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Summary: Various approaches have been proposed to control/eliminate toxic Microcystis sp. blooms including H2O2 treatments. Earlier studies showed that pre‐exposure of various algae to oxidative stress induced massive cell death when cultures were exposed to an additional H2O2 treatment. We examined the vulnerability of exponential and stationary‐phase Microcystis sp. strain MGK cultures to single and double H2O2 applications. Stationary cultures show a much higher ability to decompose H2O2 than younger cultures. Nevertheless, they are more sensitive to an additional H2O2 dose given 1–6 h after the first one. Transcript analyses following H2O2 application showed a fast rise in glutathione peroxidase abundance (227‐fold within an hour) followed by a steep decline thereafter. Other genes potentially engaged in oxidative stress were far less affected. Metabolic‐related genes were downregulated after H2O2 treatments. Among those examined, the transcript level of prk (encoding phosphoribulose kinase) was the slowest to recover in agreement with the decline in photosynthetic rate revealed by fluorescence measurements. Our findings shed light on the response of Microcystis MGK to oxidative stress suggesting that two consecutive H2O2 applications of low concentrations are far more effective in controlling Microcystis sp. population than a single dose of a higher concentration. [ABSTRACT FROM AUTHOR]

Summary: Toxic Microcystis spp. blooms constitute a serious threat to water quality worldwide. Aeromonas veronii was isolated from Microcystis sp. colonies collected in Lake Kinneret. Spent Aeromonas media inhibits the growth of Microcystis aeruginosa MGK isolated from Lake Kinneret. The inhibition was much stronger when Aeromonas growth medium contained spent media from MGK suggesting that Aeromonas recognized its presence and produced secondary metabolites that inhibit Microcystis growth. Fractionations of the crude extract and analyses of the active fractions identified several secondary metabolites including lumichrome in Aeromonas media. Application of lumichrome at concentrations as low as 4 nM severely inhibited Microcystis growth. Inactivation of aviH in the lumichrome biosynthetic pathway altered the lumichrome level in Aeromonas and the extent of MGK growth inhibition. Conversely, the initial lag in Aeromonas growth was significantly longer when provided with Microcystis spent media but Aeromonas was able to resume normal growth. The longer was pre‐exposure to Microcystis spent media the shorter was the lag phase in Aeromonas growth indicating the presence of, and acclimation to, secondary MGK metabolite(s) the nature of which was not revealed. Our study may help to control toxic Microcystis blooms taking advantage of chemical languages used in the interspecies communication. [ABSTRACT FROM AUTHOR]

Cyanobacteria possess CO2-concentrating mechanisms ( CCM) that functionally compensate for the poor affinity of their ribulose-1,5-bisphosphate carboxylase/oxygenase ( Rubisco) to CO2. It was proposed that 2-phosphoglycolate (2 PG), produced by the oxygenase activity of Rubisco and metabolized via photorespiratory routes, serves as a signal molecule for the induction of CCM-related genes under limiting CO2 level ( LC) conditions. However, in vivo evidence is still missing. Since 2 PG does not permeate the cells, we manipulated its internal concentration. Four putative phosphoglycolate phosphatases ( PGPases) encoding genes ( slr0458, sll1349, slr0586 and slr1762) were identified in the cyanobacterium S ynechocystis PCC 6803. Expression of slr0458 in E scherichia coli led to a significant rise in PGPase activity. A S ynechocystis mutant overexpressing ( OE) slr0458 was constructed. Compared with the wild type (WT), the mutant grew slower under limiting CO2 concentration and the intracellular 2 PG level was considerably smaller than in the wild type, the transcript abundance of LC-induced genes including cmpA, sbtA and ndhF3 was reduced, and the OE cells acclimated slower to LC - indicated by the delayed rise in the apparent photosynthetic affinity to inorganic carbon. Data obtained here implicated 2 PG in the acclimation of this cyanobacterium to LC but also indicated that other, yet to be identified components, are involved. [ABSTRACT FROM AUTHOR]

Summary [ABSTRACT FROM AUTHOR]

In Synechocystis sp. strain PCC 6803, over 450 genes are upregulated following transfer of the cells from a high (1–5% CO2 in air, HC) to a low level of CO2 (as in air or lower, LC). This includes sbtA, ndhF3 and cmpA involved in inorganic carbon (Ci) uptake. Earlier studies implicated NdhR in the regulation of LC-induced genes but there are indications that additional components are involved. Following extraction of proteins from cells grown under HC and (NH4)2SO4 fractionation, we have identified LexA and two AbrB-like proteins, Sll0359 and Sll0822, which bind to a fragment of the sbtA promoter. Using extracts prepared from LC-grown cells, Sll0822 did not bind to the sbtA promoter despite its presence in the cells, suggesting that it may serve as a repressor of LC-induced genes. This is supported by the fact that sbtA, ndhF3 and cmpA normally expressed only under LC in the wild-type are transcribed under both HC and LC in a Δ sll0822 mutant. When grown under HC this mutant exhibits an elevated apparent photosynthetic affinity to Ci, typically observed in the wild-type only under LC. Clearly, expression of genes essential for Ci uptake was sufficient to raise the apparent photosynthetic affinity for external Ci. [ABSTRACT FROM AUTHOR]

