Sll0751 and Sll1041 are involved in acid stress tolerance in Synechocystis sp. PCC 6803

Identification of acidic stress-responsive genes and acid tolerance engineering in Synechococcus elongatus PCC 7942

Cyanobacteria are excellent autotrophic photosynthetic chassis employed in synthetic biology, and previous studies have suggested that they have alkaline tolerance but low acid tolerance, significantly limiting their productivity as photosynthetic chassis and necessitating investigations into the acid stress resistance mechanism. In this study, differentially expressed genes were obtained by RNA sequencing-based comparative transcriptomic analysis under long-term acidic stress conditions and acidic shock treatment, in the model cyanobacterium Synechococcus elongatus PCC 7942. A pathway enrichment analysis revealed the upregulated and downregulated pathways during long-term acidic and shock stress treatment. The subsequent single gene knockout and phenotype analysis showed that under acidic stress conditions, the strains with chlL, chlN, pex, synpcc7942_2038, synpcc7942_1890, or synpcc7942_2547 knocked out grew worse than the wild type, suggesting their involvement in acid tolerance. This finding was further confirmed by introducing the corresponding genes back into the knockout mutant individually. Moreover, individual overexpression of the chlL and chlN genes in the wild type successfully improved the tolerance of S. elongatus PCC 7942 to acidic stress. This work successfully identified six genes involved in acidic stress responses, and overexpressing chIL or chIN individually successfully improved acid tolerance in S. elongatus PCC 7942, providing valuable information to better understand the acid resistance mechanism in S. elongatus PCC 7942 and novel insights into the robustness and tolerance engineering of cyanobacterial chassis. KEY POINTS: • DEGs were identified by RNA-seq based transcriptomics analysis in response to acidic stress in S. elongatus PCC 7942. • Six genes were identified to be involved in acid tolerance in S. elongatus PCC 7942. • Overexpression of chIL or chIN individually successfully improved the acid tolerance of S. elongatus PCC 7942.

Response and acclimation of cyanobacteria to acidification: A comprehensive review

Cyanobacteria, as vital components of aquatic ecosystems, face increasing challenges due to acidification driven by various anthropogenic and natural factors. Understanding how cyanobacteria adapt and respond to acidification is crucial for predicting their ecological dynamics and potential impacts on ecosystem health. This comprehensive review synthesizes current knowledge on the acclimation mechanisms and responses of cyanobacteria to acidification stress. Detailly, ecological roles of cyanobacteria were firstly briefly concluded, followed by the effects of acidification on aquatic ecosystems and cyanobacteria. Then the review focuses on the physiological, biochemical, and molecular strategies employed by cyanobacteria to cope with acidification stress, highlighting key adaptive mechanisms and their ecological implications. Finally, a summary of strategies to enhance acid resistance in cyanobacteria and future directions was discussed. Utilizing omics data and machine learning technology to build a cyanobacterial acid regulatory network allows for predicting the impact of acidification on cyanobacteria and inferring its broader effects on ecosystems. Additionally, acquiring acid-tolerant chassis cells of cyanobacteria through innovative techniques facilitates the advancement of environmentally friendly production of acidic chemicals. By synthesizing empirical evidence and theoretical frameworks, this review aims to elucidate the complex interplay between cyanobacteria and acidification stressors, providing insights for future research directions and ecosystem management strategies.

Unraveling cellular and molecular mechanisms of acid stress tolerance and resistance in marine species: New frontiers in the study of adaptation to ocean acidification

Since the industrial revolution, fossil fuel combustion has led to a 30 %-increase of the atmospheric CO₂ concentration, also increasing the ocean partial CO₂ pressure. The consequent lowered surface seawater pH is termed ocean acidification (OA) and severely affects marine life on a global scale. Cellular and molecular responses of marine species to lowered seawater pH have been studied but information on the mechanisms driving the tolerance of adapted species to comparatively low seawater pH is limited. Such information may be obtained from species inhabiting sites with naturally low water pH that have evolved remarkable abilities to tolerate such conditions. This review gathers information on current knowledge about species naturally facing low water pH conditions and on cellular and molecular adaptive mechanisms enabling the species to survive under, and even benefit from, adverse pH conditions. Evidences derived from case studies on naturally acidified systems and on resistance mechanisms will guide predictions on the consequences of future adverse OA scenarios for marine biodiversity.

