RAPID OXYGEN CYCLING IN TRICHODESMIUM-THIEBAUTII

Oxygen uptake and evolution were measured in Trichodesmium thiebautii trichomes with O-18 membrane inlet mass spectrometry. Samples were collected in the vicinity of the Bahamas Islands and eastern Caribbean Sea. Trichodesmium exhibited high rates of oxygen uptake in the dark and in the light compared with previously studied cyanobacteria. Oxygen uptake in the dark averaged 23% of the maximum gross oxygen evolution rate and was suppressed minimally at low irradiances or after the addition of cyanide. Oxygen uptake in light increased between 50 and 200 muEinst m-2 s-1 and was inhibited by 3-(3,4-)-dichlorophenyl)-1,1-dimethylurea (DCMU), indicating Mehler reaction activity. I estimate that 48% of the total photosynthetic linear electron flow went to oxygen reduction at moderate but nonsaturating irradiances. Light compensation points ranged from 50 to 300 muEinst m-2 S-1 with values highest for midday samples. Rapid oxygen cycling may contribute to the protection of nitrogenase by reducing the oxygen tension within the cell or colony during oxygenic photosynthesis and by providing a mechanism (pseudocyclic photophosphorylation) for generating excess ATP for nitrogen fixation.