphosphatidylglycerol, phosphatidylglycerol, glycolipid, and an unidentified lipid. The DNA G + C content was 63.1%. Its 16S rRNA gene sequence showed high similarities with Devosia oryziradicis G19T (99.5%; not validly published), D. yakushimensis Yak96BT (98.3%) and D. ginsengisoli Gsoil 520T (98.1%), and less than 98.1% similarities with other members of the genus Devosia. The digital DNA-DNA hybridization (dDDH) and average nucleotide identity (ANI) values between strain JXJ CY 41T and its 5 closest similar strains were 19.9-24.1% and 75.7-80.5%, respectively. Based on the data above, strain JXJ CY 41T was identified as a novel species of the genus Devosia, for which the epithet Devosia lacusdianchii sp. nov. was proposed. The type strain is JXJ CY 41T (= KCTC 72812T = CGMCC 1.17502T). Strain JXJ CY 41T exhibited different interactions with Microcystis aeruginosa FACHB-905 (Maf) under different conditions, and Maf could control the bacterial cellular density by secreting unknown specific chemical compounds according to its nutritional requirements.

Linking the impact of bacteria on phytoplankton growth with microbial community composition and co-occurrence patterns

The interactions between microalgae and bacteria have recently emerged as key control factors which might contribute to a better understanding on how phytoplankton communities assemble and respond to environmental disturbances. We analyzed partial 16S rRNA and 18S rRNA genes from a total of 42 antibiotic bioassays, where phytoplankton growth was assessed in the presence or absence of an active bacterial community. A significant negative impact of bacteria was observed in 18 bioassays, a significant positive impact was detected in 5 of the cases, and a non-detectable effect occurred in 19 bioassays. Thalasiossira spp., Chlorophytes, Vibrionaceae and Alteromonadales were relatively more abundant in the samples where a positive effect of bacteria was observed compared to those where a negative impact was observed. Phytoplankton diversity was lower when bacteria negatively affect their growth than when the effect was beneficial. The phytoplankton-bacteria co-occurrence subnetwork included many significant Chlorophyta-Alteromonadales and Bacillariophyceae-Alteromonadales positive associations. Phytoplankton-bacteria co-exclusions were not detected in the network, which contrasts with the negative effect of bacteria on phytoplankton growth frequently detected in the bioassays, suggesting strong competitive interactions. Overall, this study adds strong evidence supporting the key role of phytoplankton-bacteria interactions in the microbial communities.

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.

Exploring the phycosphere of Emiliania huxleyi: From bloom dynamics to microbiome assembly experiments

Coccolithophores have global ecological and biogeochemical significance as the most important calcifying marine phytoplankton group. The structure and selection of prokaryotic communities associated with the most abundant coccolithophore and bloom-forming species, Emiliania huxleyi, are still poorly known. In this study, we assessed the diversity of bacterial communities associated with an E. huxleyi bloom in the Celtic Sea (Eastern North Atlantic), exposed axenic E. huxleyi cultures to prokaryotic communities derived from bloom and non-bloom conditions, and followed the dynamics of their microbiome composition over one year. Bloom-associated prokaryotic communities were dominated by SAR11, Marine group II Euryarchaeota and Rhodobacterales and contained substantial proportions of known indicators of phytoplankton bloom demises such as Flavobacteriaceae and Pseudoalteromonadaceae. The taxonomic richness of bacteria derived from natural communities associated with axenic E. huxleyi rapidly shifted and then stabilized over time. The succession of microorganisms recruited from the environment was consistently dependent on the composition of the initial bacterioplankton community. Phycosphere-associated communities derived from the E. huxleyi bloom were highly similar to one another, suggesting deterministic processes, whereas cultures from non-bloom conditions show an effect of stochasticity. Overall, this work sheds new light on the importance of the initial inoculum composition in microbiome recruitment and elucidates the temporal dynamics of its composition and long-term stability.

Progress and challenges in exploring aquatic microbial communities using non-targeted metabolomics

Advances in bioanalytical technologies are constantly expanding our insights into complex ecosystems. Here, we highlight strategies and applications that make use of non-targeted metabolomics methods in aquatic chemical ecology research and discuss opportunities and remaining challenges of mass spectrometry-based methods to broaden our understanding of environmental systems.

