Biodegradation of phthalic acid esters (PAEs) by Janthinobacterium sp. strain E1 under stress conditions

Phthalates esters (PAEs) are a kind of polymeric material additives widely been added into plastics to improve products' flexibility. It can easily cause environmental pollution which are hazards to public health. In this study, we isolated an efficient PAEs degrading strain, Janthinobacterium sp. E1, and determined its degradation effect of di-2-ethylhexyl phthalate (DEHP) under stress conditions. Strain E1 showed an obvious advantage in pollutants degradation under various environmental stress conditions. Degradation halo clearly occurred around the colony of strain E1 on agar plate supplemented with triglyceride. Strain E1's esterase is a constitutively expressed intracellular enzyme. The esterase purified from strain E1 showed a higher catalytic effect on short-chain PAEs than long-chain PAEs. The input of DEHP, DBP (dibutyl phthalate) and DMP (dimethyl phthalate) into the tested soil did not change the species composition of soil prokaryotic community, but altered the dominant species in specific environmental conditions. And the community diversity and richness decreased to a certain extent. However, the diversity and richness of the microbial community were improved after the contaminated soil was treated with the strain E1. Our results also suggested that strain E1 exhibited a tremendous potential in environmental bioremediation in the real environment, which provides a new insight into the elimination of the pollutants contamination in the urban environment.

Red bioactive pigment from Himalayan Janthinobacterium sp. ERMR3:09: optimization, characterization, and potential applications

Prodigiosin is a red pigment commonly produced as a secondary metabolite by Serratia marcescens. It exhibits inherent bioactivities, including antimicrobial and anticancer, with low to no toxic effects on normal cells. The present study investigates a bioactive prodigiosin production from an atypical, red-pigmented, potentially novel Janthinobacterium sp. ERMR3:09 isolated from a glacial moraine. Statistically optimized culture parameters, i.e., w/v 1.0% glucose and 0.08% peptone as carbon and nitrogen sources, temperature 20 °C, and media pH 7, resulted in a four-fold increase in the pigment yield. The upscaled production in an 8 L volume resulted in higher pigment production within a shorter period of 48 h. The ultra-performance liquid chromatography (UPLC) analysis validated the identity of the purified pigment as prodigiosin that showed thermostability at 75 °C for 3 h. Evaluation of antimicrobial activity showed potent inhibitory effects (> 50%) against the opportunistic pathogenic fungal and Gram-positive bacterial strains. The pigment showed significant cytotoxicity (p < 0.05) towards A549 and HeLa cell lines with IC50 values of 42.2 μM and 36.11 μM, respectively. The study demonstrated that microbial communities from extreme niches can be ideal sources of bioactive pigments with immense pharmaceutical potential vital for the development of non-synthetic therapeutic agents.

Roseateles albus sp. nov., Roseateles koreensis sp. nov. and Janthinobacterium fluminis sp. nov., isolated from freshwater at Jucheon River, and emended description of Roseateles aquaticus comb. nov

Three Gram-stain-negative, facultatively anaerobic, rod-shaped, catalase-positive, oxidase-negative bacterial strains were designated as hw1T, hw8T and hw3T. Strains hw1T, hw8T and hw3T grew at 15-28 °C (optimum, 25 °C), 15-35 °C (optimum, 30 °C) and 4-28 °C (optimum, 20 °C), respectively, and at pH 7.0-12.0 (optimum, pH 9.0), pH 6.0-11.0 (optimum, pH 9.0) and 5.0-12.0 (optimum, pH 7.0), respectively. Additionally, strains hw1T and hw8T only grew when the NaCl concentration was 0 %, while strain hw3T grew at between 0 and 0.5 % (w/v; optimum, 0 %). The average nucleotide identity (ANI) values between strains hw1T, hw8T and the Roseateles type strains ranged from 73.8 to 84.2 %, while the digital DNA-DNA hybridization (dDDH) values ranged from 19.7 to 27.5 %. The ANI values between strain hw3T and the Janthinobacterium type strains ranged from 78.7 to 80.7 %, while dDDH values ranged from 22.3 to 23.0 %. The draft genomes of strains hw1T, hw8T and hw3T consisted of 5.5, 4.4 and 5.9 Mbp, with DNA G+C contents of 61.7, 61.8 and 66.0 mol%, respectively. The results of the dDDH, ANI, phylogenetic, biochemical and physiological analyses indicated that the novel strains were distinct from other members of their genera. Thus, we proposed the names Roseateles albus sp. nov. (type strain hw1T= KACC 22887T= TBRC 16613T), Roseateles koreensis sp. nov. (type strain hw8T= KACC 22885T= TBRC 16614T) and Janthinobacterium fluminis sp. nov. (type strain hw3T= KACC 22886T= TBRC 16615T).

