A phylogenetically novel cyanobacterium most closely related to Gloeobacter

Exploring the Origins and Evolution of Oxygenic and Anoxygenic Photosynthesis in Deeply Branched Cyanobacteriota

Cyanobacteriota, the sole prokaryotes capable of oxygenic photosynthesis (OxyP), occupy a unique and pivotal role in Earth's history. While the notion that OxyP may have originated from Cyanobacteriota is widely accepted, its early evolution remains elusive. Here, by using both metagenomics and metatranscriptomics, we explore 36 metagenome-assembled genomes from hot spring ecosystems, belonging to two deep-branching cyanobacterial orders: Thermostichales and Gloeomargaritales. Functional investigation reveals that Thermostichales encode the crucial thylakoid membrane biogenesis protein, vesicle-inducing protein in plastids 1 (Vipp1). Based on the phylogenetic results, we infer that the evolution of the thylakoid membrane predates the divergence of Thermostichales from other cyanobacterial groups and that Thermostichales may be the most ancient lineage known to date to have inherited this feature from their common ancestor. Apart from OxyP, both lineages are potentially capable of sulfide-driven AnoxyP by linking sulfide oxidation to the photosynthetic electron transport chain. Unexpectedly, this AnoxyP capacity appears to be an acquired feature, as the key gene sqr was horizontally transferred from later-evolved cyanobacterial lineages. The presence of two D1 protein variants in Thermostichales suggests the functional flexibility of photosystems, ensuring their survival in fluctuating redox environments. Furthermore, all MAGs feature streamlined phycobilisomes with a preference for capturing longer-wavelength light, implying a unique evolutionary trajectory. Collectively, these results reveal the photosynthetic flexibility in these early-diverging cyanobacterial lineages, shedding new light on the early evolution of Cyanobacteriota and their photosynthetic processes.

Metagenome-assembled bacterial genomes from benthic microbial mats in ice-covered Lake Vanda, Antarctica

We recovered 57 bacterial metagenome-assembled genomes (MAGs) from benthic microbial mat pinnacles from Lake Vanda, Antarctica. These MAGs provide access to genomes from polar environments and can assist in culturing and utilizing these Antarctic bacteria.

Horizontal transfer potential of antibiotic resistance genes in wastewater treatment plants unraveled by microfluidic-based mini-metagenomics

Wastewater treatment plants (WWTPs) are known to harbor antibiotic resistance genes (ARGs), which can potentially spread to the environment and human populations. However, the extent and mechanisms of ARG transfer in WWTPs are not well understood due to the high microbial diversity and limitations of molecular techniques. In this study, we used a microfluidic-based mini-metagenomics approach to investigate the transfer potential and mechanisms of ARGs in activated sludge from WWTPs. Our results show that while diverse ARGs are present in activated sludge, only a few highly similar ARGs are observed across different taxa, indicating limited transfer potential. We identified two ARGs, ermF and tla-1, which occur in a variety of bacterial taxa and may have high transfer potential facilitated by mobile genetic elements. Interestingly, genes that are highly similar to the sequences of these two ARGs, as identified in this study, display varying patterns of abundance across geographic regions. Genes similar to ermF found are widely found in Asia and the Americas, while genes resembling tla-1 are primarily detected in Asia. Genes similar to both genes are barely detected in European WWTPs. These findings shed light on the limited horizontal transfer potential of ARGs in WWTPs and highlight the importance of monitoring specific ARGs in different regions to mitigate the spread of antibiotic resistance.

