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Anoxygenic photosynthetic bacteria are common inhabitants of wastewater: we found that Rhodopseudomonas palustris and Afifella marina in eutrophic conditions only partially degraded the azo dye (50 mmol m−3), Methyl Red, but completely degraded it under specially defined conditions. The azo dye is potentially a source of both carbon and fixed nitrogen. Rhodopseudomonas palustris and Afifella marina can live heterotrophically, photoheterotrophically or photoautotrophically under anoxic conditions where they can fix N 2 if no organic nitrogen or NH 3 is available. If organic carbon sources are available or if NH 3 is present, the cells again only partially catabolised Methyl Red. In the absence of no alternative organic carbon sources and no NH 3 , the cells almost completely spectroscopically decolourised Methyl Red in 4 days. In sewage ponds the ready availability of alternative organic carbon and NH 3 would result in only partial removal of Methyl Red. Rhodopseudomonas cells responded to the availability of Methyl Red in N-free media, by increasing both Optimum irradiance and maximum ETR (E opt 276.3 μmol quanta m−2 s−1; ETR max 391.4 μmol e− g−1 BChl a s−1) compared to control cells incubated in PM media with no organic carbon source and no fixed N-source (E opt 115.2 μmol quanta m−2 s−1; ETR max = 153.0 μmol e− g−1 BChl a s−1. If no alternative C or N sources are available, Rhodopseudomonas embedded in alginate biobeads will completely and repeatedly break down Methyl Red. The marine Afifella readily broke down Methyl Red but again breakdown was only complete if alternative carbon and no fixed nitrogen sources were available. The toxicity of the breakdown products produced by photosynthetic bacteria from azo-dyes needs to be followed up. Photosynthetic bacterial-alginate biobeads have long lifetimes (Rhodopseudomonas ≈ 2 months, Afifella > 6 months) making them of great biotechnological potential. Rhodopseudomonas palustris biobeads rapidly and repeatedly metabolised Methyl Red when offered as a nutrient with no other carbon or nitrogen source. After depleting the supplied methyl when the cells were resupplied with Methyl Red rapidly metabolised it. [Display omitted] • Rhodopseudomonas palustris and Afifella marina have very wide metabolic profiles, • Photosynthetic bacteria completely degrade Methyl Red under special conditions, • Degree of degradation depends on whether alternative C&N sources are available, • Methyl Red increased Optimum irradiance (Eopt) and maximum ETR (ETRmax), • Rhodopseudomonas and Afifella in alginate biobeads catabolise Methyl Red, • Biobeads degrade slowly (Rhodopseudomonas ≈ 2, Afifella > 6 months). [ABSTRACT FROM AUTHOR]

Thylakoid membranes, the site of photochemical and electron transport reactions of oxygenic photosynthesis, are composed of a myriad of proteins, cofactors including pigments, and glycerolipids. In the non-diazotrophic cyanobacterium Synechocystis sp. PCC 6803, the size and function of thylakoid membranes are reduced under nitrogen (N) starvation but are quickly recovered after N addition to the starved cells. To understand how the functionality of thylakoid membranes is adjusted in response to N status in Synechocystis sp. PCC 6803, we examined changes in thylakoid components and the photosynthetic activity during the N starvation and recovery processes. In N-starved cells, phycobilisome content, photosystem II protein levels and the photosynthetic activity substantially decreased as compared with those in N-sufficient cells. Although the content of chlorophyll (Chl) a, total protein and total glycerolipid also decreased under the N-starved condition based on OD730 reflecting cell density, when based on culture volume, the Chl a and total protein content remained almost constant and total glycerolipid content even increased during N starvation, suggesting that cellular levels of these components decrease under the N-starved condition mainly through dilution due to cell growth. With N addition, the photosynthetic activity quickly recovered, followed by full restoration of photosynthetic pigment and protein levels. The content of phosphatidylglycerol (PG), an essential lipid constituent of both photosystems, increased faster than that of Chl a, whereas the content of glycolipids, the main constituents of the thylakoid lipid bilayer, gradually recovered after N addition. The data indicate differential regulation of PG and glycolipids during the construction of the photosynthetic machinery and regeneration of thylakoid membranes. Of note, addition of PG to the growth medium slightly accelerated the Chl a accumulation in wild-type cells during the recovery process. Because PG is required for the biosynthesis of Chl a and the formation of functional photosystem complexes, rapid PG biosynthesis in response to N acquisition may be required for the rapid formation of the photosynthetic machinery during thylakoid regeneration.

