Oxygen Isotope Fractionation during Photosynthesis in a Blue-Green and a Green Alga

Abstract

Oxygen isotope fractionation (180/160) at the natural abundance level has been measured during photosynthesis of a blue-green and a green alga. When sufficient attention is paid to removal of contaminating air 02 before and during the experiments, then the photosynthetic 02 evolved, as compared to the water 02, had an average difference of -0.36%0 for a blue-green alga and -0.80%0 for a green alga. MATERIALS AND METHODSThe blue-green alga, Agmenellum quadruplicatum, strain PR-6, was grown in modified ASP-2 (9); the green alga, Chlorella pyrenoidosa, high temperature strain TX-71105, was grown in EDTA Knops (6). Axenic growth occurred at 35 C in test tube cultures, with continuous bubbling of 1 % CO2 in air. Illumination consisted of four (two on each side of the bath) F20T12/ CWX fluorescent lamps 11 cm from the growth tubes. Cells for isotopic experiments were purposely harvested at different stages of growth by centrifugation, then injected to give various final cell concentrations (see Table I) into a sealed 4-liter bottle containing 3 liters of fresh medium, magnetically stirred. The bottle was held at 35 C and illuminated on two sides by seven 100-w tungsten lamps, about 22 cm from the center of the bath. Oxygen content of the algal suspension during the degassing phase or during periods of photosynthesis or respiration was monitored with an 02 electrode (Gilson Medical Electronics Clark-Type electrode, OX700, connected to a Keithley 150A microvoltammeter and Brown recorder), see Figure 1. The 02 electrode was routinely calibrated using air-saturated medium (checked by the Winkler method) and an 02-free solution (0.16 M Na2SO3 in 10 mM Na2B407).The removal of atmospheric 02 from an algal suspension was attained by (a) evacuation, using a water aspirator, in con-

Effects of temperature and carbon source on the isotopic fractionations associated with O2 respiration for 17O/16O and 18O/16O ratios in E. coli

“…Aerobic respiration across a range of organisms has been found to exhibit a  of 0.518 . We followed previous workers (e.g., Luz and Barkan, 2005) Stevens et al, 1975;Guy et al, 1987;Guy et al, 1993;Helman et al, 2005;Eisenstadt et al, 2010). Measureable isotopic fractionations have been observed for some algae in which the  18 O value of newly photosynthesized O 2 is up to 7‰ higher than of the source H 2 O; this has been attributed to an unknown reductive process hypothesized to occur in algal cells after O 2 formation (Eisenstadt et al, 2010).…”

Studying the oxidation of water to molecular oxygen in photosynthetic and artificial systems by time-resolved membrane-inlet mass spectrometry

“…However, Metzner (1978) suggested that instead hydrogen carbonate (bicarbonate; HCO − 3 ) is the immediate substrate for O 2 formation that is subsequently replenished by the reaction of CO 2 with H 2 O. This hypothesis was discounted for long because the isotopic equilibration between 18 O-water and HCO − 3 is too slow to account for early isotope labeling studies (Ruben et al, 1941; Stemler and Radmer, 1975; Stevens et al, 1975; Radmer and Ollinger, 1980). Due to the discovery that a carbonic anhydrase (CA) activity is associated with PSII (Lu and Stemler, 2002; Villarejo et al, 2002; Moskvin et al, 2004) the “bicarbonate-as-substrate hypothesis” needed to be re-investigated with refined expriments.…”

Photosynthetic Fractionation of the Stable Isotopes of Oxygen and Carbon

“…Previous investigators used whole cells or leaves (Dole and Jenks, 1944;Vinogradov et al, 1969;Stevens et al, 1975;Metzner et al, 1979); consequently, there was potential for simultaneous 0 2 uptake by the competing processes of respiration, photorespiration, and the Mehler reaction. Results of some of these studies were superficially consistent with a reverse isotope effect favoring the production of isotopically heavy 02.…”

“…It is interesting to note that Metzner et al (1979) observed the greatest apparent fractionation of photosyn- 01,1993 thetic O2 (+15%0) under aerobic conditions known to favor photorespiration (Canvin et al, 1980). Stevens et al (1975) examined photosynthetic O2 accumulated by cultures of a cyanobacterium and a green alga at DIC concentrations high enough to keep the rate of photorespiration negligible. The difference in 6I8O between the source water and the collected O2 was near zero, indicating that there could not be a reverse isotope effect in the photolysis of water.…”