The occurrence of a photorespiratory 2-phosphoglycolate metabolism in cyanobacteria is not clear. In the genome of the cyanobacterium Synechocystis sp. strain PCC 6803, we have identified open reading frames encoding enzymes homologous to those forming the plant-like C2 cycle and the bacterial-type glycerate pathway. To study the route and importance of 2-phosphoglycolate metabolism, the identified genes were systematically inactivated by mutagenesis. With a few exceptions, most of these genes could be inactivated without leading to a high-CO2-requiring phenotype. Biochemical characterization of recombinant proteins verified that Synechocystis harbors an active serine hydroxymethyltransferase, and, contrary to higher plants, expresses a glycolate dehydrogenase instead of an oxidase to convert glycolate to glyoxylate. The mutation of this enzymatic step, located prior to the branching of phosphoglycolate metabolism into the plant-like C2 cycle and the bacterial- like glycerate pathway, resulted in glycolate accumulation and a growth depression already at high CO2. Similar growth inhibitions were found for a single mutant in the plant-type C2 cycle and more pronounced for a double mutant affected in both the C2 cycle and the glycerate pathway after cultivation at low CO2. These results suggested that cyanobacteria metabolize phosphoglycolate by the cooperative action of the C2 cycle and the glycerate pathway. When exposed to low CO2, glycine decarboxylase knockout mutants accumulated far more glycine and lysine than wild-type cells or mutants with inactivated glycerate pathway. This finding and the growth data imply a dominant, although not exclusive, role of the C2 route in cyanobacterial phosphoglycolate metabolism. [ABSTRACT FROM AUTHOR]

Transgenic Arabidopsis thaliana and Nicotiana tabacum plants that express ictB, a gene involved in HCO[SUP-,SUB3] accumulation within the cyanobacterium Synechococcus sp. PCC 7942, exhibited significantly faster photosynthetic rates than the wild-types under limiting but not under saturating CO[SUB2] concentrations. Under conditions of low relative humidity, growth of the transgenic A. thaliana plants was considerably faster than the wild-type. This enhancement of growth was not observed under humid conditions. There was no difference in the amount of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) detected in the wild-types and their respective transgenic plants. Following activation in vitro, the activities of RubisCO from either low- or high-humidity-grown transgenic plants were similar to those observed in the wild-types. In contrast, the in vivo RubisCO activity, i.e. without prior activation, in plants grown under low humidity was considerably higher in ictB-expressing plants than in their wild-types. The CO[SUB2] compensation point in the transgenic plants that express ictB was lower than in the wild-types, suggesting that the concentration of CO[SUB2] in close proximity to RubisCO was higher. This may explain the higher activation level of RubisCO and enhanced photosynthetic activities and growth in the transgenic plants. These data indicated a potential use of ictB for the stimulation of crop yield. [ABSTRACT FROM AUTHOR]

The occurrence of a photorespiratory 2-phosphoglycolate metabolism in cyanobacteria is not clear. In the genome of the cyanobacterium Synechocystis sp. strain PCC 6803, we have identified open reading frames encoding enzymes homologous to those forming the plant-like C2 cycle and the bacterial-type glycerate pathway. To study the route and importance of 2-phosphoglycolate metabolism, the identified genes were systematically inactivated by mutagenesis. With a few exceptions, most of these genes could be inactivated without leading to a high-CO2-requiring phenotype. Biochemical characterization of recombinant proteins verified that Synechocystis harbors an active serine hydroxymethyltransferase, and, contrary to higher plants, expresses a glycolate dehydrogenase instead of an oxidase to convert glycolate to glyoxylate. The mutation of this enzymatic step, located prior to the branching of phosphoglycolate metabolism into the plant-like C2 cycle and the bacterial- like glycerate pathway, resulted in glycolate accumulation and a growth depression already at high CO2. Similar growth inhibitions were found for a single mutant in the plant-type C2 cycle and more pronounced for a double mutant affected in both the C2 cycle and the glycerate pathway after cultivation at low CO2. These results suggested that cyanobacteria metabolize phosphoglycolate by the cooperative action of the C2 cycle and the glycerate pathway. When exposed to low CO2, glycine decarboxylase knockout mutants accumulated far more glycine and lysine than wild-type cells or mutants with inactivated glycerate pathway. This finding and the growth data imply a dominant, although not exclusive, role of the C2 route in cyanobacterial phosphoglycolate metabolism. [ABSTRACT FROM AUTHOR]