The siderophilic cyanobacterium Leptolyngbya sp. strain JSC-1 acclimates to iron starvation by expressing multiple isiA-family genes

Cyanobacteria and Algal-Based Biological Life Support System (BLSS) and Planetary Surface Atmospheric Revitalizing Bioreactor Brief Concept Review

Exploring austere environments required a reimagining of resource acquisition and utilization. Cyanobacterial in situ resources utilization (ISRU) and biological life support system (BLSS) bioreactors have been proposed to allow crewed space missions to extend beyond the temporal boundaries that current vehicle mass capacities allow. Many cyanobacteria and other microscopic organisms evolved during a period of Earth’s history that was marked by very harsh conditions, requiring robust biochemical systems to ensure survival. Some species work wonderfully in a bioweathering capacity (siderophilic), and others are widely used for their nutritional power (non-siderophilic). Playing to each of their strengths and having them grow and feed off of each other is the basis for the proposed idea for a series of three bioreactors, starting from regolith processing and proceeding to nutritional products, gaseous liberation, and biofuel production. In this paper, we discuss what that three reactor system will look like, with the main emphasis on the nutritional stage.

Structure of a monomeric photosystem I core associated with iron-stress-induced-A proteins from Anabaena sp. PCC 7120

Iron-stress-induced-A proteins (IsiAs) are expressed in cyanobacteria under iron-deficient conditions. The cyanobacterium Anabaena sp. PCC 7120 has four isiA genes; however, their binding property and functional roles in PSI are still missing. We analyzed a cryo-electron microscopy structure of a PSI-IsiA supercomplex isolated from Anabaena grown under an iron-deficient condition. The PSI-IsiA structure contains six IsiA subunits associated with the PsaA side of a PSI core monomer. Three of the six IsiA subunits were identified as IsiA1 and IsiA2. The PSI-IsiA structure lacks a PsaL subunit; instead, a C-terminal domain of IsiA2 occupies the position of PsaL, which inhibits the oligomerization of PSI, leading to the formation of a PSI monomer. Furthermore, excitation-energy transfer from IsiAs to PSI appeared with a time constant of 55 ps. These findings provide insights into both the molecular assembly of the Anabaena IsiA family and the functional roles of IsiAs.

Acclimation of the photosynthetic apparatus to low light in a thermophilic Synechococcus sp. strain

Depending upon their growth responses to high and low irradiance, respectively, thermophilic Synechococcus sp. isolates from microbial mats associated with the effluent channels of Mushroom Spring, an alkaline siliceous hot spring in Yellowstone National Park, can be described as either high-light (HL) or low-light (LL) ecotypes. Strains isolated from the bottom of the photic zone grow more rapidly at low irradiance compared to strains isolated from the uppermost layer of the mat, which conversely grow better at high irradiance. The LL-ecotypes develop far-red absorbance and fluorescence emission features after growth in LL. These isolates have a unique gene cluster that encodes a putative cyanobacteriochrome denoted LcyA, a putative sensor histidine kinase; an allophycocyanin (FRL-AP; ApcD4-ApcB3) that absorbs far-red light; and a putative chlorophyll a-binding protein, denoted IsiX, which is homologous to IsiA. The emergence of FRL absorbance in LL-adapted cells of Synechococcus sp. strain A1463 was analyzed in cultures responding to differences in light intensity. The far-red absorbance phenotype arises from expression of a novel antenna complex containing the FRL-AP, ApcD4-ApcB3, which is produced when cells were grown at very low irradiance. Additionally, the two GAF domains of LcyA were shown to bind phycocyanobilin and a [4Fe-4S] cluster, respectively. These ligands potentially enable this photoreceptor to respond to a variety of environmental factors including irradiance, redox potential, and/or oxygen concentration. The products of the gene clusters specific to LL-ecotypes likely facilitate growth in low-light environments through a process called Low-Light Photoacclimation.