Characterization of Sll1558 in environmental stress tolerance of Synechocystis sp. PCC 6803

So far, the molecular mechanisms underlying the acidic-stress responses of plants are complicated and only fragmentally understood. Here, we investigated the mechanisms responsible for acidic-stress acclimation. Previously, DNA microarray analysis identified the sll1558 gene in Synechocystis sp. PCC 6803 (hereafter called Synechocystis 6803) to be upregulated following short-term acid treatment (1 h at pH 3.0). The sll1558 gene encodes uridine diphosphate-glucose pyrophosphorylase (UDP-glucose pyrophosphorylase), which catalyzes the conversion of glucose-1-phosphate into UDP-glucose. We constructed mutant cells for this gene and analyzed their phenotype. The sll1558 gene did not completely segregate in sll1558 mutant cells; thus, Sll1558 is essential for the survival of Synechocystis 6803. Besides, the partially disrupted sll1558 mutant cells were highly sensitive to acidic stress (pH 6.0) as well as other stress conditions (high salt, high osmolality, high/low temperature, and ultraviolet-B stress); the number of sll1558 transcripts increased under these conditions. UDP-glucose is used for the synthesis of various materials, such as glycolipids. From the membrane lipid composition analysis, digalactosyldiacylglycerol decreased and phosphatidylglycerol increased in the partially disrupted sll1558 mutant cells under acidic stress. These results suggest that sll1558 is important not only for the survival of Synechocystis 6803, but also for tolerance under various stress conditions.

The HlyD-like membrane fusion protein All5304 is essential for acid stress survival of the filamentous cyanobacterium Anabaena sp. PCC 7120

Acid stress is an environmental problem for plants and fresh water cyanobacteria like the filamentous, heterocyst forming species Anabaena sp. PCC 7120 (hereafter Anabaena sp.). Heterocyst differentiation, cell-cell communication and nitrogen fixation has been deeply studied in this model organism, but little is known about the cellular response of Anabaena sp. to decreased pH values, causing acid stress. ATP-binding cassette (ABC) transporters are involved in acid stress response in other bacteria, by exporting proteins responsible for survival under acidification. The genome of Anabaena sp. encodes numerous ABC transporter components, whose function is not known yet. Here, we describe the function of the gene all5304 encoding a protein with homology to membrane fusion proteins of tripartite efflux pumps driven by ABC transporters like HlyBD-TolC of Escherichia coli. The all5304 mutant shows less resistance against low pH, even though the expression of the gene is independent from the pH of the medium. We compared the exoproteome of the wild type and mutant cultures and identified three proteins-candidate substrates of the putative transporter. Including the in silico analysis of All5304, our results suggest that All5304 functions as part of an efflux pump, secreting of a protein necessary for acid tolerance in Anabaena sp.

Improved Salt Tolerance and Metabolomics Analysis of Synechococcus elongatus UTEX 2973 by Overexpressing Mrp Antiporters

The fast-growing cyanobacterium Synechococcus elongatus UTEX 2973 (Syn2973) is a promising candidate for photosynthetic microbial factory. Seawater utilization is necessary for large-scale cultivation of Syn2973 in the future. However, Syn2973 is sensitive to salt stress, making it necessary to improve its salt tolerance. In this study, 21 exogenous putative transporters were individually overexpressed in Syn2973 to evaluate their effects on salt tolerance. The results showed the overexpression of three Mrp antiporters significantly improved the salt tolerance of Syn2973. Notably, overexpressing the Mrp antiporter from Synechococcus sp. PCC 7002 improved cell growth by 57.7% under 0.4 M NaCl condition. In addition, the metabolomics and biomass composition analyses revealed the possible mechanisms against salt stress in both Syn2973 and the genetically engineered strain. The study provides important engineering strategies to improve salt tolerance of Syn2973 and is valuable for understanding mechanisms of salt tolerance in cyanobacteria.