Genome sequencing and multifaceted taxonomic analysis of novel strains of violacein-producing bacteria and non-violacein-producing close relatives

Violacein is a water-insoluble violet pigment produced by various Gram-negative bacteria. The compound and the bacteria that produce it have been gaining attention due to the antimicrobial and proposed antitumour properties of violacein and the possibility that strains producing it may have broad industrial uses. Bacteria that produce violacein have been isolated from diverse environments including fresh and ocean waters, glaciers, tropical soils, trees, fish and the skin of amphibians. We report here the isolation and characterization of six violacein-producing bacterial strains and three non-violacein-producing close relatives, each isolated from either an aquatic environment or moist food materials in northern California, USA. For each isolate, we characterized traditional phenotypes, generated and analysed draft genome sequences, and carried out multiple types of taxonomic, phylogenetic and phylogenomic analyses. Based on these analyses we assign putative identifications to the nine isolates, which include representatives of the genera Chromobacterium, Aquitalea, Iodobacter, Duganella, Massilia and Janthinobacterium. In addition, we discuss the utility of various metrics for taxonomic assignment in these groups including average nucleotide identity, whole genome phylogenetic analysis and extent of recent homologous recombination using the software program PopCOGenT.

Unique or not unique? Comparative genetic analysis of bacterial O-antigens from the Oxalobacteraceae family

Many plants and animals have symbiotic relationships with microorganisms, including bacteria. The interactions between bacteria and their hosts result in different outcomes for the host organism. The outcome can be neutral, harmful or have beneficial effects for participants. Remarkably, these relationships are not static, as they change throughout an organism's lifetime and on an evolutionary scale. One of the structures responsible for relationships in bacteria is O-antigen. Depending on the characteristics of its components, the bacteria can avoid the host's immune response or establish a mutualistic relationship with it. O-antigen is a key component in Gram-negative bacteria's outer membrane. This component facilitates interaction between the bacteria and host immune system or phages. The variability of the physical structure is caused by the genomic variability of genes encoding O-antigen synthesis components. The genes and pathways of O-polysaccharide (OPS) synthesis were intensively investigated mostly for Enterobacteriaceae species. Considering high genetic and molecular diversity of this structure even between strains, these findings may not have caught the entire variety possibly presented in non-model species. The current study presents a comparative analysis of genes associated with O-antigen synthesis in bacteria of the Oxalobacteraceae family. In contrast to existing studies based on PCR methods, we use a bioinformatics approach and compare O- antigens at the level of clusters rather than individual genes. We found that the O-antigen genes of these bacteria are represented by several clusters located at a distance from each other. The greatest similarity of the clusters is observed within individual bacterial genera, which is explained by the high variability of O-antigens. The study describes similarities of OPS genes inherent to the family as a whole and also considers individual unique cases of O-antigen genetic variability inherent to individual bacteria.

Functional characterization of a novel violacein biosynthesis operon from Janthinobacterium sp. B9-8

Violacein is a secondary metabolite mainly produced by Gram-negative bacteria that is formed from tryptophan by five enzymes encoded by a single operon. It is a broad-spectrum antibacterial pigment with various important biological activities such as anti-tumor, antiviral, and antioxidative effects. The newly discovered violacein operon vioABCDE was identified in the genome of the extremophile Janthinobacterium sp. B9-8. The key enzyme-encoding genes were cloned to construct the multigene coexpression plasmids pET-vioAB and pRSF-vioCDE. The violacein biosynthesis pathway was heterologously introduced into engineered Escherichia coli VioABCDE and VioABCDE-SD. The factors affecting violacein production, including temperature, pH, inoculum size, carbon and nitrogen source, precursor, and inducers were investigated. The violacein titer of VioABCDE-SD reached 107 mg/L in a two-stage fermentation process, representing a 454.4% increase over the original strain. The violacein operon from B9-8 provides a new microbial gene source for the analysis of the violacein synthesis mechanism, and the constructed engineering E. coli strains lay a foundation for the efficient and rapid synthesis of other natural products.Key points• The newly discovered violacein operon vioABCDE was identified in the genome of the extremophile Janthinobacterium sp. B9-8.• The violacein synthesis pathway was reconstructed in E. coli using two compatible plasmids.• A two-stage fermentation process was optimized for improved violacein accumulation.

