Physiological and thylakoid proteome analyses of Anabaena sp. PCC 7120 for monitoring the photosynthetic responses under cadmium stress

Abstract

Photosynthetic organisms are highly susceptible to cadmium (Cd) as it interferes with the structural and functional aspects of the photosynthesis. As a major group of photosynthetic prokaryotes, cyanobacteria are also affected by Cd. The inhibitory effects of Cd on cyanobacterial photosynthesis have been studied from an early stage, but the mechanism of Cd toxicity is still unclear. Therefore, we investigated the photosynthetic responses of Anabaena sp. PCC 7120 under Cd stress (LC50) at physiological and thylakoid proteome levels via chlorophyll a fluorescence measurements and blue native (BN)-SDS PAGE, respectively. The findings revealed that Cd exposure triggered the intracellular ROS production and negatively affected the photosynthetic performance. With Cd exposure, effective photochemical quantum yield of PSII (Y(II)) and photochemical quenching efficiency (qP and qL) were significantly reduced, whereas the non-regulated energy dissipation (Y(NO)) was increased. Fast fluorescence kinetic measurements showed that Cd exposure resulted in the inactivation of PSII reaction centres, which interfered with the transfer of energy from antenna complexes to reaction centre, decreased the ability to convert excitation energy into electron transport, increased thermal dissipation, and thus decreased linear electron flow efficiency. Conversely, the efficiency of electron transfer from intermediate carriers to final PSI acceptors was increased, which might be related to the induction of PSI-based cyclic electron flow. This assumption was confirmed by an enhanced post-illumination fluorescence transient. Consistent with these observations, BN-SDS PAGE showed that Cd exposure significantly decreased PSII subunit proteins accumulation, but slightly increased PSI proteins accumulation. Additionally, only minor reductions in the accumulation of ATP synthase/NDH-1 and Cytochrome b6f subunit proteins were observed. The transcript levels of most of the selected subunit proteins were in accordance with BN-SDS PAGE. Collectively, our experiments show that a shift to PSI-based cyclic electron flow was a protective strategy of Anabaena sp. PCC 7120 against Cd-induced PSII photo-damage.