The Nostoc-Gunnera symbiosis: carbon fixation and translocation.

The in vitro specific activity of ribulose‐1,5‐bisphosphate carboxylase (Rubisco; EC 4. 1. 1. 39) and the dark and light in vivo CO2 fixation activities were determined in the cyanobiont of Gunnera. Compared to the free‐living isolate Nostoc PCC 9231, the in vitro Rubisco activity was high, while the in vivo CO2 fixation was very low. Light did not significantly influence CO2 fixation if the cyanobiont was left in the sliced Gunnera tissues, while a small light stimulation was found for CO2 fixation of the freshly‐isolated cyanobiont. The adjacent non‐infected Gunnera tissue showed a very low CO2 fixation. A rapid translocation of fixed 14CO2 from leaves towards apical parts of the plant was apparent, in particular to the symbiotic tissue. The 14C label appeared mainly in soluble form in this tissue and was rapidly catabolised as shown by 14C chase experiments. Also, short‐term experiments revealed that maximum 14C accumulation occurred in the symbiotic tissue showing the highest rates of nitrogen fixation (Söderbäck et al. 1990), about 10–15 mm from the plant apex. The data were taken to indicate that there is a modification in the photosynthetic light reaction of the cyanobiont and that the cyanobiont lives heterotrophically in the dark on photo‐synthate rapidly delivered from nearby leaves of the host plant. [ABSTRACT FROM AUTHOR]