Comparative Genomics Reveals Insights into Induction of Violacein Biosynthesis and Adaptive Evolution in Janthinobacterium

Violacein has different bioactive properties conferring distinct selective advantages, such as defense from predation and interspecific competition. Adaptation of Janthinobacterium to diverse habitats likely leads to variation in violacein production among phylogenetically closely related species inhabiting different environments, yet genomic mechanisms and the influence of adaptive evolution underpinning violacein biosynthesis in Janthinobacterium are not clear. In this study, we performed genome sequencing, comparative genomic analysis, and phenotypic characterization to investigate genomic factors regulating violacein production in nine Janthinobacterium strains, including a type strain from soil and eight strains we isolated from terrestrial subsurface sediment and groundwater. Results show that although all nine Janthinobacterium strains are phylogenetically closely related and contain genes essential for violacein biosynthesis, they vary in carbon usage and violacein production. Sediment and groundwater strains are weak violacein producers and possess far fewer secondary metabolite biosynthesis genes, indicating genome adaptation compared to soil strains. Further examination suggests that quorum sensing (QS) may play an important role in regulating violacein in Janthinobacterium: the strains exhibiting strong potential in violacein production possess both N-acyl-homoserine lactone (AHL) QS and Janthinobacterium QS (JQS) systems in their genomes, while weaker violacein-producing strains harbor only the JQS system. Preliminary tests of spent media of two Janthinobacterium strains possessing both AHL QS and JQS systems support the potential role of AHLs in inducing violacein production in Janthinobacterium. Overall, results from this study reveal potential genomic mechanisms involved in violacein biosynthesis in Janthinobacterium and provide insights into evolution of Janthinobacterium for adaptation to oligotrophic terrestrial subsurface environment.

IMPORTANCE Phylogenetically closely related bacteria can thrive in diverse environmental habitats due to adaptive evolution. Genomic changes resulting from adaptive evolution lead to variations in cellular function, metabolism, and secondary metabolite biosynthesis. The most well-known secondary metabolite produced by Janthinobacterium is the purple-violet pigment violacein. To date, the mechanisms of induction of violacein biosynthesis in Janthinobacterium is not clear. Comparative genome analysis of closely related Janthinobacterium strains isolated from different environmental habitats not only reveals potential mechanisms involved in induction of violacein production by Janthinobacterium but also provides insights into the survival strategy of Janthinobacterium for adaptation to oligotrophic terrestrial subsurface environment.

Glacier ice archives nearly 15,000-year-old microbes and phages

BACKGROUND

Glacier ice archives information, including microbiology, that helps reveal paleoclimate histories and predict future climate change. Though glacier-ice microbes are studied using culture or amplicon approaches, more challenging metagenomic approaches, which provide access to functional, genome-resolved information and viruses, are under-utilized, partly due to low biomass and potential contamination.

RESULTS

We expand existing clean sampling procedures using controlled artificial ice-core experiments and adapted previously established low-biomass metagenomic approaches to study glacier-ice viruses. Controlled sampling experiments drastically reduced mock contaminants including bacteria, viruses, and free DNA to background levels. Amplicon sequencing from eight depths of two Tibetan Plateau ice cores revealed common glacier-ice lineages including Janthinobacterium, Polaromonas, Herminiimonas, Flavobacterium, Sphingomonas, and Methylobacterium as the dominant genera, while microbial communities were significantly different between two ice cores, associating with different climate conditions during deposition. Separately, ~355- and ~14,400-year-old ice were subject to viral enrichment and low-input quantitative sequencing, yielding genomic sequences for 33 vOTUs. These were virtually all unique to this study, representing 28 novel genera and not a single species shared with 225 environmentally diverse viromes. Further, 42.4% of the vOTUs were identifiable temperate, which is significantly higher than that in gut, soil, and marine viromes, and indicates that temperate phages are possibly favored in glacier-ice environments before being frozen. In silico host predictions linked 18 vOTUs to co-occurring abundant bacteria (Methylobacterium, Sphingomonas, and Janthinobacterium), indicating that these phages infected ice-abundant bacterial groups before being archived. Functional genome annotation revealed four virus-encoded auxiliary metabolic genes, particularly two motility genes suggest viruses potentially facilitate nutrient acquisition for their hosts. Finally, given their possible importance to methane cycling in ice, we focused on Methylobacterium viruses by contextualizing our ice-observed viruses against 123 viromes and prophages extracted from 131 Methylobacterium genomes, revealing that the archived viruses might originate from soil or plants.