Thylakoid membranes, the site of photochemical and electron transport reactions of oxygenic photosynthesis, are composed of a myriad of proteins, cofactors including pigments, and glycerolipids. In the non-diazotrophic cyanobacterium Synechocystis sp. PCC 6803, the size and function of thylakoid membranes are reduced under nitrogen (N) starvation but are quickly recovered after N addition to the starved cells. To understand how the functionality of thylakoid membranes is adjusted in response to N status in Synechocystis sp. PCC 6803, we examined changes in thylakoid components and the photosynthetic activity during the N starvation and recovery processes. In N-starved cells, phycobilisome content, photosystem II protein levels and the photosynthetic activity substantially decreased as compared with those in N-sufficient cells. Although the content of chlorophyll (Chl) a , total protein and total glycerolipid also decreased under the N-starved condition based on OD730 reflecting cell density, when based on culture volume, the Chl a and total protein content remained almost constant and total glycerolipid content even increased during N starvation, suggesting that cellular levels of these components decrease under the N-starved condition mainly through dilution due to cell growth. With N addition, the photosynthetic activity quickly recovered, followed by full restoration of photosynthetic pigment and protein levels. The content of phosphatidylglycerol (PG), an essential lipid constituent of both photosystems, increased faster than that of Chl a , whereas the content of glycolipids, the main constituents of the thylakoid lipid bilayer, gradually recovered after N addition. The data indicate differential regulation of PG and glycolipids during the construction of the photosynthetic machinery and regeneration of thylakoid membranes. Of note, addition of PG to the growth medium slightly accelerated the Chl a accumulation in wild-type cells during the recovery process. Because PG is required for the biosynthesis of Chl a and the formation of functional photosystem complexes, rapid PG biosynthesis in response to N acquisition may be required for the rapid formation of the photosynthetic machinery during thylakoid regeneration. [ABSTRACT FROM AUTHOR]

Bacteria routinely utilize two-component signal transduction pathways to sense and alter gene expression in response to environmental cues. While cyanobacteria express numerous two-component systems, these pathways do not regulate all of the genes within many of the identified abiotic stress-induced regulons. Electron transport inhibitors combined with western analysis and measurement of chlorophyll a fluorescent yield, using pulse amplitude modulation fluorometry, were used to detect the effect of a diverse range of abiotic stresses on the redox status of the photosynthetic electron transport chain and the accumulation and degradation of the Synechocystis sp. PCC 6803 DEAD box RNA helicase, CrhR. Alterations in CrhR abundance were tightly correlated with the redox poise of the electron transport chain between Q A and cytochrome b 6 f , with reduction favoring CrhR accumulation. The results provide evidence for an alternative, convergent sensing mechanism mediated through the redox poise of Q B /PQH 2 that senses multiple, divergent forms of abiotic stress and regulates accumulation of CrhR. The RNA helicase activity of CrhR could then function as a post-translational effector to regulate downstream gene expression. The potential for a related system in Staphylococcus aureus and higher plant chloroplasts suggest convergent sensing mechanisms may be evolutionarily conserved and occur more widely than anticipated. Unlabelled Image • The DEAD-box RNA-helicase, CrhR, is induced by diverse abiotic stresses. • CrhR expression correlates with the redox potential of the electron transport chain. • Redox poise acts as a convergent sensing hub in response to abiotic stress. • CrhR potentially functions as a post-transciptional effector of gene expression. • Similar systems suggest convergent sensing is more widespread than anticipated. [ABSTRACT FROM AUTHOR]