Growth promotion of Sargassum fusiforme by epiphytic microbes is dependent on the extent of interspecific interactions of the microbial community

Profound growth differences such as seedling length and biomass are often observed during the cultivation of Sargassum fusiforme despite the absence of detectable variance in abiotic factors that could have affected this process. This highlights the importance of biotic factors such as epiphytic microbiota in controlling seedling growth. Yet, how, and to what extent microbial activities can affect host growth in the presence of seawater flow and continuous erosion remains debatable. Particularly, the contribution of microbial network interactions to the growth of macroalgae remains poorly understood. This study aimed to compare the physicochemical properties of S. fusiforme seedlings via 16S rRNA gene Illumina sequencing-based profiling of the epiphytic microbial communities of seedlings with different lengths. Significantly different epiphytic bacterial communities were observed among S. fusiforme seedlings of different lengths. The result showed that community from longer seedlings maintained higher bacterial diversity with the taxa Gammaproteobacteria, Burkholderiales, Alteromonadales, Vibrionaceae, Ralstonia, Colwelliaceae, and Thalassotalea being selectively enriched. More importantly, microbial interspecific interactions, which were predominantly positive, were enhanced consistently in communities of the longer seedlings, indicative of reinforced prevalent and mutually cooperative relationships among the microorganisms associated with S. fusiforme seedlings of greater length. Furthermore, longer seedlings also displayed up-regulation of microbial functional potentials involved in N fixation and mineralization, P mineralization and transportation, and ion transportation compared with shorter ones. Lastly, stochastic processes dominated the community assembly of the epiphytic microorganisms. These findings could provide new insights into the relationship between microbial communities and growth in S. fusiforme seedlings and enable us to predict the community diversity and assembly of macroalgae-associated microbial communities. This could have important implications for linking microbial community diversity and network interactions to their host productivity.

Microbiome and photoperiod interactively determine thermal sensitivity of polar and temperate diatoms

The effect of temperature on ectothermic organisms in the context of climate change has long been considered in isolation (i.e. as a single driver). This is challenged by observations demonstrating that temperature-dependent growth is correlated to further factors. However, little is known how the chronobiological history of an organism reflected in its adaptation to re-occurring cyclic patterns in its environment (e.g. annual range of photoperiods in its habitat) and biotic interactions with its microbiome, contribute to shaping its realized niche. To address this, we conducted a full-factorial microcosm multi-stressor experiment with the marine diatoms Thalassiosira gravida (polar) and Thalassiosira rotula (temperate) across multiple levels of temperature (4°C; 9°C; 13.5°C) and photoperiod (4 h; 16 h; 24 h), both in the presence or absence of their microbiomes. While temperature-dependent growth of the temperate diatom was constrained by short and long photoperiods, the polar diatom coped with a 24 h photoperiod up to its thermal optimum (9°C). The algal microbiomes particularly supported host growth at the margins of their respective fundamental niches except for the combination of the warmest temperature tested at 24 h photoperiod. Overall, this study demonstrates that temperature tolerances may have evolved interactively and that the mutualistic effect of the microbiome can only be determined once the multifactorial abiotic niche is defined.

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.

Polyunsaturated Aldehydes Profile in the Diatom Cyclotella cryptica Is Sensitive to Changes in Its Phycosphere Bacterial Assemblages

Diatoms are responsible for the fixation of ca. 20% of the global CO2 and live associated with bacteria that utilize the organic substances produced by them. Current research trends in marine microbial ecology show which diatom and bacteria interact mediated through the production and exchange of infochemicals. Polyunsaturated aldehydes (PUA) are organic molecules released by diatoms that are considered to have infochemical properties. In this work, we investigated the possible role of PUA as a mediator in diatom-bacteria interactions. To this end, we compare the PUA profile of a newly isolated oceanic PUA producer diatom, Cyclotella cryptica, co-cultured with and without associated bacteria at two phosphate availability conditions. We found that the PUA profile of C. cryptica cultured axenically was different than its profile when it was co-cultured with autochthonous (naturally associated) and non-autochthonous bacteria (unnaturally inoculated). We also observed that bacterial presence significantly enhanced diatom growth and that C. cryptica modulated the percentage of released PUA in response to the presence of bacteria, also depending on the consortium type. Based on our results, we propose that this diatom could use released PUA as a specific organic matter sign to attract beneficial bacteria for constructing its own phycosphere, for more beneficial growth.