Cyanobacteriochromes from Gloeobacterales Provide New Insight into the Diversification of Cyanobacterial Photoreceptors

The phytochrome superfamily comprises three groups of photoreceptors sharing a conserved GAF (cGMP-specific phosphodiesterases, cyanobacterial adenylate cyclases, and formate hydrogen lyase transcription activator FhlA) domain that uses a covalently attached linear tetrapyrrole (bilin) chromophore to sense light. Knotted red/far-red phytochromes are widespread in both bacteria and eukaryotes, but cyanobacteria also contain knotless red/far-red phytochromes and cyanobacteriochromes (CBCRs). Unlike typical phytochromes, CBCRs require only the GAF domain for bilin binding, chromophore ligation, and full, reversible photoconversion. CBCRs can sense a wide range of wavelengths (ca. 330-750 nm) and can regulate phototaxis, second messenger metabolism, and optimization of the cyanobacterial light-harvesting apparatus. However, the origins of CBCRs are not well understood: we do not know when or why CBCRs evolved, or what selective advantages led to retention of early CBCRs in cyanobacterial genomes. In the current work, we use the increasing availability of genomes and metagenome-assembled-genomes from early-branching cyanobacteria to explore the origins of CBCRs. We reaffirm the earliest branches in CBCR evolution. We also show that early-branching cyanobacteria contain late-branching CBCRs, implicating early appearance of CBCRs during cyanobacterial evolution. Moreover, we show that early-branching CBCRs behave as integrators of light and pH, providing a potential unique function for early CBCRs that led to their retention and subsequent diversification. Our results thus provide new insight into the origins of these diverse cyanobacterial photoreceptors.

Biogeographic distribution of five Antarctic cyanobacteria using large-scale k-mer searching with sourmash branchwater

Cyanobacteria form diverse communities and are important primary producers in Antarctic freshwater environments, but their geographic distribution patterns in Antarctica and globally are still unresolved. There are however few genomes of cultured cyanobacteria from Antarctica available and therefore metagenome-assembled genomes (MAGs) from Antarctic cyanobacteria microbial mats provide an opportunity to explore distribution of uncultured taxa. These MAGs also allow comparison with metagenomes of cyanobacteria enriched communities from a range of habitats, geographic locations, and climates. However, most MAGs do not contain 16S rRNA gene sequences, making a 16S rRNA gene-based biogeography comparison difficult. An alternative technique is to use large-scale k-mer searching to find genomes of interest in public metagenomes. This paper presents the results of k-mer based searches for 5 Antarctic cyanobacteria MAGs from Lake Fryxell and Lake Vanda, assigned the names Phormidium pseudopriestleyi FRX01, Microcoleus sp. MP8IB2.171, Leptolyngbya sp. BulkMat.35, Pseudanabaenaceae cyanobacterium MP8IB2.15, and Leptolyngbyaceae cyanobacterium MP9P1.79 in 498,942 unassembled metagenomes from the National Center for Biotechnology Information (NCBI) Sequence Read Archive (SRA). The Microcoleus sp. MP8IB2.171 MAG was found in a wide variety of environments, the P. pseudopriestleyi MAG was found in environments with challenging conditions, the Leptolyngbyaceae cyanobacterium MP9P1.79 MAG was only found in Antarctica, and the Leptolyngbya sp. BulkMat.35 and Pseudanabaenaceae cyanobacterium MP8IB2.15 MAGs were found in Antarctic and other cold environments. The findings based on metagenome matches and global comparisons suggest that these Antarctic cyanobacteria have distinct distribution patterns ranging from locally restricted to global distribution across the cold biosphere and other climatic zones.

Genomic profiles of four novel cyanobacteria MAGs from Lake Vanda, Antarctica: insights into photosynthesis, cold tolerance, and the circadian clock