CONCLUSIONS

Together, these efforts further microbial and viral sampling procedures for glacier ice and provide a first window into viral communities and functions in ancient glacier environments. Such methods and datasets can potentially enable researchers to contextualize new discoveries and begin to incorporate glacier-ice microbes and their viruses relative to past and present climate change in geographically diverse regions globally. Video Abstract.

Janthinobacterium violaceinigrum sp. nov., Janthinobacterium aquaticum sp. nov. and Janthinobacterium rivuli sp. nov., isolated from a subtropical stream in China

Four strains assigned the names FT13W^T, FT14W, FT58W^T and FT68W^T were isolated from a subtropical stream in PR China. All the strains were Gram-stain-negative, catalase- and oxidase-positive, rod-shaped and motile with flagella. Comparisons based on 16S rRNA gene sequences showed that strains FT13W^T, FT14W, FT58W^T and FT68W^T belonged to genus Janthinobacterium and shared 16S rRNA gene similarities in the range of 98.8-99.7 % with Janthinobacterium lividum DSM 1522^T, Janthinobacterium agaricidamnosum DSM 9628^T and 'Janthinobacterium svalbardensis JA-1', respectively. The calculated pairwise average nucleotide identity (ANI) values among the genomes of above seven strains were in the range of 79.0-92.2 %, except that the ANI value was 96.8 % between strain FT13W^T and FT14W. The respiratory quinone of strains FT13W^T, FT14W, FT58W^T and FT68W^T was determined to be Q-8. The major fatty acids were C_16 : 1 ω7c, C_16 : 0, C_18 : 1 ω7c and C_12 : 0. The polar lipids included phosphatidylethanolamine, phosphatidylglycerol and one unidentified phospholipid. The genome sizes of strains FT13W^T, FT14W, FT58W^T and FT68W^T were 6.45, 6.38, 5.73 and 6.37 Mbp with G+C contents of 63.4, 63.7, 61.6 and 63.1 mol%, respectively. Combining phenotypic, biochemical, genotypic and ANI data, strain FT13W^T and FT14W should belong to the same species. The four strains were considered to represent three novel species within genus Janthinobacterium, for which the names Janthinobacterium violaceinigrum sp. nov. (type strain FT13W^T=GDMCC 1.1638^T=KACC 21319^T), Janthinobacterium aquaticum sp. nov. (FT58W^T=GDMCC 1.1676^T=KACC 21468^T) and Janthinobacterium rivuli sp. nov. (FT68W^T=GDMCC 1.1677^T=KACC 21469^T) are proposed.

Janthinobacterium CG23_2: Comparative Genome Analysis Reveals Enhanced Environmental Sensing and Transcriptional Regulation for Adaptation to Life in an Antarctic Supraglacial Stream

As many bacteria detected in Antarctic environments are neither true psychrophiles nor endemic species, their proliferation in spite of environmental extremes gives rise to genome adaptations. Janthinobacterium sp. CG23_2 is a bacterial isolate from the Cotton Glacier stream, Antarctica. To understand how Janthinobacterium sp. CG23_2 has adapted to its environment, we investigated its genomic traits in comparison to genomes of 35 published Janthinobacterium species. While we hypothesized that genome shrinkage and specialization to narrow ecological niches would be energetically favorable for dwelling in an ephemeral Antarctic stream, the genome of Janthinobacterium sp. CG23_2 was on average 1.7 ± 0.6 Mb larger and predicted 1411 ± 499 more coding sequences compared to the other Janthinobacterium spp. Putatively identified horizontal gene transfer events contributed 0.92 Mb to the genome size expansion of Janthinobacterium sp. CG23_2. Genes with high copy numbers in the species-specific accessory genome of Janthinobacterium sp. CG23_2 were associated with environmental sensing, locomotion, response and transcriptional regulation, stress response, and mobile elements-functional categories which also showed molecular adaptation to cold. Our data suggest that genome plasticity and the abundant complementary genes for sensing and responding to the extracellular environment supported the adaptation of Janthinobacterium sp. CG23_2 to this extreme environment.