Characterization of ABC transporter genes, sll1180, sll1181, and slr1270, involved in acid stress tolerance of Synechocystis sp. PCC 6803

Over 50 ATP-binding cassette (ABC) transporter-related genes are detected in the Synechocystis sp. PCC 6803 genome by genome sequence analysis. Deletion mutants of other substrate-unknown ABC transporter genes were screened for their acid stress sensitivities in a low-pH medium to identify ABC transporters involved in acid resistance. We found that a mutant of sll1180 encoding proteins with homology to HlyB in Escherichia coli (E.coli) is more sensitive to acid stress than wild-type (WT) cells and analyzed the abundance of expression of the genes in WT cells under acid stress condition by quantitative real-time reverse transcriptase-polymerase chain reaction. sll1180 expression increased in the WT cells after acid stress treatment. Immunofluorescence revealed that Sll1180 localized in the plasma membrane. These results suggest that Sll1180 has an important role in the growth of Synechocystis sp. PCC 6803 under acid stress conditions. HlyB, HlyD, and TolC complex transport HlyA in E.coli; therefore, we searched for genes corresponding to these in Synechocystis sp. PCC 6803. A BlastP search suggested that HlyA, HlyD, and TolC proteins had homology to Sll1951, Sll1181, and Slr1270. Therefore, we constructed deletion mutant of these genes. sll1181 and slr1270 mutant cells revealed acid stress sensitivity. The bacterial two-hybrid analysis showed that Sll1180 interacted with Sll1181 and Sll1951. Dot blot analysis of Sll1951-His revealed that the sll1180 and sll1181 mutant cells did not transport Sll1951-His from the cytoplasm to the extracellular matrix. These results suggest that Sll1180 and Sll1181 transport Sll1951 and that Sll1951-outside of the cells-might be a key factor in acid stress tolerance.

Tangled evolutionary processes with commonality and diversity in plastidial glycolipid synthesis in photosynthetic organisms

In photosynthetic organisms, the photosynthetic membrane constitutes a scaffold for light-harvesting complexes and photosynthetic reaction centers. Three kinds of glycolipids, namely monogalactosyldiacylglycerol, digalactosyldiacylglycerol, and sulfoquinovosyldiacylglycerol, constitute approximately 80-90% of photosynthetic membrane lipids and are well conserved from tiny cyanobacteria to the leaves of huge trees. These glycolipids perform a wide variety of functions beyond biological membrane formation. In particular, the capability of adaptation to harsh environments through regulation of membrane glycolipid composition is essential for healthy growth and development of photosynthetic organisms. The genome analysis and functional genetics of the model seed plant Arabidopsis thaliana have yielded many new findings concerning the biosynthesis, regulation, and functions of glycolipids. Nevertheless, it remains to be clarified how the complex biosynthetic pathways and well-organized functions of glycolipids evolved in early and primitive photosynthetic organisms, such as cyanobacteria, to yield modern photosynthetic organisms like land plants. Recently, genome data for many photosynthetic organisms have been made available as the fruit of the rapid development of sequencing technology. We also have reported the draft genome sequence of the charophyte alga Klebsormidium flaccidum, which is an intermediate organism between green algae and land plants. Here, we performed a comprehensive phylogenic analysis of glycolipid biosynthesis genes in oxygenic photosynthetic organisms including K. flaccidum. Based on the results together with membrane lipid analysis of this alga, we discuss the evolution of glycolipid synthesis in photosynthetic organisms. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.