Cyanobacteria in polar environments face environmental challenges, including cold temperatures and extreme light seasonality with small diurnal variation, which has implications for polar circadian clocks. However, polar cyanobacteria remain underrepresented in available genomic data, and there are limited opportunities to study their genetic adaptations to these challenges. This paper presents four new Antarctic cyanobacteria metagenome-assembled genomes (MAGs) from microbial mats in Lake Vanda in the McMurdo Dry Valleys in Antarctica. The four MAGs were classified as Leptolyngbya sp. BulkMat.35, Pseudanabaenaceae cyanobacterium MP8IB2.15, Microcoleus sp. MP8IB2.171, and Leptolyngbyaceae cyanobacterium MP9P1.79. The MAGs contain 2.76 Mbp - 6.07 Mbp, and the bin completion ranges from 74.2-92.57%. Furthermore, the four cyanobacteria MAGs have average nucleotide identities (ANIs) under 90% with each other and under 77% with six existing polar cyanobacteria MAGs and genomes. This suggests that they are novel cyanobacteria and demonstrates that polar cyanobacteria genomes are underrepresented in reference databases and there is continued need for genome sequencing of polar cyanobacteria. Analyses of the four novel and six existing polar cyanobacteria MAGs and genomes demonstrate they have genes coding for various cold tolerance mechanisms and most standard circadian rhythm genes with the Leptolyngbya sp. BulkMat.35 and Leptolyngbyaceae cyanobacterium MP9P1.79 contained kaiB3, a divergent homolog of kaiB.

Hapalosiphonacean cyanobacteria (Nostocales) thrived amid emerging embryophytes in an early Devonian (407-million-year-old) landscape

Cyanobacteria have a long evolutionary history, well documented in marine rocks. They are also abundant and diverse in terrestrial environments; however, although phylogenies suggest that the group colonized land early in its history, paleontological documentation of this remains limited. The Rhynie chert (407 Ma), our best preserved record of early terrestrial ecosystems, provides an opportunity to illuminate aspects of cyanobacterial diversity and ecology as plants began to radiate across the land surface. We used light microscopy and super-resolution confocal laser scanning microscopy to study a new population of Rhynie cyanobacteria; we also reinvestigated previously described specimens that resemble the new fossils. Our study demonstrates that all are part of a single fossil species belonging to the Hapalosiphonaceae (Nostocales). Along with other Rhynie microfossils, these remains show that the accommodation of morphologically complex cyanobacteria to terrestrial ecosystems transformed by embryophytes was well underway more than 400 million years ago.

Novel diversity of polar Cyanobacteria revealed by genome-resolved metagenomics

Benthic microbial mats dominated by Cyanobacteria are important features of polar lakes. Although culture-independent studies have provided important insights into the diversity of polar Cyanobacteria, only a handful of genomes have been sequenced to date. Here, we applied a genome-resolved metagenomics approach to data obtained from Arctic, sub-Antarctic and Antarctic microbial mats. We recovered 37 metagenome-assembled genomes (MAGs) of Cyanobacteria representing 17 distinct species, most of which are only distantly related to genomes that have been sequenced so far. These include (i) lineages that are common in polar microbial mats such as the filamentous taxa Pseudanabaena, Leptolyngbya, Microcoleus/Tychonema and Phormidium; (ii) the less common taxa Crinalium and Chamaesiphon; (iii) an enigmatic Chroococcales lineage only distantly related to Microcystis; and (iv) an early branching lineage in the order Gloeobacterales that is distributed across the cold biosphere, for which we propose the name Candidatus Sivonenia alaskensis. Our results show that genome-resolved metagenomics is a powerful tool for expanding our understanding of the diversity of Cyanobacteria, especially in understudied remote and extreme environments.

GUN4 appeared early in cyanobacterial evolution

Photosynthesis relies on chlorophylls, which are synthesized via a common tetrapyrrole trunk pathway also leading to heme, vitamin B12, and other pigmented cofactors. The first committed step for chlorophyll biosynthesis is insertion of magnesium into protoporphyrin IX by magnesium chelatase. Magnesium chelatase is composed of H-, I-, and D-subunits, with the tetrapyrrole substrate binding to the H-subunit. This subunit is rapidly inactivated in the presence of substrate, light, and oxygen, so oxygenic photosynthetic organisms require mechanisms to protect magnesium chelatase from similar loss of function. An additional protein, GUN4, binds to the H-subunit and to tetrapyrroles. GUN4 has been proposed to serve this protective role via its ability to bind linear tetrapyrroles (bilins). In the current work, we probe the origins of bilin binding by GUN4 via comparative phylogenetic analysis and biochemical validation of a conserved bilin-binding motif. Based on our results, we propose that bilin-binding GUN4 proteins arose early in cyanobacterial evolution and that this early acquisition represents an ancient adaptation for maintaining chlorophyll biosynthesis in the presence of light and oxygen.