Microbiota encompassing putative spoilage bacteria in retail packaged broiler meat and commercial broiler abattoir

It is well established, that certain bacteria within the Brochothrix, Carnobacterium, Lactobacillus, Lactococcus, and Leuconostoc genera have an important role in the spoilage of chill stored poultry meat packaged in modified atmosphere. However, little is known about the role of microorganisms that are difficult to culture and the microbiota during poultry spoilage. We combined traditional cultivation and culture-independent 16S rRNA amplicon sequencing to investigate the microbiota encompassing putative bacteria of whole broiler meat, packaged in modified atmosphere, during and exceeding shelf-life. Samples were taken from 6 flocks during independent slaughter days. Additional samples were analysed from the production line. There was a significant difference in the microbial community structure of 80%O₂/20%CO₂ retail packaged broiler meat during different times of shelf-life, mainly due to an increase of species within the Brochothrix, Carnobacterium, Vagococcus, and Janthinobacterium genera. These genera were already detected four to eight days after slaughter. However, no significant difference between flocks with respect to the microbiota encompassing putative spoilage bacteria was observed when examined in retail packaged broilers, slaughtered at the same abattoir on different days. Our study also showed that lactic acid bacteria within the Vagococcus genus can constitute a dominating part of the later shelf-life microbiota in fresh whole broiler meat packaged in 80%O₂/20%CO₂ modified atmosphere. A single operational taxonomic unit (OTU) assigned as Janthinobacterium lividum, an occasional spoiler of meat products, was identified as a major part of the microbiota in late shelf life broiler meat and swab samples in the cooling facility at the slaughter house production line. The combination of traditional cultivation and culture-independent methods provided a great insight into the microbiota of broiler meat during shelf-life and identified a potential point of contamination in the production line for cold tolerant Janthinobacterium.

Molecular Keys to the Janthinobacterium and Duganella spp. Interaction with the Plant Pathogen Fusarium graminearum

Janthinobacterium and Duganella are well-known for their antifungal effects. Surprisingly, almost nothing is known on molecular aspects involved in the close bacterium-fungus interaction. To better understand this interaction, we established the genomes of 11 Janthinobacterium and Duganella isolates in combination with phylogenetic and functional analyses of all publicly available genomes. Thereby, we identified a core and pan genome of 1058 and 23,628 genes. All strains encoded secondary metabolite gene clusters and chitinases, both possibly involved in fungal growth suppression. All but one strain carried a single gene cluster involved in the biosynthesis of alpha-hydroxyketone-like autoinducer molecules, designated JAI-1. Genome-wide RNA-seq studies employing the background of two isolates and the corresponding JAI-1 deficient strains identified a set of 45 QS-regulated genes in both isolates. Most regulated genes are characterized by a conserved sequence motif within the promoter region. Among the most strongly regulated genes were secondary metabolite and type VI secretion system gene clusters. Most intriguing, co-incubation studies of J. sp. HH102 or its corresponding JAI-1 synthase deletion mutant with the plant pathogen Fusarium graminearum provided first evidence of a QS-dependent interaction with this pathogen.

Volatiles in Inter-Specific Bacterial Interactions

The importance of volatile organic compounds for functioning of microbes is receiving increased research attention. However, to date very little is known on how inter-specific bacterial interactions effect volatiles production as most studies have been focused on volatiles produced by monocultures of well-described bacterial genera. In this study we aimed to understand how inter-specific bacterial interactions affect the composition, production and activity of volatiles. Four phylogenetically different bacterial species namely: Chryseobacterium, Dyella, Janthinobacterium, and Tsukamurella were selected. Earlier results had shown that pairwise combinations of these bacteria induced antimicrobial activity in agar media whereas this was not the case for monocultures. In the current study, we examined if these observations were also reflected by the production of antimicrobial volatiles. Thus, the identity and antimicrobial activity of volatiles produced by the bacteria were determined in monoculture as well in pairwise combinations. Antimicrobial activity of the volatiles was assessed against fungal, oomycetal, and bacterial model organisms. Our results revealed that inter-specific bacterial interactions affected volatiles blend composition. Fungi and oomycetes showed high sensitivity to bacterial volatiles whereas the effect of volatiles on bacteria varied between no effects, growth inhibition to growth promotion depending on the volatile blend composition. In total 35 volatile compounds were detected most of which were sulfur-containing compounds. Two commonly produced sulfur-containing volatile compounds (dimethyl disulfide and dimethyl trisulfide) were tested for their effect on three target bacteria. Here, we display the importance of inter-specific interactions on bacterial volatiles production and their antimicrobial activities.

