Selective and Reversible Inhibition of Active CO₂ Transport by Hydrogen Sulfide in a Cyanobacterium

Abstract

The active transport of CO2 in the cyanobacterium Synechococcus UTEX 625 was inhibited by H2S. Treatment of the cells with up to 150 micromolar H2S + HS-at pH 8.0 had little effect on Nat-dependent HC03-transport or photosynthetic O2 evolution, but CO2 transport was inhibited by more than 90%. CO2 transport was restored when H2S was removed by flushing with N2. At constant total H2S + HS-concentrations, inhibition of CO2 transport increased as the ratio of H2S to HS-increased, suggesting a direct role for H2S in the inhibitory process. Hydrogen sulfide does not appear to serve as a substrate for transport. In the presence of H2S and Na -dependent HC03-transport, the extracellular CO2 concentration rose considerably above its equilibrium level, but was maintained far below its equilibrium level in the absence of H2S. The inhibition of CO2 transport, therefore, revealed an ongoing leakage from the cells of CO2 which was derived from the intracellular dehydration of HC03-which itself had been recently transported into the cells. Normally, leaked CO2 is efficiently transported back into the cell by the CO2 transport system, thus maintaining the extracellular CO2 concentration near zero. It is suggested that CO2 transport not only serves as a primary means of inorganic carbon acquisition for photosynthesis but also serves as a means of recovering CO2 lost from the cell. A schematic model describing the relationship between the CO2 and HCO3-transport systems is presented. 8,13,20) and Anabaena variabilis ( 11) MATERIALS AND METHODS Organisms and GrowthThe unicellular cyanobacterium Synechococcus leopoliensis UTEX 625 (University of Texas Culture Collection, Austin, TX) was grown with air-bubbling (0.05% v/v CO2) in unbuffered Allen's medium at 30°C as described previously (6). Experimental ConditionsCells were washed three times by centrifugation (1 min at l0,OOOg, Beckman microfuge B) and resuspended (7-9 sg Chl * mL-') in 25 mm BTP/HC1 buffer of the appropriate pH.The buffer contained about 15 uM DIC and 5 Mm Na+ as contaminants. Washed cells (6 mL) were subsequently placed in a thermostated (30°C) glass reaction vessel (20 mm diameter) and briefly purged with a stream of N2 to reduce the [02] to less than 75 AM. The chamber was then closed to the atmosphere and the cells were allowed to temperature equilibrate for several minutes in darkness. The reaction vessel contained a port for the inlet capillary of a mass spectrometer and the cell suspension was continuously stirred with a magnetic stirrer during measurements. Light was provided by a 387 www.plantphysiol.org on May 10, 2018 -Published by Downloaded from

High Affinity Transport of CO2 in the Cyanobacterium Synechococcus UTEX 625

“…The concentration of the stock CO2 was measured with MS as described previously (14) and found to be between 3.6 and 3.9 mM.…”

“…It now appears that both CO2 and HCO3-serve as substrates for a DIC transport system (1,3,13,14,18,22,31). Transport is light dependent, requires Na+, and is blocked by inhibitors of energy metabolism such as DCMU, carbonyl cyanide mchlorophenylhydrazone, and diethylstilbestrol (13,18,21,23,28).…”

“…Several mechanisms for DIC transport have been proposed to account for these observations (3,14,25,27). In low DICgrown cells of Synechococcus PCC 7942 (3,25) a "front-end" mechanism with vectorial, CA-like properties is thought to convert extracellular HCO3-to CO2 for use by a C02-utilizing transporter.…”

“…With Synechococcus UTEX 625, we have recently found that carbonyl sulfide (22) and H2S (14) inhibit CO2 transport without greatly affecting Na+-dependent HCO3-transport, and monensin, Li+, and the lack of millimolar concentrations of Na+ inhibit HCO3-transport much more effectively than HC03-transport can be separated into discrete events in this cyanobacterium, we have proposed that two distinct paths for DIC entry into the cell exist (14). One is constitutive and utilizes CO2 from the medium, whereas the other is inducible and utilizes HCO3-from the medium.…”

Influence of hydrogen sulfide on the composition of cell wall proteins of Saccharomyces cerevisiae yeast

“…Встановлено, що в порівняно невисоких концентраціях він пригнічує лю-мінесценцію в бактерій Vibrio fischeri (більше 0,1 мМ) [21], оксигенний фотосинтез у ціанобактерій (від 5 мкМ) [14,15,24]. Гідроген сульфід знижує швидкість погли-нання кисню, що може свідчити про інгібування дихального метаболізму цією спо-лукою, та пригнічує активність ізоцитратдегідрогенази (КФ 1.1.1.41) у клітинах дріжджів Saccharomyces cerevisiae та Pichia guilliermondii [1].…”