Environmental predictors of electroactive bacterioplankton in small boreal lakes

Extracellular electron transfer (EET) by electroactive bacteria in anoxic soils and sediments is an intensively researched subject, but EET's function in planktonic ecology has been less considered. Following the discovery of an unexpectedly high prevalence of EET genes in a bog lake's bacterioplankton, we hypothesized that the redox capacities of dissolved organic matter (DOM) enrich for electroactive bacteria by mediating redox chemistry. We developed the bioinformatics pipeline FEET (Find EET) to identify and summarize predicted EET protein-encoding genes from metagenomics data. We then applied FEET to 36 bog and thermokarst lakes and correlated gene occurrence with environmental data to test our predictions. Our results provide indirect evidence that DOM may participate in bacterioplankton EET. We found a similarly high prevalence of genes encoding putative EET proteins in most of these lakes, where oxidative EET strongly correlated with DOM. Numerous novel clusters of multiheme cytochromes that may enable EET were identified. Taxa previously not considered EET-capable were found to carry EET genes. We propose that EET and DOM interactions are of ecologically important to bacterioplankton in small boreal lakes, and that EET, particularly by methylotrophs and anoxygenic phototrophs, should be further studied and incorporated into methane emission models of melting permafrost.

Evidence for an early green/red photocycle that precedes the diversification of GAF domain photoreceptor cyanobacteriochromes

Phytochromes are linear tetrapyrrole-binding photoreceptors in eukaryotes and bacteria, primarily responding to red and far-red light signals reversibly. Among the GAF domain-based phytochrome superfamily, cyanobacteria-specific cyanobacteriochromes show various optical properties covering the entire visible region. It is unknown what physiological demands drove the evolution of cyanobacteriochromes in cyanobacteria. Here, we utilize ancestral sequence reconstruction and biochemical verification to show that the resurrected ancestral cyanobacteriochrome proteins reversibly respond to green and red light signals. pH titration analyses indicate that the deprotonation of the bound phycocyanobilin chromophore is crucial to perceive green light. The ancestral cyanobacteriochromes show only modest thermal reversion to the green light-absorbing form, suggesting that they evolved to sense the incident green/red light ratio. Many cyanobacteria can utilize green light for photosynthesis using phycobilisome light-harvesting complexes. The green/red sensing cyanobacteriochromes may have allowed better acclimation to changing light environments by rearranging the absorption capacity of the phycobilisome through chromatic acclimation.

An updated classification of cyanobacterial orders and families based on phylogenomic and polyphasic analysis

Cyanobacterial taxonomy is facing a period of rapid changes thanks to the ease of 16S rRNA gene sequencing and established workflows for description of new taxa. Since the last comprehensive review of the cyanobacterial system in 2014 until 2021, at least 273 species in 140 genera were newly described. These taxa were mainly placed into previously defined orders and families although several new families were proposed. However, the classification of most taxa still relied on hierarchical relationships inherited from the classical morphological taxonomy. Similarly, the obviously polyphyletic orders such as Synechococcales and Oscillatoriales were left unchanged. In this study, the rising number of genomic sequences of cyanobacteria and well-described reference strains allowed us to reconstruct a robust phylogenomic tree for taxonomic purposes. A less robust but better sampled 16S rRNA gene phylogeny was mapped to the phylogenomic backbone. Based on both these phylogenies, a polyphasic classification throughout the whole phylum of Cyanobacteria was created, with ten new orders and fifteen new families. The proposed system of cyanobacterial orders and families relied on a phylogenomic tree but still employed phenotypic apomorphies where possible to make it useful for professionals in the field. It was, however, confirmed that morphological convergence of phylogenetically distant taxa was a frequent phenomenon in cyanobacteria. Moreover, the limited phylogenetic informativeness of the 16S rRNA gene, resulting in ambiguous phylogenies above the genus level, emphasized the integration of genomic data as a prerequisite for the conclusive taxonomic placement of a vast number of cyanobacterial genera in the future.