Microbial dynamics and properties of aerobic granules developed in a laboratory-scale sequencing batch reactor with an intermediate filamentous bulking stage

Aerobic granules offer enhanced biological nutrient removal and are compact and dense structures resulting in efficient settling properties. Granule instability, however, is still a challenge as understanding of the drivers of instability is poorly understood. In this study, transient instability of aerobic granules, associated with filamentous outgrowth, was observed in laboratory-scale sequencing batch reactors (SBRs). The transient phase was followed by the formation of stable granules. Loosely bound, dispersed, and pinpoint seed flocs gradually turned into granular flocs within 60 days of SBR operation. In stage 1, the granular flocs were compact in structure and typically 0.2 mm in diameter, with excellent settling properties. Filaments appeared and dominated by stage 2, resulting in poor settleability. By stage 3, the SBRs were selected for larger granules and better settling structures, which included filaments that became enmeshed within the granule, eventually forming structures 2-5 mm in diameter. Corresponding changes in sludge volume index were observed that reflected changes in settleability. The protein-to-polysaccharide ratio in the extracted extracellular polymeric substance (EPS) from stage 1 and stage 3 granules was higher (2.8 and 5.7, respectively), as compared to stage 2 filamentous bulking (1.5). Confocal laser scanning microscopic (CLSM) imaging of the biomass samples, coupled with molecule-specific fluorescent staining, confirmed that protein was predominant in stage 1 and stage 3 granules. During stage 2 bulking, there was a decrease in live cells; dead cells predominated. Denaturing gradient gel electrophoresis (DGGE) fingerprint results indicated a shift in bacterial community composition during granulation, which was confirmed by 16S rRNA gene sequencing. In particular, Janthinobacterium (known denitrifier and producer of antimicrobial pigment) and Auxenochlorella protothecoides (mixotrophic green algae) were predominant during stage 2 bulking. The chitinolytic activity of Chitinophaga is likely antagonistic towards Auxenochlorella and may have contributed to stage 3 stable granule formation. Rhodanobacter, known to support complete denitrification, were predominant in stage 1 and stage 3 granules. The relative abundance of Rhodanobacter coincided with high protein concentrations in EPS, suggesting a role in microbial aggregation and granule formation.

Winter bloom of a rare betaproteobacterium in the Arctic Ocean

Extremely low abundance microorganisms (members of the "rare biosphere") are believed to include dormant taxa, which can sporadically become abundant following environmental triggers. Yet, microbial transitions from rare to abundant have seldom been captured in situ, and it is uncertain how widespread these transitions are. A bloom of a single ribotype (≥99% similarity in the 16S ribosomal RNA gene) of a widespread betaproteobacterium (Janthinobacterium sp.) occurred over 2 weeks in Arctic marine waters. The Janthinobacterium population was not detected microscopically in situ in January and early February, but suddenly appeared in the water column thereafter, eventually accounting for up to 20% of bacterial cells in mid February. During the bloom, this bacterium was detected at open water sites up to 50 km apart, being abundant down to more than 300 m. This event is one of the largest monospecific bacterial blooms reported in polar oceans. It is also remarkable because Betaproteobacteria are typically found only in low abundance in marine environments. In particular, Janthinobacterium were known from non-marine habitats and had previously been detected only in the rare biosphere of seawater samples, including the polar oceans. The Arctic Janthinobacterium formed mucilagenous monolayer aggregates after short (ca. 8 h) incubations, suggesting that biofilm formation may play a role in maintaining rare bacteria in pelagic marine environments. The spontaneous mass occurrence of this opportunistic rare taxon in polar waters during the energy-limited season extends current knowledge of how and when microbial transitions between rare and abundant occur in the ocean.