Antibiotic-Induced Recruitment of Specific Algae-Associated Microbiome Enhances the Adaptability of Chlorella vulgaris to Antibiotic Stress and Incidence of Antibiotic Resistance

Insights into the symbiotic relation between eukaryotic hosts and their microbiome lift the curtain on the crucial roles of microbes in host fitness, behavior, and ecology. However, it remains unclear whether and how abiotic stress shapes the microbiome and further affects host adaptability. This study first investigated the effect of antibiotic exposure on behavior across varying algae taxa at the community level. Chlorophyta, in particular Chlorella vulgaris, exhibited remarkable adaptability to antibiotic stress, leading to their dominance in phytoplankton communities. Accordingly, we isolated C. vulgaris strains and compared the growth of axenic and nonaxenic ones under antibiotic conditions. The positive roles of antibiotics in algal growth were apparent only in the presence of bacteria. Results of 16S rRNA sequencing further revealed that antibiotic challenges resulted in the recruitment of specific bacterial consortia in the phycosphere, whose functions were tightly linked to the host growth promotion and adaptability enhancement. In addition, the algal phycosphere was characterized with 47-fold higher enrichment capability of antibiotic resistance genes (ARGs) than the surrounding water. Under antibiotic stress, specific ARG profiles were recruited in C. vulgaris phycosphere, presumably driven by the specific assembly of bacterial consortia and mobile genetic elements induced by antibiotics. Moreover, the antibiotics even enhanced the dissemination potential of the bacteria carrying ARGs from the algal phycosphere to broader environmental niches. Overall, this study provides an in-depth understanding into the potential functional significance of antibiotic-mediated recruitment of specific algae-associated bacteria for algae adaptability and ARG proliferation in antibiotic-polluted waters.

Temporal variability in the growth-enhancing effects of different bacteria within the microbiome of the diatom Actinocyclus sp

Reciprocal metabolite exchanges between diatoms and bacteria can enhance the growth of both partners and therefore fundamentally influence aquatic ecosystem productivity. Here, we examined the growth-promoting capabilities of 15 different bacterial isolates from the bacterial community associated with the marine diatom Actinocyclus sp. and investigated the magnitude and timing of their effect on the growth of this diatom. In the presence of its microbiome, Actinocyclus sp. growth was significantly enhanced relative to axenic cultures. Co-culture with each of the 15 bacterial isolates examined here (seven Rhodobacteraceae, four Vibrionaceae, two Pseudoalteromonadaceae, one Oceanospirillaceae and one Alteromonadaceae) increased the growth of the diatom host, with four isolates inducing rates of growth that were similar to those delivered by the diatom's full microbiome. However, the timing and duration of this effect differed between the different bacteria tested. Indeed, one Rhodobacteraceae and one Alteromonadaceae enhanced Actinocyclus sp. cell numbers between days 0-6 after co-incubation, five other Rhodobacteraceae promoted diatom cell numbers the most between days 8-12, whilst four Vibrionaceae, one Oceanospirillaceae and one Rhodobacteraceae enhanced Actinocyclus sp. cell abundance between days 14-16. These results are indicative of a succession of the growth-enhancing effects delivered by diverse bacteria throughout the Actinocyclus sp. life cycle, which will likely deliver sustained growth benefits to the diatom when its full microbiome is present.

Bacteria and microalgae associations in periphyton-mechanisms and biotechnological opportunities

Phototrophic and heterotrophic microorganisms coexist in complex and dynamic structures called periphyton. These structures shape the biogeochemistry and biodiversity of aquatic ecosystems. In particular, microalgae-bacteria interactions are a prominent focus of study by microbial ecologists and can provide biotechnological opportunities for numerous applications (i.e. microalgal bloom control, aquaculture, biorefinery, and wastewater bioremediation). In this review, we analyze the species dynamics (i.e. periphyton formation and factors determining the prevalence of one species over another), coexisting communities, exchange of resources, and communication mechanisms of periphytic microalgae and bacteria. We extend periphyton mathematical modelling as a tool to comprehend complex interactions. This review is expected to boost the applicability of microalgae-bacteria consortia, by drawing out knowledge from natural periphyton.

