Functional profiling of cyanobacterial genomes and its role in ecological adaptations

Insights into cyanobacterial blooms through the lens of omics

Cyanobacteria are oxygen-producing photosynthetic bacteria that convert carbon dioxide into biomass upon exposure to sunlight. However, favorable conditions cause harmful cyanobacterial blooms (HCBs), which are the dense accumulation of biomass at the water surface or subsurface, posing threats to freshwater ecosystems and human health. Understanding the mechanisms underlying cyanobacterial bloom formation is crucial for effective management. In this regard, recent advancements in omics technologies have provided valuable insights into HCBs, which have raised expectations to develop more effective control methods in the near future. This literature review aims to present the genomic architecture, adaptive mechanisms, microbial interactions, and ecological impacts of HCBs through the lens of omics. Genomic analysis indicates that the genome plasticity of cyanobacteria has enabled their resilience and effective adaptation to environmental changes. Transcriptomic investigations have revealed that cyanobacteria use various strategies for adapting to environmental stress. Additionally, metagenomic and metatranscriptomic analyses have emphasized the significant role of the microbial community in regulating HCBs. Finally, we offer perspectives on potential opportunities for further research in this field.

Microbial Characterization of Arctic Glacial Ice Cores with a Semiautomated Life Detection System

The search for extant microbial life will be a major focus of future astrobiology missions; however, no direct extant life detection instrumentation is included in current missions to Mars. In this study, we developed the semiautomated MicroLife detection platform that collects and processes environmental samples, detects biosignatures, and characterizes microbial activity. This platform is composed of a drill for sample collection, a redox dye colorimetric system for microbial metabolic activity detection and assessment (μMAMA [microfluidics Microbial Activity MicroAssay]), and a MinION sequencer for biosignature detection and characterization of microbial communities. The MicroLife platform was field-tested on White Glacier on Axel Heiberg Island in the Canadian high Arctic, with two extracted ice cores. The μMAMA successfully detected microbial metabolism from the ice cores within 1 day of incubation. The MinION sequencing of the ice cores and the positive μMAMA card identified a microbial community consistent with cold and oligotrophic environments. Furthermore, isolation and identification of microbial isolates from the μMAMA card corroborated the MinION sequencing. Together, these analyses support the MicroLife platform's efficacy in identifying microbes natively present in cryoenvironments and detecting their metabolic activity. Given our MicroLife platform's size and low energy requirements, it could be incorporated into a future landed platform or rovers for life detection.

Trichotorquatus salinus sp. nov. (Oculatellaceae, Cyanobacteria) from a Saltern of Gomso, Republic of Korea

Six strains of subaerial cyanobacteria were isolated from a Gomso saltern in the Republic of Korea, all of which were studied using morphological and molecular traits. Trichotorquatus salinus sp. nov. was studied using a light microscope (LM), transmission electron microscope (TEM), 16S rRNA, 16S–23S ITS region, and ecological data. T. salinus is a thin and simple filament with a false branch and a firm collar sheath. The phylogenetic analyses of 16S rRNA revealed that Trichotorquatus formed a monophyletic lineage and the strains of T. salinus formed a distinct clade among the species in the genus Trichotorquatus. In the statistical analysis, the inter-species genetic distance of the five species of Trichotorquatus, including T. salinus, is shown to be greater than the distance of the previously reported species of Trichotorquatus. Additionally, 16S–23S ITS gene sequences between T. salinus and four species of Trichotorquatus showed dissimilarities of 55.3–59.4%. In the secondary structure of 16S–23S ITS region (type 2 operon), D1–D1′, Box-B, and V3 helix of T. salinus were different from the other taxa in the genus Trichotorquatus. These results demonstrate that T. salinus sp. nov. has unique morphological, ecological, and molecular traits. Therefore, we propose that T. salinus sp. nov. is a novel species belonging to the genus Trichotorquatus.

Retrograde and anterograde signaling in the crosstalk between chloroplast and nucleus

The chloroplast is a complex cellular organelle that not only performs photosynthesis but also synthesizes amino acids, lipids, and phytohormones. Nuclear and chloroplast genetic activity are closely coordinated through signaling chains from the nucleus to chloroplast, referred to as anterograde signaling, and from chloroplast to the nucleus, named retrograde signaling. The chloroplast can act as an environmental sensor and communicates with other cell compartments during its biogenesis and in response to stress, notably with the nucleus through retrograde signaling to regulate nuclear gene expression in response to developmental cues and stresses that affect photosynthesis and growth. Although several components involved in the generation and transmission of plastid-derived retrograde signals and in the regulation of the responsive nuclear genes have been identified, the plastid retrograde signaling network is still poorly understood. Here, we review the current knowledge on multiple plastid retrograde signaling pathways, and on potential plastid signaling molecules. We also discuss the retrograde signaling-dependent regulation of nuclear gene expression within the frame of a multilayered network of transcription factors.

