Carbon Oxysulfide Inhibition of the CO₂-Concentrating Process of Unicellular Green Algae

Abstract

Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, 02 evolution, dissolved inorganic carbon accumulation, and photosynthetic CO2 fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a C02-concentrating process. This COS inhibition can be attributed to inhibition of extenal HC03-conversion to CO2 and OH-by the carbonic anhydrase component of an active CO2 pump. At a low pH of 5 to 6, COS stimulated 02 evolution during photosynthesis by algae with low CO2 in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO2. Although COS had less effect on HC03-accumulation at pH 9 by a HCO3-pump in Scenedesmus, COS reduced 02 evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO2 and H2S, its inhibition of the CO2 pump activity and photosynthesis is not accurate, when measured by 02 evolution, by NaH'4CO3 accumulation, or by 14CO2 fixation. Unicellular green algae, Chlamydomonas, (3, 18), Dunaliella (12), Chlorella (17), Scenesdesmus (1 1, 22), and species of Cyanobacteria (1), when grown or adapted at air levels of C02, form a DIC3-concentrating mechanism, which is suppressed when grown with elevated levels of CO2 (1-5% CO2 in air v/v). Depending on the pH of the medium and algal species, both HCO3-and/or CO2 may enter the cells. Many unicellular green algae have only a CO2 pump, but Scenedesmus has both a CO2 pump and HC03-pump (22

Relationships between carbonyl sulfide (COS) and CO2 during leaf gas exchange

“…This is significant because both CO 2 and COS react with CA and could act as competitive inhibitors, or become toxic at very high concentrations (Miller et al. , 1989; Goyal et al. , 1991).…”

Oxygen inhibition of dissolved inorganic carbon uptake in unicellular green algae

“…Air-adapted cells of unicellular green algae accumulate DIC internally to a level several fold higher than that of the external medium, normally measured by the accumulation of unfixed DIC using the silicone oil centrifugation technique Goyal et al 1992). In the protocols for the DIC-uptake experiment, cells were first allowed to photosynthesize in an oxygen electrode to deplete all internal and external DIC.…”

“…One of the problems in assaying the presence of the DIC pump by the silicone oil centrifugation procedure is the inconsistency in the data on the amount and time of saturation of intracellular DIC Goyal and Tolbert 1989;Goyal et al 1992). Therefore, we have re-evaluated experimental protocols for assaying DIC-pump activity by the silicone oil centrifugation procedure.…”

Factors affecting production of COS and CS2 in Leucaena and Mimosa species

“…The toxicity of COS to humans, insects, * Fax no: + 17065420914 nematodes, and bacteria is unknown. COS is toxic to green algae (Goyal et al, 1992), but otherwise neither COS nor CS2 is toxic to plants at low concentrations (Taylor and Selvidge, 1984).…”