Cellular Damage of Bacteria Attached to Senescent Phytoplankton Cells as a Result of the Transfer of Photochemically Produced Singlet Oxygen: A Review

Several studies set out to explain the presence of high proportions of photooxidation products of cis-vaccenic acid (generally considered to be of bacterial origin) in marine environments. These studies show that these oxidation products result from the transfer of singlet oxygen from senescent phytoplankton cells to the bacteria attached to them in response to irradiation by sunlight. This paper summarizes and reviews the key findings of these studies, i.e., the demonstration of the process at work and the effect of different parameters (intensity of solar irradiance, presence of bacterial carotenoids, and presence of polar matrices such as silica, carbonate, and exopolymeric substances around phytoplankton cells) on this transfer. A large part of this review looks at how this type of alteration of bacteria can affect the preservation of algal material in the marine environment, especially in polar regions where conditions drive increased transfer of singlet oxygen from sympagic algae to bacteria.

Ship Hull-Fouling Diatoms on Korean Research Vessels Revealed by Morphological and Molecular Methods, and Their Environmental Implications

Ship biofouling is one of the main vectors for the introduction and global spread of non-indigenous organisms. Diatoms were the early colonizers of ship hulls; however, their community composition on ships is poorly understood. Herein, we investigated the diatom community on the hull samples collected from two Korean research vessels Isabu (IRV) and Onnuri (ORV) on September 2 and November 10, 2021, respectively. IRV showed low cell density (345 cells/cm2) compared to ORV (778 cells/cm2). We morphologically identified more than 15 species of diatoms from the two research vessels (RVs). The microalgae in both RVs were identified as Amphora, Cymbella, Caloneis, Halamphora, Navicula, Nitzschia, and Plagiogramma. Of them, the genus Halamphora was found to be predominant. However, both RVs had a varied dominant species with a significant difference in body size; Halamphora oceanica dominated at IRV, and Halamphora sp. at ORV, respectively. Molecular cloning showed similar results to morphological analysis, in which Halamphora species dominated in both RVs. The hull-attached species were distinct from species found in the water column. These results revealed diatoms communities that are associated with ship hull-fouling at an early stage of biofilm formation. Moreover, ships arriving from different regions could show some variation in species composition on their hull surfaces, with the potential for non-indigenous species introduction.

A bacterial symbiont in the gill of the marine scallop Argopecten irradians irradians metabolizes dimethylsulfoniopropionate

Microbial lysis of dimethylsulfoniopropionate (DMSP) is a key step in marine organic sulfur cycling and has been recently demonstrated to play an important role in mediating interactions between bacteria, algae, and zooplankton. To date, microbes that have been found to lyse DMSP are largely confined to free-living and surface-attached bacteria. In this study, we report for the first time that a symbiont (termed "Rhodobiaceae bacterium HWgs001") in the gill of the marine scallop Argopecten irradians irradians can lyse and metabolize DMSP. Analysis of 16S rRNA gene sequences suggested that HWgs001 accounted for up to 93% of the gill microbiota. Microscopic observations suggested that HWgs001 lived within the gill tissue. Unlike symbionts of other bivalves, HWgs001 belongs to Alphaproteobacteria rather than Gammaproteobacteria, and no genes for carbon fixation were identified in its small genome. Moreover, HWgs001 was found to possess a dddP gene, responsible for the lysis of DMSP to acrylate. The enzymatic activity of dddP was confirmed using the heterologous expression, and in situ transcription of the gene in scallop gill tissues was demonstrated using reverse-transcription PCR. Together, these results revealed a taxonomically and functionally unique symbiont, which represents the first-documented DMSP-metabolizing symbiont likely to play significant roles in coastal marine ecosystems.

Harmful cyanobacteria-diatom/dinoflagellate blooms and their cyanotoxins in freshwaters: A nonnegligible chronic health and ecological hazard

Human and ecological health depends on the vitality of freshwater systems, but these are increasingly threatened by cyanotoxins released from harmful algal blooms (HABs). Periodic cyanotoxin production, although undesirable, may be tolerable when there is enough time for cyanotoxins to degrade and dissipate in the environment, but the year-round presence of these toxins will be a chronic health for humans and ecosystems. The purpose of this critical review is to document the seasonal shifts of algal species and their ecophysiological acclimatation to dynamic environmental conditions. We discuss how these conditions will create successive occurrences of algal blooms and the release of cyanotoxins into freshwater. We first review the most common cyanotoxins, and evaluate the multiple ecological roles and physiological functions of these toxins for algae. Then, the annual recurring patterns HABs are considered in the context of global change, which demonstrates the capacity for algal blooms to shift from seasonal to year-round growth regimes that are driven by abiotic and biotic factors, leading to chronic loading of freshwaters with cyanotoxins. At last, we illustrate the impacts of HABs on the environment by compiling four health issues and four ecology issues emanating from their presence in the that covers atmosphere, aquatic ecosystems and terrestrial ecosystems. Our study highlights the annual patterns of algal blooms, and proposes that a "perfect storm" of events is lurking that will cause the 'seasonal toxicity' to become a full-blown, 'chronic toxicity' in the context of the deterioration of HABs, highlighting a non-negligible chronic health and ecological hazard.

