Species-specific roles of sulfolipid metabolism in acclimation of photosynthetic microbes to sulfur-starvation stress

Preferential phosphatidylglycerol synthesis via phosphorus supply through rRNA degradation in the cyanobacterium, Synechocystis sp. PCC 6803, under phosphate-starved conditions

Photosynthetic organisms often encounter phosphorus (P) limitation in natural habitats. When faced with P limitation, seed plants degrade nucleic acids and extra-plastid phospholipids to remobilize P, thereby enhancing their internal-P utilization efficiency. Although prokaryotic and eukaryotic photosynthetic organisms decrease the content of phosphatidylglycerol (PG) under P-limited conditions, it remains unclear whether PG is degraded for P remobilization. Moreover, information is limited on internal-P remobilization in photosynthetic microbes. This study investigates internal-P remobilization under P-starvation (-P) conditions in a cyanobacterium, Synechocystis sp. PCC 6803, focusing on PG and nucleic acids. Our results reveal that the PG content increases by more than double in the -P culture, indicating preferential PG synthesis among cellular P compounds. Simultaneously, the faster increases of glycolipids counteract this PG increase, which decreases the PG proportion in total lipids. Two genes, glpD and plsX, contribute to the synthesis of diacylglycerol moieties in glycerolipids, with glpD also responsible for the polar head group synthesis in PG. The mRNA levels of both glpD and plsX are upregulated during -P, which would cause the preferential metabolic flow of their P-containing substrates toward glycerolipid synthesis, particularly PG synthesis. Meanwhile, we find that RNA accounts for 62% of cellular P, and that rRNA species, which makes up the majority of RNA, are degraded under -P conditions to less than 30% of their initial levels. These findings emphasize the importance of PG in -P-acclimating cell growth and the role of rRNA as a significant internal-P source for P remobilization, including preferential PG synthesis.

slr2103, a homolog of type-2 diacylglycerol acyltransferase genes, for plastoquinone-related neutral lipid synthesis and NaCl-stress acclimatization in a cyanobacterium, Synechocystis sp. PCC 6803

A cyanobacterium, Synechocystis sp. PCC 6803, contains a lipid with triacylglycerol-like TLC mobility but its identity and physiological roles remain unknown. Here, on ESI-positive LC-MS² analysis, it is shown that the triacylglycerol-like lipid (lipid X) is related to plastoquinone and can be grouped into two subclasses, X_a and X_b, the latter of which is esterified by 16:0 and 18:0. This study further shows that a Synechocystis homolog of type-2 diacylglycerol acyltransferase genes, slr2103, is essential for lipid X synthesis: lipid X disappears in a Synechocystis slr2103-disruptant whereas it appears in an slr2103-overexpressing transformant (OE) of Synechococcus elongatus PCC 7942 that intrinsically lacks lipid X. The slr2103 disruption causes Synechocystis cells to accumulate plastoquinone-C at an abnormally high level whereas slr2103 overexpression in Synechococcus causes the cells to almost completely lose it. It is thus deduced that slr2103 encodes a novel acyltransferase that esterifies 16:0 or 18:0 with plastoquinone-C for the synthesis of lipid X_b. Characterization of the slr2103-disruptant in Synechocystis shows that slr2103 contributes to sedimented-cell growth in a static culture, and to bloom-like structure formation and its expansion by promoting cell aggregation and floatation upon imposition of saline stress (0.3-0.6 M NaCl). These observations provide a basis for elucidation of the molecular mechanism of a novel cyanobacterial strategy to acclimatize to saline stress, and one for development of a system of seawater-utilization and economical harvesting of cyanobacterial cells with high-value added compounds, or blooming control of toxic cyanobacteria.

Diacylglyceryl-N,N,N-trimethylhomoserine-dependent lipid remodeling in a green alga, Chlorella kessleri

Membrane lipid remodeling contributes to the environmental acclimation of plants. In the green lineage, a betaine lipid, diacylglyceryl-N,N,N-trimethylhomoserine (DGTS), is included exclusively among green algae and nonflowering plants. Here, we show that the green alga Chlorella kessleri synthesizes DGTS under phosphorus-deficient conditions through the eukaryotic pathway via the ER. Simultaneously, phosphatidylcholine and phosphatidylethanolamine, which are similar to DGTS in their zwitterionic properties, are almost completely degraded to release 18.1% cellular phosphorus, and to provide diacylglycerol moieties for a part of DGTS synthesis. This lipid remodeling system that substitutes DGTS for extrachloroplast phospholipids to lower the P-quota operates through the expression induction of the BTA1 gene. Investigation of this lipid remodeling system is necessary in a wide range of lower green plants for a comprehensive understanding of their phosphorus deficiency acclimation strategies.