Cyanobacterial Bioenergetics in Relation to Cellular Growth and Productivity

Cyanobacteria, the evolutionary originators of oxygenic photosynthesis, have the capability to convert CO2, water, and minerals into biomass using solar energy. This process is driven by intricate bioenergetic mechanisms that consist of interconnected photosynthetic and respiratory electron transport chains coupled. Over the last few decades, advances in physiochemical analysis, molecular genetics, and structural analysis have enabled us to gain a more comprehensive understanding of cyanobacterial bioenergetics. This includes the molecular understanding of the primary energy conversion mechanisms as well as photoprotective and other dissipative mechanisms that prevent photodamage when the rates of photosynthetic output, primarily in the form of ATP and NADPH, exceed the rates that cellular assimilatory processes consume these photosynthetic outputs. Despite this progress, there is still much to learn about the systems integration and the regulatory circuits that control expression levels for optimal cellular abundance and activity of the photosynthetic complexes and the cellular components that convert their products into biomass. With an improved understanding of these regulatory principles and mechanisms, it should be possible to optimally modify cyanobacteria for enhanced biotechnological purposes.

Complete Genome Sequencing of a Novel Gloeobacter Species from a Waterfall Cave in Mexico

Only two complete genomes of the cyanobacterial genus Gloeobacter from two very different regions of the world currently exist. Here, we present the complete genome sequence of a third member of the genus isolated from a waterfall cave in Mexico. Analysis of the average nucleotide identities (ANIs) between published Gloeobacter genomes revealed that the complete genome of this new member is only 92.7% similar to Gloeobacter violaceus and therefore we determined it to be a new species. We propose to name this new species Gloeobacter morelensis after the location in Mexico where it was isolated. The complete genome consists of one circular chromosome (4,921,229 bp), one linear plasmid (172,328 bp), and one circular plasmid (8,839 bp). Its genome is the largest of all completely sequenced genomes of Gloeobacter species. Pangenomic comparisons revealed that G. morelensis encodes 759 genes not shared with other Gloeobacter species. Despite being more closely related to G. violaceus, it features an extremely divergent psbA gene encoding an atypical D1 core subunit of Photosystem II previously only found within the genome of Gloeobacter kilaueensis. In addition, we detected evidence of concerted evolution of psbA genes encoding identical D1 in all three Gloeobacter genomes, a characteristic that seems widespread in cyanobacteria and may therefore be traced back to their last common ancestor.

Characterizing the Uncultivated Microbial Minority: towards Understanding the Roles of the Rare Biosphere in Microbial Communities

Microbial communities are frequently numerically dominated by just a few species. Often, the long “tail” of the rank-abundance plots of microbial communities constitutes the so-called “rare biosphere,” microorganisms that are highly diverse but are typically found in low abundance in these communities. Their presence in microbial communities has only recently become apparent with advances in high-throughput sequencing technologies. Despite their low numbers, they are thought to play important roles in their communities and may function as potential members to keep the communities intact and resilient. Their phylogenetic diversity also means that they are important subjects for better understanding the interplay between microbial diversity and evolution. I propose that more efforts should be put into characterizing these poorly understood and mostly unknown microbial lineages that hold vast potentials for our understanding of microbial diversity, ecology, and evolution of life on this planet.