Algae obscura: The potential of rare species as model systems

Model organism research has provided invaluable knowledge about foundational biological principles. However, most of these studies have focused on species that are in high abundance, easy to cultivate in the lab, and represent only a small fraction of extant biodiversity. Here, we present three examples of rare algae with unusual features that we refer to as "algae obscura." The Cyanidiophyceae (Rhodophyta), Glaucophyta, and Paulinella (rhizarian) lineages have all transitioned out of obscurity to become models for fundamental evolutionary research. Insights have been gained into the prevalence and importance of eukaryotic horizontal gene transfer, early Earth microbial community dynamics, primary plastid endosymbiosis, and the origin of Archaeplastida. By reviewing the research that has come from the exploration of these organisms, we demonstrate that underappreciated algae have the potential to help us formulate, refine, and substantiate core hypotheses and that such organisms should be considered when establishing future model systems.

Dimethylsulfoniopropionate decorated cryogels as synthetic spatially structured habitats of marine bacterial communities

In microbial consortia bacteria often settle on other organisms that provide nutrients and organic material for their growth. This is true for the plankton where microalgae perform photosynthesis and exude metabolites that feed associated bacteria. The investigation of such processes is difficult since algae provide bacteria with a spatially structured environment with a gradient of released organic material that is hard to mimic. Here we introduce the design and synthesis of a cryogel-based microstructured habitat for bacteria that provides dimethylsulfoniopropionate (DMSP) as a carbon and sulfur source for growth. DMSP, a widely distributed metabolite released by algae, is thereby made available for bacteria in a biomimetic manner. Based on a novel DMSP derived building block (DMSP-HEMA), we synthesized cryogels providing structured surfaces for settlement and delivering the organic material fueling bacterial growth. By monitoring bacterial settlement and performance we show that the cryogels represent microbial arenas mimicking the ecological situation in the plankton.

Vitamin B12 is not shared by all marine prototrophic bacteria with their environment

Vitamin B₁₂ (cobalamin, herein B₁₂) is an essential cofactor involved in amino acid synthesis and carbon resupply to the TCA cycle for most prokaryotes, eukaryotic microorganisms, and animals. Despite being required by most, B₁₂ is produced by only a minor fraction of prokaryotes and therefore leads to complex interaction between prototrophs and auxotrophs. However, it is unknown how B₁₂ is provided by prototrophs to auxotrophs. In this study, 33 B₁₂ prototrophic alphaproteobacterial strains were grown in co-culture with Thalassiosira pseudonana, a B₁₂ auxotrophic diatom, to determine the bacterial ability to support the growth of the diatom by sharing B₁₂. Among these strains, 18 were identified to share B₁₂ with the diatom, while nine were identified to retain B₁₂ and not support growth of the diatom. The other bacteria either shared B₁₂ with the diatom only with the addition of substrate or inhibited the growth of the diatom. Extracellular B₁₂ measurements of B₁₂-provider and B₁₂-retainer strains confirmed that the cofactor could only be detected in the environment of the tested B₁₂-provider strains. Intracellular B₁₂ was measured by LC-MS and showed that the concentrations of the different B₁₂-provider as well as B₁₂-retainer strains differed substantially. Although B₁₂ is essential for the vast majority of microorganisms, mechanisms that export this essential cofactor are still unknown. Our results suggest that a large proportion of bacteria that can synthesise B₁₂ de novo cannot share the cofactor with their environment.

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.

Chlamydomonas reinhardtii, a Reference Organism to Study Algal-Microbial Interactions: Why Can't They Be Friends?

The stability and harmony of ecological niches rely on intricate interactions between their members. During evolution, organisms have developed the ability to thrive in different environments, taking advantage of each other. Among these organisms, microalgae are a highly diverse and widely distributed group of major primary producers whose interactions with other organisms play essential roles in their habitats. Understanding the basis of these interactions is crucial to control and exploit these communities for ecological and biotechnological applications. The green microalga Chlamydomonas reinhardtii, a well-established model, is emerging as a model organism for studying a wide variety of microbial interactions with ecological and economic significance. In this review, we unite and discuss current knowledge that points to C. reinhardtii as a model organism for studying microbial interactions.