Glyceroglycolipid Metabolism Regulations under Phosphate Starvation Revealed by Transcriptome Analysis in Synechococcus elongatus PCC 7942

Glyceroglycolipids, abundant in cyanobacteria's photosynthetic membranes, present bioactivities and pharmacological activities, and can be widely used in the pharmaceutical industry. Environmental factors could alter the contents and compositions of cyanobacteria glyceroglycolipids, but the regulation mechanism remains unclear. Therefore, the glyceroglycolipids contents and the transcriptome in Synechococcus elongatus PCC 7942 were analyzed under phosphate starvation. Under phosphate starvation, the decrease of monogalactosyl diacylglycerol (MGDG) and increases of digalactosyl diacylglycerol (DGDG) and sulfoquinovosyl diacylglycerol (SQDG) led to a decrease in the MGDG/DGDG ratio, from 4:1 to 5:3, after 12 days of cultivation. However, UDP-sulfoquinovose synthase gene sqdB, and the SQDG synthase gene sqdX, were down-regulated, and the decreased MGDG/DGDG ratio was later increased back to 2:1 after 15 days of cultivation, suggesting the regulation of glyceroglycolipids on day 12 was based on the MGDG/DGDG ratio maintaining glyceroglycolipid homeostasis. There are 12 differentially expressed transcriptional regulators that could be potential candidates related to glyceroglycolipid regulation, according to the transcriptome analysis. The transcriptome analysis also suggested post-transcriptional or post-translational regulations in glyceroglycolipid synthesis. This study provides further insights into glyceroglycolipid metabolism, as well as the scientific basis for glyceroglycolipid synthesis optimization and cyanobacteria glyceroglycolipids utilization via metabolic engineering.

Characterizing metabolic stress-induced phenotypes of Synechocystis PCC6803 with Raman spectroscopy

BACKGROUND

During their long evolution, Synechocystis sp. PCC6803 developed a remarkable capacity to acclimate to diverse environmental conditions. In this study, Raman spectroscopy and Raman chemometrics tools (RametrixTM) were employed to investigate the phenotypic changes in response to external stressors and correlate specific Raman bands with their corresponding biomolecules determined with widely used analytical methods.

METHODS

Synechocystis cells were grown in the presence of (i) acetate (7.5-30 mM), (ii) NaCl (50-150 mM) and (iii) limiting levels of MgSO4 (0-62.5 mM) in BG-11 media. Principal component analysis (PCA) and discriminant analysis of PCs (DAPC) were performed with the RametrixTM LITE Toolbox for MATLABⓇ. Next, validation of these models was realized via RametrixTM PRO Toolbox where prediction of accuracy, sensitivity, and specificity for an unknown Raman spectrum was calculated. These analyses were coupled with statistical tests (ANOVA and pairwise comparison) to determine statistically significant changes in the phenotypic responses. Finally, amino acid and fatty acid levels were measured with well-established analytical methods. The obtained data were correlated with previously established Raman bands assigned to these biomolecules.

RESULTS

Distinguishable clusters representative of phenotypic responses were observed based on the external stimuli (i.e., acetate, NaCl, MgSO4, and controls grown on BG-11 medium) or its concentration when analyzing separately. For all these cases, RametrixTM PRO was able to predict efficiently the corresponding concentration in the culture media for an unknown Raman spectra with accuracy, sensitivity and specificity exceeding random chance. Finally, correlations (R > 0.7) were observed for all amino acids and fatty acids between well-established analytical methods and Raman bands.

Comparative membrane proteomics reveal contrasting adaptation strategies for coastal and oceanic marine Synechococcus cyanobacteria

Marine cyanobacteria genus Synechococcus are among the most abundant and widespread primary producers in the open ocean. Synechococcus strains belonging to different clades have adapted distinct strategies for growth and survival across a range of marine conditions. Clades I and IV are prevalent in colder, mesotrophic, coastal waters, while clades II and III prefer warm, oligotrophic open oceans. To gain insight into the cellular resources these unicellular organisms invest in adaptation strategies we performed shotgun membrane proteomics of four Synechococcus spp. strains namely CC9311 (clade I), CC9605 (clade II), WH8102 (clade III) and CC9902 (clade IV). Comparative membrane proteomes analysis demonstrated that CC9902 and WH8102 showed high resource allocation for phosphate uptake, accounting for 44% and 38% of overall transporter protein expression of the species. WH8102 showed high expression of the iron uptake ATP-binding cassette binding protein FutA, suggesting that a high binding affinity for iron is possibly a key adaptation strategy for some strains in oligotrophic ocean environments. One protein annotated as a phosphatase 2c (Sync_2505 and Syncc9902_0387) was highly expressed in the coastal mesotrophic strains CC9311 and CC9902, constituting 14%-16% of total membrane protein, indicating a vital, but undefined function, for strains living in temperate mesotrophic environments.