Insights from cyanobacterial genomic and transcriptomic analyses into adaptation strategies in terrestrial environments

Phylogenomic analysis of Nostoc sp. MG11, a terrestrial cyanobacterium, and some terrestrial and freshwater Nostoc strains showed that the terrestrial strains grouped together in a distinctive clade, which reveals the effect of habitat on shaping Nostoc genomes. Terrestrial strains showed larger genomes and had higher predicted CDS contents than freshwater strains. Comparative genomic analysis demonstrated that genome expansion in the terrestrial Nostoc is supported by an increase in copy number of the core genes and acquisition of shared genes. Transcriptomic profiling analysis under desiccation stress revealed that Nostoc sp. MG11 protected its cell by induction of catalase, proteases, sucrose synthase, trehalose biosynthesis and maltodextrin utilization genes and maintained its normal metabolism during this condition by up-regulation of genes related to phycobilisomes and light reactions of photosynthesis, CO₂ fixation and protein metabolism. These results provide insights into the strategies related to survival and adaptation of Nostoc strains to terrestrial environments.

Microbialites show a distinct cyanobacterial phylogenetic structure and functional redundancy in Bacalar lagoon and Cenote Azul sinkhole, Yucatan peninsula, Mexico

Cyanobacterial components of microbialites from two geographically close systems, the Bacalar lagoon (BL) and the Cenote Azul sinkhole (CA) in Quintana Roo, Mexico, were characterized. BL and CA systems were studied along a longitudinal gradient (north to south) and a depth gradient (5 to 30 m), respectively. Microscopic observations, 16S rRNA amplicon sequencing, and shotgun metagenomics were used to characterize Cyanobacteria. Both systems showed similar metabolic/functional profiles but harbored completely different cyanobacterial taxa. BL was dominated by Nostocales, including a population of previously undescribed Chakia sp., while CA was dominated by an unknown taxon of Chroococcales, comprising 70% of relative abundance through all depths. Interestingly, cyanobacterial assemblages in microbialites exhibited phylogenetic overdispersion in most of the BL sites, while CA sites exhibited phylogenetic clustering, these differences were attributed to depth/light conditions and possibly different times of geological formation for BL and CA systems.

Sigma Factor Modulation for Cyanobacterial Metabolic Engineering

Sigma (σ) factors are key regulatory proteins that control the transcription initiation in prokaryotes. In response to environmental or developmental cues, σ factors initiate the transcription of necessary genes responsible for maintaining a life-sustaining metabolic balance. Due to the significant role of σ factors in bacterial metabolism, their rational engineering for commercial metabolite production in photoautotrophic, cyanobacterial cells is a desirable venture. As cyanobacterial genomes typically encode multiple σ factors, effective execution of metabolic engineering efforts largely relies on uncovering the complicated gene regulatory network and further characterization of the members of σ factor regulatory circuits. This review outlines the prospects of σ factor in metabolic engineering of cyanobacteria, summarizes the challenges in the path towards an efficient strain construction and highlights the genomic context of putative regulators of cyanobacterial σ factors.

Viruses of freshwater bloom-forming cyanobacteria: genomic features, infection strategies and coexistence with the host

Freshwater bloom-forming cyanobacteria densely grow in the aquatic environments, leading to an increase in the viral-contact rate. They possess numerous antiviral genes, as well as cell differentiation- and physiological performance-related genes, owing to genome expansion. Their genomic features and unique lifestyles suggest that they coexist with cyanoviruses in ways different from marine cyanobacteria. Furthermore, genome contents of isolated freshwater bloom-forming cyanobacterial viruses have little in common with those of marine cyanoviruses studied to date. They lack the marine cyanoviral hallmark genes that sustain photosynthetic activity and redirect host metabolism to viral reproduction; therefore, they are predicted to share metabolisms and precursor pools with host cyanobacteria to ensure efficient viral reproduction and avoid nutrient deficiencies and antiviral response. Additionally, cyanovirus-cyanobacteria coexistence strategies may change as bloom density increases. Diverse genotypic populations of cyanoviruses and hosts coexist and fluctuate under high viral-contact rate conditions, leading to their rapid coevolution through antiviral responses. The ancestral and newly evolved genotypes coexist, thereby expanding the diversity levels of host and viral populations. Bottleneck events occurring due to season-related decreases in bloom-forming species abundance provide each genotype within cyanobacterial population an equal chance to increase in prevalence during the next bloom and enhance further diversification.