Novel Gloeobacterales spp. from Diverse Environments across the Globe

Photosynthetic Cyanobacteria and their descendants are the only known organisms capable of oxygenic photosynthesis. Their metabolism permanently changed the Earth’s surface and the evolutionary trajectory of life, but little is known about their evolutionary history. Genomes of the Gloeobacterales, an order of deeply divergent photosynthetic Cyanobacteria, may hold clues about the evolutionary process. However, there are only three published genomes within this order, and it is difficult to make broad inferences based on such little data. Here, I describe five species within the Gloeobacterales retrieved from publicly available databases and examine their photosynthetic gene content and the environments in which Gloeobacterales genomes and 16S rRNA gene sequences are found. The Gloeobacterales contain reduced photosystems and inhabit cold, wet-rock, and low-light environments. They are likely present in low abundances due to their low growth rate. Future searches for Gloeobacterales should target these environments, and samples should be deeply sequenced to capture the low-abundance taxa. Publicly available databases contain undescribed taxa within the Gloeobacterales. However, searching through all available data with current methods is computationally expensive. Therefore, new methods must be developed to search for these and other evolutionarily important taxa. Once identified, these novel photosynthetic Cyanobacteria will help illuminate the origin and evolution of oxygenic photosynthesis.

IMPORTANCE Early branching photosynthetic Cyanobacteria such as the Gloeobacterales may provide clues into the evolutionary history of oxygenic photosynthesis, but there are few genomes or cultured taxa from this order. Five new metagenome-assembled genomes suggest that members of the Gloeobacterales all contain reduced photosystems and lack genes associated with thylakoids and circadian rhythms. Their distribution suggests that they may thrive in environments that are marginal for other species, including wet-rock and cold environments. These traits may aid in the discovery and cultivation of novel species in this clade.

Emendation of the Coccoid Cyanobacterial Genus Gloeocapsopsis and Description of the New Species Gloeocapsopsis diffluens sp. nov. and Gloeocapsopsis dulcis sp. nov. Isolated From the Coastal Range of the Atacama Desert (Chile)

The taxonomy of coccoid cyanobacteria, such as Chroococcidiopsis, Pleurocapsa, Chroococcus, Gloeothece, Gloeocapsa, Gloeocapsopsis, and the related recent genera Sinocapsa and Aliterella, can easily be intermixed when solely compared on a morphological basis. There is still little support on the taxonomic position of some of the addressed genera, as genetic information is available only for a fraction of species that have been described solely on morphology. Modern polyphasic approaches that combine classic morphological investigations with DNA-based molecular analyses and the evaluation of ecological properties can disentangle these easily confusable unicellular genera. By using such an approach, we present here the formal description of two novel unicellular cyanobacterial species that inhabit the Coastal Range of the Atacama Desert, Gloeocapsopsis dulcis (first reported as Gloeocapsopsis AAB1) and Gloeocapsopsis diffluens. Both species could be clearly separated from previously reported species by 16S rRNA and 16S–23S ITS gene sequencing, the resulting secondary structures, p-distance analyses of the 16S–23S ITS, and morphology. For avoiding further confusions emendation of the genus Gloeocapsopsis as well as epitypification of the type species Gloeocapsopsis crepidinum based on the strain LEGE06123 were conducted.

Cyanobacteria and biogeochemical cycles through Earth history

Cyanobacteria are the only prokaryotes to have evolved oxygenic photosynthesis, transforming the biology and chemistry of our planet. Genomic and evolutionary studies have revolutionized our understanding of early oxygenic phototrophs, complementing and dramatically extending inferences from the geologic record. Molecular clock estimates point to a Paleoarchean origin (3.6-3.2 billion years ago, bya) of the core proteins of Photosystem II (PSII) involved in oxygenic photosynthesis and a Mesoarchean origin (3.2-2.8 bya) for the last common ancestor of modern cyanobacteria. Nonetheless, most extant cyanobacteria diversified after the Great Oxidation Event (GOE), an environmental watershed ca. 2.45 bya made possible by oxygenic photosynthesis. Throughout their evolutionary history, cyanobacteria have played a key role in the global carbon cycle.

Microbial biodiversity: A newly isolated cyanobacterium sheds light on the evolution of photosynthesis

A newly isolated cyanobacterium found growing in close association with a tropical, non-vascular plant has been cultured and its genome sequenced. Its lineage is well over a billion years old and gives insights into the evolutionary origin of oxygenic photosynthesis.

