Active Transport of Inorganic Carbon Increases the Rate of O₂ Photoreduction by the Cyanobacterium Synechococcus UTEX 625

Abstract

Chlorophyll a fluorescence ofSynechococcus UTEX 625 was quenched during the transport of inorganic carbon, even when CO2 rfxation was inhibited by iodoacetamide. Measurements with a pulse modulation fluorometer showed that at least 75% of the quenching was due to oxidation of QA, the primary acceptor of photosystem II. Mass spectrometry revealed that transport of inorganic carbon increased the rate of 02 photoreduction. Hence, 02 could serve as an electron acceptor to allow oxidation of QA even in the absence of CO2 fixation.The active transport of HC03-or CO2 causes quenching of Chl a fluorescence in Synechococcus UTEX 625 even when CO2 fixation is inhibited by iodoacetamide (14, 16). Quenching does not occur in the presence of DCMU which suggests that the quenching is due to oxidation of the acceptor QA of PSII (14). In the presence ofiodoacetamide, CO2 reduction cannot be a means for oxidation of QA (14, 16), but it is well established that 02 can also be reduced by noncycic electron transport (1,4,6 In this communication, we present evidence that an increase in qQ is the major reason for the drop in Chl a fluorescence yield resulting from DIC transport. We also show that DIC transport increases the rate of 02 photoreduction. MATERIALS AND METHODSGrowth and Preparation of Cells. Synechococcus UTEX 625 was grown with air bubbling as previously described (5). Cells were washed and resuspended in 25.0 mm BTP/23.5 mM HCI buffer, pH 8.0, which contained low levels (about 15 gM) of DIC (13).Fluorimetry. The fluorescence yield ofChl was monitored with the modulation fluorometer described by Schreiber et al. (21) (PAM-101, H. Walz, D-8521, Effeltrich, FRG). Actinic light to drive DIC uptake and CO2 fixation was from a tungsten-halogen projector lamp with PFR varied with neutral density filters. Fluorescence yield was monitored with a weak (about 2 ME. m2. s_') pulse modulated beam (frequency, I05 s-) and the oxidation state of QA was measured at 20 s intervals with a high intensity white-light flash (1600 uE.m-2.s' for 500 ms). The PAM-101 amplifier system only monitors the fluorescence emission elicited by the modulated light beam, therefore, during the 500 ms saturating flash of actinic light, only changes in fluorescence emission due to changes in the oxidation state ofQA are recorded (21). Fluorescence yield was monitored at the same time as 02 and DIC fluxes were being measured by MS. The Fm was determined in the presence of 20 uM DCMU (14). The Fo was estimated with the modulated light beam at a very low PFR and a frequency of 1.6 x 10-3 s-'. For cyanobacteria, we defined F, which is essentially the same as (F,)m of Schreiber et al. (21), as .Mass Spectrometry. Dissolved 1602, 1802, '2C02, and "3CO2 (m/z = 32, 36, 44, and 45) in the cell suspension (details in figure legends) were sequentially measured at 7-to 15-s intervals using a magnetic sector mass spectrometer (VG Gas Analysis, Middlewich, England, model MM 14-80 SC) equipped with a membrane inlet system (15). Data were stored on magnetic disc fo...

Evolved physiological responses of phytoplankton to their integrated growth environment

“…Bicarbonate accumulation in the cytoplasm is thought to have an ATP : HCO K 3 or NADH : CO 2 cost ratio of 1 (but see Ritchie et al 1996). This ATP demand for CCM operation can be supplied directly by thylakoid pathways described in §5 (figure 6), including the pseudocyclic pathway (Mehler; Miller et al 1988;Sü ltemeyer et al 1993). Rubisco levels are also elevated under low CO 2 conditions to overcome slow catalytic rates (Tortell et al 2000), representing an additional resource cost to the cell.…”

Inorganic Carbon-Stimulated O2 Photoreduction Is Suppressed by NO2- Assimilation in Air-Grown Cells of Synechococcus UTEX 625

“…When CO, fixation was inhibited by IAC or glycolaldehyde, net O, evolution was not observed, but the C,-stimulated rates of O, evolution and O, reduction were nearly equal to the rates of O, evolution observed with C,-saturated photosynthesis (Miller et al, 1988). The occurrence of photochemical quenching with O, photoreduction when CO, fixation was inhibited clearly indicated that linear electron transport was stimulated by the development of the intracellular Ci pool (Miller et al, 1991;Badger and Schreiber, 1993).…”

“…Calibration for CO, was performed by injecting small amounts of K,',C03 in the stoppered cuvette in BTP/HCl buffer and calculating the equilibrium concentration of CO, at 30°C and pH 8 (Miller et al, 1988). The mass spectrometer was also calibrated by injecting the K,CO, into 0.2 N HC1 in the reaction cuvette.…”

“…Concentratrons of dissolved I6O,, "O,, and I3CO2 (m/z 32, 36, and 451, respectively) in the 6 mL of cell suspension in the reaction chamber were measured using a magnetic sector mass spectrometer (VG Gas Analysis, Middbewich, UK; model MM 14-80 SC) equipped with a membrane inlet connected to the chamber as described previously (Miller et al, 1988). Calibration for CO, was performed by injecting small amounts of K,',C03 in the stoppered cuvette in BTP/HCl buffer and calculating the equilibrium concentration of CO, at 30°C and pH 8 (Miller et al, 1988).…”

“…Photoreduction of O, in cyanobacteria such as Syneckococcus UTEX 625 is greatly stimulated when an internal Ci pool is allowed to develop in the light, whether CO, fixation is permitted or not (Miller et al, 1988). When CO, fixation was inhibited by IAC or glycolaldehyde, net O, evolution was not observed, but the C,-stimulated rates of O, evolution and O, reduction were nearly equal to the rates of O, evolution observed with C,-saturated photosynthesis (Miller et al, 1988).…”