Changes in supramolecular organization of cyanobacterial thylakoid membrane complexes in response to far-red light photoacclimation

Cyanobacteria are ubiquitous in nature and have developed numerous strategies that allow them to live in a diverse range of environments. Certain cyanobacteria synthesize chlorophylls d and f to acclimate to niches enriched in far-red light (FRL) and incorporate paralogous photosynthetic proteins into their photosynthetic apparatus in a process called FRL-induced photoacclimation (FaRLiP). We characterized the macromolecular changes involved in FRL-driven photosynthesis and used atomic force microscopy to examine the supramolecular organization of photosystem I associated with FaRLiP in three cyanobacterial species. Mass spectrometry showed the changes in the proteome of Chroococcidiopsis thermalis PCC 7203 that accompany FaRLiP. Fluorescence lifetime imaging microscopy and electron microscopy reveal an altered cellular distribution of photosystem complexes and illustrate the cell-to-cell variability of the FaRLiP response.

Treatment of polyacrylamide-polluted wastewater using a revolving algae biofilm reactor: Pollutant removal performance and microbial community characterization

Industries such as oil mining face challenges in the treatment of polyacrylamide (PAM)-containing wastewater produced during petroleum extraction. The feasibility of using revolving algae biofilm (RAB) reactors to treat PAM-contaminated wastewater for simultaneous removal of carbon and nitrogen was evaluated. The presence or absence of external nitrogen sources had a significant impact on the treatment effect of the RAB system. With the additional N source, the PAM, COD, TOC, and TN removal rates were 64.1 ± 2.0, 58 ± 1.5, 34.5 ± 1.5, and 85 ± 6.0%, respectively. High-throughput sequencing showed that the biofilms on RAB reactors contained a variety of bacteria, cyanobacteria, and green algae, degrading PAM through various mechanisms. The results of infrared spectroscopy analysis indicate that the product of these processes was carboxylic acid. Based on these results, it was concluded that RAB systems can be effectively applied to the treatment of polymer-containing wastewater.

Molecular organizations and function of iron-stress-induced-A protein family in Anabaena sp. PCC 7120

Iron-stress-induced-A proteins (IsiAs) are expressed in cyanobacteria under iron-deficient conditions, and surround photosystem I (PSI) trimer with a ring formation. A cyanobacterium Anabaena sp. PCC 7120 has four isiA genes; however, it is unknown how the IsiAs are associated with PSI. Here we report on molecular organizations and function of the IsiAs in this cyanobacterium. A deletion mutant of the isiA1 gene was constructed, and the four types of thylakoids were prepared from the wild-type (WT) and ΔisiA1 cells under iron-replete (+Fe) and iron-deficient (-Fe) conditions. Immunoblotting analysis exhibits a clear expression of the IsiA1 in the WT-Fe. The PSI-IsiA1 supercomplex is found in the WT-Fe, and excitation-energy transfer from IsiA1 to PSI is verified by time-resolved fluorescence analyses. Instead of the IsiA1, both IsiA2 and IsiA3 are bound to PSI monomer in the ΔisiA1-Fe. These findings provide insights into multiple-expression system of the IsiA family in this cyanobacterium.

Two-step treatment of brewery wastewater using electrocoagulation and cyanobacteria-based cultivation

This study combines electrocoagulation (EC) and cyanobacteria-based cultivation for the two-step treatment of brewery wastewater (BW), with the aim to develop a viable alternative to conventional activated sludge technology. The first step applied EC as a pretreatment method, using different electrode materials (aluminum and iron), to remove color and some pollutant load from the BW. After 30 min of EC treatment, decolorization of BW exceeded 80% for both electrode materials and a 100% reduction of total suspended solids was achieved. In the second step, the electrochemically pretreated BW was used as substrate for a cyanobacteria-based cultivation. After 15 days of cultivation total biomass concentrations (containing up to 50% carbohydrates) reached 525.0 mg L^-1 and 740.0 mg L^-1, for aluminum- and iron-pretreated BW, respectively. Moreover, the cyanobacterial community assimilated most of the residual aluminum and iron produced by the EC process, therefore verifying its bioremediation abilities. The combined process also proved effective at pollutant removal (89.1%, 100%, 89.4%, 98.5% and 91.6% of nitrate, ammonium, total Kjeldahl nitrogen, total phosphorus and chemical oxygen demand, respectively). The two-stage treatment method proposed could offer a promising alternative to conventional BW treatment technologies as it combines both efficiency and sustainability.

