Photosynthesis and Cell Growth Trigger Degradation of Phycobilisomes during Nitrogen Limitation

Abstract

Under nitrogen (N)-limited conditions, the non-N2-fixing cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803) actively grows during early stages of starvation by performing photosynthesis but gradually stops the growth and eventually enters dormancy to withstand long-term N limitation. During N limitation, Synechocystis 6803 cells degrade the large light-harvesting antenna complex phycobilisomes (PBSs) presumably to avoid excess light absorption and to reallocate available N to essential functions for growth and survival. These two requirements may be driving forces for PBS degradation during N limitation, but how photosynthesis and cell growth affect PBS degradation remains unclear. To address this question, we examined involvements of photosynthesis and cell growth in PBS degradation during N limitation. Treatment of photosynthesis inhibitors and shading suppressed PBS degradation and caused non-bleaching of cells during N limitation. Limitations of photosynthesis after initial gene responses to N limitation suppressed PBS degradation, implying that photosynthesis affects PBS degradation in a post-translational manner. In addition, limitations of cell growth by inhibition of peptidoglycan and fatty acid biosynthesis, low growth temperature and phosphorous starvation suppressed PBS degradation during N limitation. Because decreased photosynthetic activity led to decreased cell growth, and vice versa, photosynthesis and cell growth would inseparably intertwine each other and affect PBS degradation during N limitation in a complex manner. Our data shed light on the coordination mechanisms among photosynthesis, cell growth and PBS degradation during N limitation.

The Unique Light-Harvesting System of the Algal Phycobilisome: Structure, Assembly Components, and Functions

“…Degradation of PBS plays an important role in photoprotection, cell maintenance, growth, and development in a constantly changing environment. During nitrogen limitation, PBSs in cells are degraded to avoid excessive light uptake and to allocate effective nitrogen to functions essential for growth and survival [56]. As a large nutrient reserve, the degradation of PBSs can provide essential amino acids for metabolic processes, and low levels of photosynthesis and loss of pigments are essential for cell survival during nitrogen, sulphur, or phosphorus depletion.…”