Photoreduction of O₂ Primes and Replaces CO₂ Assimilation

Abstract

A mass spectrometer with a membrane inlet system was used to monitor directly gaseous components in a suspension of algae. Using labeled oxygen, we observed that during the frrst 20 seconds of illumination after a dark period, when no net 02 evolution or CO2 uptake was observed, 02 evolution was normal but completely compensated by 02 uptake. Similarly, when CO2 uptake was totally or partially inhibited, 02 evolution proceeded at a high (near maximal) rate. Under all conditions, 02 uptake balanced that fraction of the 02 evolution which could not be accounted for by CO2 uptake.From these observations we concluded that 02 and CO2 are in direct competition for photosynthetically generated reducing power, with 02 being the main electron acceptor during the induction process and under other conditions in which CO2 reduction cannot keep pace with 02 evolution. The After a period of darkness, photosynthesis has a distinct induction phase in which there is a lag (zero rate) of net 02 evolution and its associated CO2 uptake (14,17,19). It is unlikely that the lag is due to the O2-evolving system; in isolated chloroplasts, 02 evolution is virtually maximal from the onset of illumination. In this communication we will describe experiments in which a mass spectrometer with a special inlet and a fast mass stepper system was used to monitor simultaneously 02 evolution, 02 uptake, and CO2 uptake by algae during and after the induction period. A preliminary report of some of these experiments was presented earlier (15). MATERIALS AND METHODSThe mass spectrometer inlet system used in these experiments is similar to the one developed earlier in this laboratory (5). The reaction vessel is a 1. trometer vacuum system through a circular window (-16 mm2) of 1 mil silicone rubber membrane (MEM 213, General Electric Co.) that forms part of the wall of the cavity; it transmits only dissolved gases and some water vapor from the liquid phase.Since this sampling system does not require equilibration between a gas and a liquid phase, the mass spectrometer signals rather accurately reflect the gaseous components in the reaction vessel. The time response of this system is limited by the mass spectrometer, its associated vacuum system, and the transport characteristics of the membrane; the response time of our present system end-to-end is 2 to 3 sec.The quadropole mass spectrometer (Extranuclear Laboratories, Inc.) is programmed using a peak selection stepper system developed in-house. This system monitors up to eight masses, stepped cyclically. In the experiments described below we moni- Algae suspensions were illuminated using a 1000-w projector lamp focussed on the reaction vessel. To minimize heating effects, the light beam passed through a 30-cm water bath containing copper sulfate and an OG-3 (Schott) filter.The usual assay procedure (unless stated otherwise) was as follows. Five ml of cells (cell density 0.3%, v/v) were concentrated to 0.1 ml by centrifugation and transferred to the reaction vessel using a Pasteur pipette (15...

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

“…In one of their experiments, it is apparent that the fastest ratesof 02 uptake occurred whenCO2 fixation (as monitored by 1602 evolution) was the lowest (12 (19). It was postulated that the rapid photoreduction of 02 after a period of darkness served as a priming reaction for the photosynthetic apparatus (19 Unlike previous work with cyanobacteria (3,8,9,12), recent work by Sueltemeyer et al (23,24) with Chlamydomonas has demonstrated an increased rate of 02 uptake with increased DIC concentration. It was concluded that the increased rate of 02 uptake was due to enhanced photoreduction in a Mehler-type reaction, and that extra ATP needed for increased rates of DIC uptake was synthesized as a result (23,24).…”

“…They found that light increased the rateof 1802 uptake, but inhibition occurred athigh DIC concentrations (12). In one of their experiments, it is apparent that the fastest ratesof 02 uptake occurred whenCO2 fixation (as monitored by 1602 evolution) was the lowest (12 (19). It was postulated that the rapid photoreduction of 02 after a period of darkness served as a priming reaction for the photosynthetic apparatus (19 Unlike previous work with cyanobacteria (3,8,9,12), recent work by Sueltemeyer et al (23,24) with Chlamydomonas has demonstrated an increased rate of 02 uptake with increased DIC concentration.…”

“…Involvement of RuBP oxygenase in the 02 uptake process is ruled out because uptake continued in the presence of iodoacetamide (Fig. 2B), which would have prevented RuBP regeneration (19). Also, uptake was not inhibited by 500 IAM CN-, a concentration that completely inhibited CO2 fixation, presumably by inhibiting the RuBP carboxylase/oxygenase (25).…”

Tansley Review No. 112

“…In Scenedesmus, however, electron transport rates are similar regardless of whether CO # or O # is used as an electron acceptor, even at saturating light intensities (Radmer & Kok, 1976). This implies either a lack of photosynthetic control in these organisms or the existence of an unidentified ATP sink able to regenerate ADP and phosphate consumed during O # reduction.…”

“…3. However, as pointed out previously, an ATP sink or uncoupling mechanism has to be invoked to support rates of O # reduction that are comparable to rates of CO # fixation (Radmer & Kok, 1976).…”