Transcriptome Analysis Reveals IsiA-Regulatory Mechanisms Underlying Iron Depletion and Oxidative-Stress Acclimation in Synechocystis sp. Strain PCC 6803

Microorganisms in nature are commonly exposed to various stresses in parallel. The isiA gene encodes an iron stress-induced chlorophyll-binding protein which is significantly induced under iron starvation and oxidative stress. Acclimation of oxidative stress and iron deficiency was investigated using a regulatory mutant of the Synechocystis sp. strain PCC 6803. In this study, the ΔisiA mutant grew more slowly in oxidative-stress and iron depletion conditions compared to the wild-type (WT) counterpart under the same conditions. Thus, we performed transcriptome sequencing (RNA-seq) analysis of the WT strain and the ΔisiA mutant under double-stress conditions to obtain a comprehensive view of isiA-regulated genes. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses showed significant differences between the WT strain and ΔisiA mutant, mainly related to photosynthesis and the iron-sulfur cluster. The deletion of isiA affects the expression of various genes that are involved in cellular processes and structures, such as photosynthesis, phycobilisome, and the proton-transporting ATPase complex. Weighted gene coexpression network analysis (WGCNA) demonstrated three functional modules in which the turquoise module was negatively correlated with superoxide dismutase (SOD) activity. Coexpression network analysis identified several hub genes of each module. Cotranscriptional PCR and reads coverage using the Integrative Genomics Viewer demonstrated that isiA, isiB, isiC, ssl0461, and dfp belonged to the isi operon. Three sRNAs related to oxidative stress were identified. This study enriches our knowledge of IsiA-regulatory mechanisms under iron deficiency and oxidative stress.IMPORTANCE This study analyzed the impact of isiA deletion on the transcriptomic profile of Synechocystis The isiA gene encodes an iron stress-induced chlorophyll-binding protein, which is significantly induced under iron starvation. The deletion of isiA affects the expression of various genes that are involved in photosynthesis and ABC transporters. WGCNA revealed three functional modules in which the blue module was correlated with oxidative stress. We further demonstrated that the isi operon contained the following five genes: isiA, isiB, isiC, ssl0461, and dfp by cotranscriptional PCR. Three sRNAs were identified that were related to oxidative stress. This study enhances our knowledge of IsiA-regulatory mechanisms under iron deficiency and oxidative stress.

Metabolic versatility in a modern lineage of cyanobacteria from terrestrial hot springs

The extent of oxygenated environments on the early Earth was much lower than today, and cyanobacteria were critical players in Earth's shift from widespread anoxia to oxygenated surface environments. Extant cyanobacteria that aggregate into cones, tufts and ridges are used to understand the long record of photosynthesis and microbe-mineral interactions during times when oxygen was much lower, i.e., the Archean and the Proterozoic. To better understand the metabolic versatility and physiological properties of these organisms, we examined publicly available genomes of cyanobacteria from modern terrestrial hydrothermal systems and a newly sequenced genome of a cyanobacterium isolated from conical and ridged microbialites that grow in occasionally sulfidic hydrothermal springs in Yellowstone National Park, USA. Phylogenomic analyses reveal that cyanobacteria from globally distributed terrestrial and shallow marine hydrothermal systems form a monophyletic clade within the Cyanobacteria phylum. Comparative genomics of this clade reveals the genetic capacity for oxygenic photosynthesis that uses photosystems I and II, and anoxygenic photosynthesis that uses a putative sulfide quinone reductase to oxidize sulfide and bypass photosystem II. Surprisingly large proportions of the newly sequenced genome from Yellowstone National Park are also dedicated to secondary metabolite production (15.1-15.6%), of which ∼6% can be attributed to antibiotic production and resistance genes. All this may be advantageous to benthic, mat-forming photosynthesizers that have to compete for light and nutrients in sporadically or permanently sulfidic environments, and may have also improved the tolerance of ancient counterparts of these cyanobacteria to sulfidic conditions in benthic communities that colonized the coastal margins in the Archean and the Proterozoic.