A novel thylakoid-less isolate fills a billion-year gap in the evolution of Cyanobacteria

Cyanobacteria have played pivotal roles in Earth's geological history, especially during the rise of atmospheric oxygen. However, our ability to infer the early transitions in Cyanobacteria evolution has been limited by their extremely lopsided tree of life-the vast majority of extant diversity belongs to Phycobacteria (or "crown Cyanobacteria"), while its sister lineage, Gloeobacteria, is depauperate and contains only two closely related species of Gloeobacter and a metagenome-assembled genome. Here, we describe a new cultured member of Gloeobacteria, Anthocerotibacter panamensis, isolated from a tropical hornwort. Anthocerotibacter diverged from Gloeobacter over 1.4 Ga ago and has low 16S rDNA identities with environmental samples. Our ultrastructural, physiological, and genomic analyses revealed that this species possesses a unique combination of traits that are exclusively shared with either Gloeobacteria or Phycobacteria. For example, similar to Gloeobacter, it lacks thylakoids and circadian clock genes, but the carotenoid biosynthesis pathway is typical of Phycobacteria. Furthermore, Anthocerotibacter has one of the most reduced gene sets for photosystems and phycobilisomes among Cyanobacteria. Despite this, Anthocerotibacter is capable of oxygenic photosynthesis under a wide range of light intensities, albeit with much less efficiency. Given its key phylogenetic position, distinct trait combination, and availability as a culture, Anthocerotibacter opens a new window to further illuminate the dawn of oxygenic photosynthesis.

Genetic, Genomics, and Responses to Stresses in Cyanobacteria: Biotechnological Implications

Cyanobacteria are widely-diverse, environmentally crucial photosynthetic prokaryotes of great interests for basic and applied science. Work to date has focused mostly on the three non-nitrogen fixing unicellular species Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002, which have been selected for their genetic and physiological interests summarized in this review. Extensive "omics" data sets have been generated, and genome-scale models (GSM) have been developed for the rational engineering of these cyanobacteria for biotechnological purposes. We presently discuss what should be done to improve our understanding of the genotype-phenotype relationships of these models and generate robust and predictive models of their metabolism. Furthermore, we also emphasize that because Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002 represent only a limited part of the wide biodiversity of cyanobacteria, other species distantly related to these three models, should be studied. Finally, we highlight the need to strengthen the communication between academic researchers, who know well cyanobacteria and can engineer them for biotechnological purposes, but have a limited access to large photobioreactors, and industrial partners who attempt to use natural or engineered cyanobacteria to produce interesting chemicals at reasonable costs, but may lack knowledge on cyanobacterial physiology and metabolism.

Metabolic Capacity of the Antarctic Cyanobacterium Phormidium pseudopriestleyi That Sustains Oxygenic Photosynthesis in the Presence of Hydrogen Sulfide

Sulfide inhibits oxygenic photosynthesis by blocking electron transfer between H₂O and the oxygen-evolving complex in the D1 protein of Photosystem II. The ability of cyanobacteria to counter this effect has implications for understanding the productivity of benthic microbial mats in sulfidic environments throughout Earth history. In Lake Fryxell, Antarctica, the benthic, filamentous cyanobacterium Phormidium pseudopriestleyi creates a 1–2 mm thick layer of 50 µmol L⁻¹ O₂ in otherwise sulfidic water, demonstrating that it sustains oxygenic photosynthesis in the presence of sulfide. A metagenome-assembled genome of P. pseudopriestleyi indicates a genetic capacity for oxygenic photosynthesis, including multiple copies of psbA (encoding the D1 protein of Photosystem II), and anoxygenic photosynthesis with a copy of sqr (encoding the sulfide quinone reductase protein that oxidizes sulfide). The genomic content of P. pseudopriestleyi is consistent with sulfide tolerance mechanisms including increasing psbA expression or directly oxidizing sulfide with sulfide quinone reductase. However, the ability of the organism to reduce Photosystem I via sulfide quinone reductase while Photosystem II is sulfide-inhibited, thereby performing anoxygenic photosynthesis in the presence of sulfide, has yet to be demonstrated.