An assessment of photosynthetic downregulation in cyanobacteria from the Gunnera–Nostoc symbiosis

Abstract

Summary • The transition of Nostoc colonies from a free-living to a symbiotic condition, in the Nostoc–Gunnera association, involves a downregulation of photosynthesis, which is not associated with an altered heterocyst frequency or a loss of photosynthetic pigments. • Fluorescence assessments and measurements of Hill activity using the artificial electron acceptor 2,6-dichloro-p-benzoquinine (DCIP) indicate a major reduction in photosystem II (PSII) photochemistry in symbiotic vegetative cells. • The downregulation of PSII activity in symbiotic cells was associated with a smaller effective pool size of the primary electron acceptors (QA) and plastoquinone (PQ), and a reduction in the efficiency of light utilization by individual PSII units. The reduction in PSII electron transport was correlated with a partial degradation of the D1 protein and the presence of C-terminal degradation products. • While it has been proposed that PSII is absent in the symbiotic state, our results suggest that PSII units are present but their photochemical efficiency is much reduced. We suggest that modification to the D1 protein and a decrease in the effective pool size of the primary electron acceptors of PSII may signal a shift from an autotrophic to a heterotrophic mode of metabolism.