Comparison of Antimicrobial Resistance and Pan-Genome of Clinical and Non-Clinical Enterococcus cecorum from Poultry Using Whole-Genome Sequencing

Enterococcus cecorum is an emerging avian pathogen, particularly in chickens, but can be found in both diseased (clinical) and healthy (non-clinical) poultry. To better define differences between E. cecorum from the two groups, whole-genome sequencing (WGS) was used to identify and compare antimicrobial resistance genes as well as the pan-genome among the isolates. Eighteen strains selected from our previous study were subjected to WGS using Illumina MiSeq and comparatively analyzed. Assembled contigs were analyzed for resistance genes using ARG-ANNOT. Resistance to erythromycin was mediated by ermB, ermG, and mefA, in clinical isolates and ermB and mefA, in non-clinical isolates. Lincomycin resistance genes were identified as linB, lnuB, lnuC, and lnuD with lnuD found only in non-clinical E. cecorum; however, lnuB and linB were found in only one clinical isolate. For both groups of isolates, kanamycin resistance was mediated by aph3-III, while tetracycline resistance was conferred by tetM, tetO, and tetL. No mutations or known resistance genes were found for isolates resistant to either linezolid or chloramphenicol, suggesting possible new mechanisms of resistance to these drugs. A comparison of WGS results confirmed that non-clinical isolates contained more resistance genes than clinical isolates. The pan-genome of clinical and non-clinical isolates resulted in 3651 and 4950 gene families, respectively, whereas the core gene sets were comprised of 1559 and 1534 gene families in clinical and non-clinical isolates, respectively. Unique genes were found more frequently in non-clinical isolates than clinical. Phylogenetic analysis of the isolates and all the available complete and draft genomes showed no correlation between healthy and diseased poultry. Additional genomic comparison is required to elucidate genetic factors in E. cecorum that contribute to disease in poultry.

Comparative assessment of the therapeutic drug targets of C. botulinum ATCC 3502 and C. difficile str. 630 using in silico subtractive proteomics approach

Growing antimicrobial resistance of the pathogens against multiple drugs posed a serious threat to the human health worldwide. This fueled the need of identifying the novel therapeutic targets that can be used for developing new class of the drugs. Recently, there is a substantial rise in the rate of Clostridium infections as well as in the emergence of virulent and antibiotic resistant strains. Hence, there is an urgent need for the identification of potential therapeutic targets and the development of new drugs for the treatment and prevention of Clostridium infections. In the present study, a combinatorial approach involving systems biology and comparative genomics strategy was tested against Clostridium botulinum ATCC 3502 and Clostridium difficile str. 630 pathogens, to render potential therapeutic target at qualitative and quantitative level. This resulted in the identification of five common (present in both the pathogens, 34 in C. botulinum ATCC 3502 and 42 in C. difficile str. 630) drug targets followed by virtual screening-based identification of potential inhibitors employing molecular docking and molecular dynamics simulations. The identified targets will provide a solid platform for the designing of novel wide-spectrum lead compounds capable of inhibiting their catalytic activities against multidrug-resistant Clostridium in the near future.

Cyanobacterial phylogenetic analysis based on phylogenomics approaches render evolutionary diversification and adaptation: an overview of representative orders

Phylogenetic studies based on a definite set of marker genes usually reconstruct evolutionary relationships among the prokaryotic species. Based on specific target sequences, such studies represent variations and allow identification of similarities or dissimilarities in organisms. With the advent of completely sequenced genomes and accumulation of information on whole prokaryotic genomes, phylogenetic reconstructions should be considered more reliable if they are ideally based on entire genomes to resolve phylogenetic interest. We applied phylogenomics approaches taking into account completely sequenced cyanobacterial genomes to reconstruct underlying species that represented major taxonomic classes and belonged to distinctly different habitats (freshwater, marine, soils, and rocks). We did not rely on describing phylogeny of all representative class of cyanobacterial species on the basis of only ribosomal gene, 16S rDNA gene. In contrast, we analyzed combined molecular marker and phylogenomics approaches (genome alignment, gene content and gene order, composition vector and protein domain content) for accurately inferring phylogenetic relationship of species. We have shown that this approach reflects the impact of evolution on the organisms and considers connects with the ecological adaptation in cyanobacteria in different habitats. Analysis revealed that the members from marine habitat occupy different profile than those from freshwater. Impact of GC content and genomic repetitiveness over the diversification of cyanobacterial species and their possible role in adaptation was also reflected. Members occupying similar habitats cover more evolutionary distance together and also evolve various strategies for adaptation and survival either through genomic repetitiveness or preferences for genes of particular functions or modified GC content. Genomes undergo different changes for their adaptation in diverse habitats.