Energy transfer from chlorophyll f to the trapping center in naturally occurring and engineered Photosystem I complexes

Certain cyanobacteria can thrive in environments enriched in far-red light (700-800 nm) due to an acclimation process known as far-red light photoacclimation (FaRLiP). During FaRLiP, about 8% of the Chl a molecules in the photosystems are replaced by Chl f and a very small amount of Chl d. We investigated the spectroscopic properties of Photosystem I (PSI) complexes isolated from wild-type (WT) Synechococcus sp. PCC 7335 and a chlF mutant strain (lacking Chl f synthase) grown in white and far-red light (WL-PSI and FRL-PSI, respectively). WT-FRL-PSI complexes contain Chl f and Chl a but not Chl d. The light-minus dark difference spectrum of the trapping center at high spectral resolution indicates that the special pair in WT-FRL-PSI consists of Chl a molecules with maximum bleaching at 703-704 nm. The action spectrum for photobleaching of the special pair showed that Chl f molecules absorbing at wavelengths up to 800 nm efficiently transfer energy to the trapping center in FRL-PSI complexes to produce a charge-separated state. This is ~ 50 nm further into the near IR than WL-PSI; Chl f has a quantum yield equivalent to that of Chl a in the antenna, i.e., ~ 1.0. PSI complexes from Synechococcus 7002 carrying 3.8 Chl f molecules could promote photobleaching of the special pair by energy transfer at wavelengths longer than WT PSI complexes. Results from these latter studies are directly relevant to the issue of whether introduction of Chl f synthase into plants could expand the wavelength range available for oxygenic photosynthesis in crop plants.

Diverse Chromatic Acclimation Processes Regulating Phycoerythrocyanin and Rod-Shaped Phycobilisome in Cyanobacteria

Cyanobacteria have evolved various photoacclimation processes to perform oxygenic photosynthesis under different light environments. Chromatic acclimation (CA) is a widely recognized and ecologically important type of photoacclimation, whereby cyanobacteria alter the absorbing light colors of a supermolecular antenna complex called the phycobilisome. To date, several CA variants that regulate the green-absorbing phycoerythrin (PE) and/or the red-absorbing phycocyanin (PC) within the hemi-discoidal form of phycobilisome have been characterized. In this study, we identified a unique CA regulatory gene cluster encoding yellow-green-absorbing phycoerythrocyanin (PEC) and a rod-membrane linker protein (CpcL) for the rod-shaped form of phycobilisome. Using the cyanobacterium Leptolyngbya sp. PCC 6406, we revealed novel CA variants regulating PEC (CA7) and the rod-shaped phycobilisome (CA0), which maximize yellow-green light-harvesting capacity and balance the excitation of photosystems, respectively. Analysis of the distribution of CA gene clusters in 445 cyanobacteria genomes revealed eight CA variants responding to green and red light, which are classified based on the presence of PEC, PE, cpcL, and CA photosensor genes. Phylogenetic analysis further suggested that the emergence of CA7 was a single event and preceded that of heterocystous strains, whereas the acquisition of CA0 occurred multiple times. Taken together, these results offer novel insights into the diversity and evolution of the complex cyanobacterial photoacclimation mechanisms.

Characterization of chlorophyll f synthase heterologously produced in Synechococcus sp. PCC 7002

