PARENT SESSION
Posters P2D Marine photosynthesis and production. Abstracts (711-719)

Light dependent oxygen consumption in nitrogen fixing Trichodesmium . Allen Milligan^*,1, Ilana Berman-Frank², Yoram Gerchman³, Charles Dismukes³, Paul Falkowski¹, ¹ Institute of Marine and Coastal Studies, New Brunswick, New Jersey, USA² Life Sciences Department, Ramat-Gan, Israel³ Department of Chemistry, Princeton, New Jersey, USA

ABSTRACT- Nitrogen fixation is catalysed by the enzyme nitrogenase, which is irreversibly inactivated by molecular oxygen in vitro. In cyanobacteria, oxygen may be consumed through respiration and/or two light dependent reactions, the oxygenase activity of RubisCO (photorespiration) and the photosynthetic reduction of O₂ termed pseudocyclic photophosphorylation or the Mehler reaction. The cyanobacterium Trichodesmium is unusual in that it has no heterocysts and is able to fix N₂ while evolving O₂. In Trichodesmium, nitrogenase is compartmentalized in a fraction of the cells along a trichome. However, active photosynthetic components (such as PSI and PSII complexes, Rubisco, carboxysomes) are found in all cells, even those harboring nitrogenase. Protection against oxygen in Trichodesmium is a complex interaction comprising a spatial and temporal segregation of the photosynthetic, respiratory and nitrogen fixation processes. In addition, oxygen consumption through activity of photosystem I (Mehler activity) was hypothesized to provide additional protection for nitrogenase (Kana 1993, Berman-Frank et al. 2001). We provide evidence that Mehler activity is greatly enhanced under nitrogen-fixing growth relative to cultures grown with nitrate as a nitrogen source. Comparison of rates reveals that N₂-fixing Trichodesmium consumes O2 through Mehler activity at unprecedented rates ( ca. 50% of gross O₂ production) which is about twice as fast as Synechococcus (ca. 25% of gross O₂ production). When cultures are grown on nitrate, early light period Mehler activity is similar to N2 fixing cultures but quickly drops to low and constant rates through the rest of the light period. Mehler rates are about 10% of gross production when nitrate is used as a nitrogen source.

KEY WORDS: Nirtogen fixation, Mehler, Trichodesmium