Insights Into Limnothrix sp. Metabolism Based on Comparative Genomics

Currently only four genome sequences for Limnothrix spp. are publicly available, and information on the genetic properties of cyanobacteria belonging to this genus is limited. In this study, we report the draft genome of Limnothrix sp. CACIAM 69d, isolated from the reservoir of a hydroelectric dam located in the Amazon ecosystem, from where cyanobacterial genomic data are still scarce. Comparative genomic analysis of Limnothrix revealed the presence of key enzymes in the cyanobacterial central carbon metabolism and how it is well equipped for environmental sulfur and nitrogen acquisition. Additionally, this work covered the analysis of Limnothrix CRISPR-Cas systems, pathways related to biosynthesis of secondary metabolites and assembly of extracellular polymeric substances and their exportation. A trans-AT PKS gene cluster was identified in two strains, possibly related to the novel toxin Limnothrixin biosynthesis. Overall, the draft genome of Limnothrix sp. CACIAM 69d adds new data to the small Limnothrix genome library and contributes to a growing representativeness of cyanobacterial genomes from the Amazon region. The comparative genomic analysis of Limnothrix made it possible to highlight unique genes for each strain and understand the overall features of their metabolism.

Phenotypic and genotypic characterisation of an unique indigenous hypersaline unicellular cyanobacterium, Euhalothece sp.nov

A novel halotolerant species of cyanobacterium of the order Chroococcales was isolated from hypersaline estuary in Kwa-Zulu Natal, South Africa. A comprehensive polyphasic approach viz., cell morphology, pigment composition and complete genome sequence analysis was conducted to elucidate the taxonomic position of the isolated strain. The blue-green oval to rod-shaped cells were 14-18 μm in size, and contained a high amount of phycocyanin pigments. The strain was moderate thermotolerant/alkalitolerant halophile with the optimum conditions for growth at 35 °C, pH 8.5 and 120 g/l of NaCl. Based on 16S rRNA gene sequence phylogeny, the strain was related to members of the 'Euhalothece' subcluster (99%). The whole genome sequence was determined, and the annotated genes showed a 90% sequence similarity to the gas-vacuolate, spindle-shaped Dactylococcopsis salina PCC 8305. The size of the genome was determined to be 5,113,178 bp and contained 4332 protein-coding genes and 69 RNA genes with a G + C content of 46.7%. Genes encoding osmoregulation, oxidative stress, heat shock, persister cells, and UV-absorbing secondary metabolites, among others, were identified. Based on the phylogenetic analysis of the 16S rRNA gene sequences, physiological data, pigment compositions and genomic data, the strain is considered to represent a novel species of Euhalothece.

The effect of extrinsic mortality on genome size evolution in prokaryotes

Mortality has a significant role in prokaryotic ecology and evolution, yet the impact of variations in extrinsic mortality on prokaryotic genome evolution has received little attention. We used both mathematical and agent-based models to reveal how variations in extrinsic mortality affect prokaryotic genome evolution. Our results suggest that the genome size of bacteria increases with increased mortality. A high extrinsic mortality increases the pool of free resources and shortens life expectancy, which selects for faster reproduction, a phenotype we called 'scramblers'. This phenotype is realised by the expansion of gene families involved in nutrient acquisition and metabolism. In contrast, a low mortality rate increases an individual's life expectancy, which results in natural selection favouring tolerance to starvation when conditions are unfavourable. This leads to the evolution of small, streamlined genomes ('stayers'). Our models predict that large genomes, gene family expansion and horizontal gene transfer should be observed in prokaryotes occupying ecosystems exposed to high abiotic stress, as well as those under strong predator- and/or pathogen-mediated selection. A comparison of genome size of cyanobacteria in relatively stable marine versus more turbulent freshwater environments corroborates our predictions, although other factors between these environments could also be responsible.