Photosynthesis and Photorespiration in Algae

Abstract

The CO2 exchange of several species of fresh water and marine algae was measured in the laboratory to determine whether photorespiration occurs in these organisms. The algae were positioned as thin layers on filter paper and the CO2 exchange determined in an open gas exchange system. In either 21 or 1% 02 there was little difference between 14CO2 and 12CO2 uptake. Apparent photosynthesis was the same in 2, 21, or 50% 02. The compensation points of all algae were less than 10 id 1-1.CO2 or 14CO2 evolution into C02-free air in the light was always less than the corresponding evolution in darkness. These observadons are inconsistent with the proposal that photorespiration exists in these algae.Algae are C3 plants but there is still controversy regarding the existence or nature of photorespiration (7,8,33). Many algae have been shown to produce glycolate (28,33) and to possess the enzymes of the glycolate pathway (28, 33). The inhibition of photosynthesis by 02 or the Warburg effect was first shown in algae, and has since been extensively studied (2, 34). Some workers, however, have found no effect of 02 on CO2 fixation in algae (1, 16, 18) and many algae show low compensation points (4,25,35) and release less CO2 in the light compared to the dark (7,17).In higher plants, CO2 evolution and CO2 exchange are the most extensively studied aspects of photorespiration (36). In algae, although CO2 evolution during photosynthesis has been implied (33), few studies have been performed on CO2 exchange because of the difficulties imposed by the aqueous medium. In view of the paucity of information and the diversity of observations and opinions on CO2 exchange in algae, we decided to investigate the CO2 exchange of several species of algae using an open gas analysis system such as that used for measurement of CO2 exchange in higher plants (15,26). MATERIALS AND METHODSThe following algae were obtained from the Indiana University Culture Collection, and were grown on Gorham's culture medium No. 11 (21) Scotia. These marine species were grown on a shaker without aeration with a 16-hr photoperiod.All species were grown at 25 C and a quantum flux density of 80 to 100 ,ueinsteins m-2 sec-'. All cultures were axenic with the exception of Mougeotia. Cells were harvested during linear growth phase, and concentrated, where necessary, by settling and decanting the medium. Algae were suspended as an "artificial leaf" (12) to facilitate gas exchange and rapid changes in gas composition. The cells were filtered on Whatman No. 3 filter paper, producing an even double layer of cells (4 x 4 cm). This square was cut out and enclosed in the leaf chamber for the gas analysis measurements. The total volume of medium on the filter paper was approximately 0.3 ml. Similar results were obtained when the algae were suspended on 10-gm Nitex nylon mesh (B and S. H.The open gas analysis system was as previously described (15, 26). Measurements were started about 30 min after the algae were placed in the leaf chamber because the rate of apparent ...

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