Inputs of seabird guano alter microbial growth, community composition and the phytoplankton-bacterial interactions in a coastal system

Seabird guano enters coastal waters providing bioavailable substrates for microbial plankton, but their role in marine ecosystem functioning remains poorly understood. Two concentrations of the water soluble fraction (WSF) of gull guano were added to different natural microbial communities collected in surface waters from the Ría de Vigo (NW Spain) in spring, summer, and winter. Samples were incubated with or without antibiotics (to block bacterial activity) to test whether gull guano stimulated phytoplankton and bacterial growth, caused changes in taxonomic composition, and altered phytoplankton-bacteria interactions. Alteromonadales, Sphingobacteriales, Verrucomicrobia and diatoms were generally stimulated by guano. Chlorophyll a (Chl a) concentration and bacterial abundance significantly increased after additions independently of the initial ambient nutrient concentrations. Our study demonstrates, for the first time, that the addition of guano altered the phytoplankton-bacteria interaction index from neutral (i.e. phytoplankton growth was not affected by bacterial activity) to positive (i.e. phytoplankton growth was stimulated by bacterial activity) in the low-nutrient environment occurring in spring. In contrast, when environmental nutrient concentrations were high, the interaction index changed from positive to neutral after guano additions, suggesting the presence of some secondary metabolite in the guano that is needed for phytoplankton growth, which would otherwise be supplied by bacteria.

Phototroph-heterotroph interactions during growth and long-term starvation across Prochlorococcus and Alteromonas diversity

Due to their potential impact on ecosystems and biogeochemistry, microbial interactions, such as those between phytoplankton and bacteria, have been studied intensively using specific model organisms. Yet, to what extent interactions differ between closely related organisms, or how these interactions change over time, or culture conditions, remains unclear. Here, we characterize the interactions between five strains each of two globally abundant marine microorganisms, Prochlorococcus (phototroph) and Alteromonas (heterotroph), from the first encounter between individual strains and over more than a year of repeated cycles of exponential growth and long-term nitrogen starvation. Prochlorococcus-Alteromonas interactions had little effect on traditional growth parameters such as Prochlorococcus growth rate, maximal fluorescence, or lag phase, affecting primarily the dynamics of culture decline, which we interpret as representing cell mortality and lysis. The shape of the Prochlorococcus decline curve and the carrying capacity of the co-cultures were determined by the phototroph and not the heterotroph strains involved. Comparing various mathematical models of culture mortality suggests that Prochlorococcus death rate increases over time in mono-cultures but decreases in co-cultures, with cells potentially becoming more resistant to stress. Our results demonstrate intra-species differences in ecologically relevant co-culture outcomes. These include the recycling efficiency of N and whether the interactions are mutually synergistic or competitive. They also highlight the information-rich growth and death curves as a useful readout of the interaction phenotype.

Deciphering interactions between the marine dinoflagellate Prorocentrum lima and the fungus Aspergillus pseudoglaucus

The comprehension of microbial interactions is one of the key challenges in marine microbial ecology. This study focused on exploring chemical interactions between the toxic dinoflagellate Prorocentrum lima and a filamentous fungal species, Aspergillus pseudoglaucus, which has been isolated from the microalgal culture. Such interspecies interactions are expected to occur even though they were rarely studied. Here, a co-culture system was designed in a dedicated microscale marine-like condition. This system allowed to explore microalgal-fungal physical and metabolic interactions in presence and absence of the bacterial consortium. Microscopic observation showed an unusual physical contact between the fungal mycelium and dinoflagellate cells. To delineate specialized metabolome alterations during microalgal-fungal co-culture metabolomes were monitored by high-performance liquid chromatography coupled to high-resolution mass spectrometry. In-depth multivariate statistical analysis using dedicated approaches highlighted (1) the metabolic alterations associated with microalgal-fungal co-culture, and (2) the impact of associated bacteria in microalgal metabolome response to fungal interaction. Unfortunately, only a very low number of highlighted features were fully characterized. However, an up-regulation of the dinoflagellate toxins okadaic acid and dinophysistoxin 1 was observed during co-culture in supernatants. Such results highlight the importance to consider microalgal-fungal interactions in the study of parameters regulating toxin production.