In diverse terrestrial cyanobacteria, Far-Red Light Photoacclimation (FaRLiP) promotes extensive remodeling of the photosynthetic apparatus, including photosystems (PS)I and PSII and the cores of phycobilisomes, and is accompanied by the concomitant biosynthesis of chlorophyll (Chl) d and Chl f. Chl f synthase, encoded by chlF, is a highly divergent paralog of psbA; heterologous expression of chlF from Chlorogloeopsis fritscii PCC 9212 led to the light-dependent production of Chl f in Synechococcus sp. PCC 7002 (Ho et al., Science 353, aaf9178 (2016)). In the studies reported here, expression of the chlF gene from Fischerella thermalis PCC 7521 in the heterologous system led to enhanced synthesis of Chl f. N-terminally [His]₁₀-tagged ChlF⁷⁵²¹ was purified and identified by immunoblotting and tryptic-peptide mass fingerprinting. As predicted from its sequence similarity to PsbA, ChlF bound Chl a and pheophytin a at a ratio of ~ 3-4:1, bound β-carotene and zeaxanthin, and was inhibited in vivo by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Cross-linking studies and the absence of copurifying proteins indicated that ChlF forms homodimers. Flash photolysis of ChlF produced a Chl a triplet that decayed with a lifetime (1/e) of ~ 817 µs and that could be attributed to intersystem crossing by EPR spectroscopy at 90 K. When the chlF⁷⁵²¹ gene was expressed in a strain in which the psbD1 and psbD2 genes had been deleted, significantly more Chl f was produced, and Chl f levels could be further enhanced by specific growth-light conditions. Chl f synthesized in Synechococcus sp. PCC 7002 was inserted into trimeric PSI complexes.

Morphophysiological and transcriptome analysis reveals a multiline defense system enabling cyanobacterium Leptolyngbya strain JSC-1 to withstand iron induced oxidative stress

Iron intoxications induce severe oxidative stress by producing reactive oxygen species (ROS) in cyanobacteria, leading to membrane lipid peroxidation, altered morphology, impaired photosynthesis and other oxidative stress injuries. Given these stresses, mitigation of ROS is a prerequisite for all aerobic organisms. Study of siderophilic cyanobacterium Leptolyngbya strain JSC-1 inhabiting iron-rich hot springs may provide insight into the mechanism of iron homeostasis and alleviation of oxidative stress. In this study, we investigated the morphophysiological and molecular mechanisms enabling this cyanobacterium to cope with iron-induced oxidative stress. Strain JSC-1 biomineralized extracellular iron via an exopolymeric sheath (acting as a first line of defense) and intracellular iron via polyphosphate inclusions (second line of defense), thus minimizing the burden of free ferric ions. Physiological parameters, SOD, CAT and POD activities, bacterioferritin and total protein contents fluctuated in response to iron elevation, displaying a third line of defense to mitigate ROS. Differential gene expression analysis of JSC-1 indicated up-regulation of 94 and 125 genes and down-regulation of 89 and 183 genes at low (4 μM) and high (400 μM) iron concentration, respectively. The differentially expressed genes (DEGs) were enriched in 100 KEGG pathways and were found to be involved in lipopolysaccharide and fatty acid biosynthesis, starch, sucrose, chlorophyll and other metabolic pathways. Together with metabolic reprogramming (fourth line of defense), JSC-1 established a unique multiline defense system that allows JSC-1 to withstand severe oxidative stress. These findings also provide insight into potential survival strategies of ancient microorganisms inhabiting similar environment present in early earth history.

Characterization of the genuine type 2 chromatic acclimation in the two Geminocystis cyanobacteria

Certain cyanobacteria can adjust the wavelengths of light they absorb by remodeling their photosynthetic antenna complex phycobilisome via a process called chromatic acclimation (CA). Although several types of CA have been reported, the diversity of the molecular mechanisms of CA among the cyanobacteria phylum is not fully understood. Here, we characterized the molecular process of CA of Geminocystis sp. strains National Institute of Environmental Studies (NIES)-3708 and NIES-3709. Absorption and fluorescence spectroscopy revealed that both strains dramatically alter their phycoerythrin content in response to green and red light. Whole-genome comparison revealed that the two strains share the typical phycobilisome structure consisting of a central core and peripheral rods, but they differ in the number of rod linkers of phycoerythrin and thus have differing capacity for phycoerythrin accumulation. RNA sequencing analysis suggested that the length of phycoerythrin rods in each phycobilisome is strictly regulated by the green light and red light-sensing CcaS/R system, whereas the total number of phycobilisomes is governed by the excitation-balancing system between phycobilisomes and photosystems. We reclassify the conventional CA types based on the genome information and designate CA of the two strains as genuine type 2, where components of phycoerythrin, but not rod-membrane linker of phycocyanin, are regulated by the CcaS/R system.