Photoheterotrophic Fluxome in Synechocystis sp. Strain PCC 6803 and Its Implications for Cyanobacterial Bioenergetics

Abstract

This study investigated metabolic responses in Synechocystis sp. strain PCC 6803 to photosynthetic impairment. We used 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU; a photosystem II inhibitor) to block O 2 evolution and ATP/NADPH generation by linear electron flow. Based on 13 C-metabolic flux analysis ( 13 C-MFA) and RNA sequencing, we have found that Synechocystis sp. PCC 6803 employs a unique photoheterotrophic metabolism. First, glucose catabolism forms a cyclic route that includes the oxidative pentose phosphate (OPP) pathway and the glucose-6-phosphate isomerase (PGI) reaction. Glucose-6-phosphate is extensively degraded by the OPP pathway for NADPH production and is replenished by the reversed PGI reaction. Second, the Calvin cycle is not fully functional, but RubisCO continues to fix CO 2 and synthesize 3-phosphoglycerate. Third, the relative flux through the complete tricarboxylic acid (TCA) cycle and succinate dehydrogenase is small under heterotrophic conditions, indicating that the newly discovered cyanobacterial TCA cycle (via the ␥-aminobutyric acid pathway or ␣-ketoglutarate decarboxylase/succinic semialdehyde dehydrogenase) plays a minimal role in energy metabolism. Fourth, NAD(P)H oxidation and the cyclic electron flow (CEF) around photosystem I are the two main ATP sources, and the CEF accounts for at least 40% of total ATP generation from photoheterotrophic metabolism (without considering maintenance loss). This study not only demonstrates a new topology for carbohydrate oxidation but also provides quantitative insights into metabolic bioenergetics in cyanobacteria. C yanobacteria, which first appeared in shallow marine settings as early as 3 billion years ago (1, 2), are now widely distributed in diverse nutrient and light environments (3-5). They can perform oxygenic photosynthesis and respiration simultaneously in the same compartment (6, 7). Cyanobacteria contain two photosystems to harvest light energy. Photosystem II (PSII) splits water and transports electrons sequentially through plastoquinone (PQ), cytochrome b 6 f, plastocyanin, and photosystem I (PSI), forming a linear electron flow (LEF) to produce ATP and NADPH. Alternatively, a cyclic electron flow (CEF) runs around PSI to generate ATP (8, 9). The cyanobacterial CEF involves respiratory electron transport reactions and PSI enzymes. NAD(P)H dehydrogenase complex (NDH-1) oxidizes NADPH and provides electrons for the PQ pool. Then the electrons from PQ flow to PSI and ferredoxin to regenerate NADPH via ferredoxin-NADP ϩ reductase (FNR). The CEF can regulate ATP and NADPH ratios for CO 2 fixation and respiration in plants (10). Cyanobacteria have a much larger PSI content than plants. For example, the PSI/PSII ratio in Synechocystis sp. strain PCC 6803 is about 5, suggesting the significant role of PSI in energy metabolism (6). To decipher photosynthesis mechanisms, researchers often use the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) to block the PQ binding site of PSII so that the LEF is inactivated. Under DCMU treatm...

Cyanobacterial carbon metabolism: Fluxome plasticity and oxygen dependence

“…S6), showing its transcription under the dark condition. A previous study also revealed that RuBisCO can fix CO 2 without a fully functional Calvin cycle, as might be found in the dark condition (You et al, ). NaH 13 CO 3 is also incorporated into aspartate and glutamate (6–8%), derivatives from the TCA cycle, through the PEP carboxylase reaction.…”

“…Numbers after condition symbol designate independent data sets. References: (D), this study; H1, (You et al, ); H2, (Nakajima et al, ); M1, (You et al, ); M2, (Nakajima et al, ); A1, (Young et al, ); and A2, (Xiong et al, ). The fluxes are shown in their absolute values in mmol/g DW/h.…”

“…Based on our data and previous studies, we compared Synechocystis 6803 fluxomes under various conditions (Fig. ): dark chemoheterotrophic, photoheterotrophic, photomixotrophic, and photoautotrophic conditions (Nakajima et al, ; Xiong et al, ; You et al, , ; Young et al, ). The fluxes are shown as the absolute values in separate 3D bar plots and reveal cyanobacterial metabolic responses to environmental changes.…”

Cyanobacterial photo-driven mixotrophic metabolism and its advantages for biosynthesis

“…Moreover, the herbicide DCMU specifically blocks the PQ binding site of PSII and interrupts the water splitting. Under such inhibitory circumstances, cyanobacteria can up-regulate both glucose oxidation (using the OPP pathway) and the CEF around PSI to compensate for the requirement of NADPH and ATP, respectively [39]. In this case, cyanobacterial RuBisCO is still functioning for CO 2 fixation.…”

Biophotonics for Biofuel Upgradation

“…Oscillatoria limnetica is capa-ble of anaerobic, sulphide dependent photo-assimilation of carbon dioxide [15]. Rana et al (2014) has been reported that fresh cow dung is a good source of different cyanobacterial species [16].…”