Macromolecular and physiological consequences of interactive photosynthetic acclimation to light and carbon availability in the cyanobacterium Synechococcus elongatus.

摘要

Photosynthetic organisms must acclimate to multiple, fluctuating environmental factors, including light and nutrient status. Acclimation to changes in one factor has consequences for how an organism can acclimate to changes in another. I studied how high (high Ci) and low inorganic carbon (low C i) concentrations affected growth, photosynthesis, and organization of the photosynthetic apparatus, and then how Ci influences photosynthetic acclimation to changes in light in the freshwater cyanobacterium Synechococcus elongatus (PCC7942). Induced Carbon Concentrating Mechanisms (CCMs) and reorganised electron transport systems allow cells to achieve similar photosynthetic and growth rates under continuous light under high and low Ci. Under changing light, however, high Ci cells quickly exploited increased light and outgrew low Ci cells, which were constrained to pre-light shift growth rates for many hours. Macromolecular acclimation to high light occurred primarily through decreases of functional PSII per PSI and PSII per Rubisco. High Ci cells achieved increased macromolecular pools of photosystems, Rubisco, and nitrogen metabolism by reallocating biosynthesis away from their large initial investment into phycobilisomes. Low Ci cells, however, focused a more limited reallocation from a smaller phycobilisome pool primarily to Rubisco. Before and immediately after the light shift both high and low Ci cells had more potential for PSII electron transport than was realised by net O2 evolution. In high light, high Ci cells acclimated to a lower excess of PSII electron transport over realised net O2 evolution, while that excess remained large in low Ci cells, possibly to power CCMs. A similar high-light acclimation occurred in high Ci cells exposed to rapidly fluctuating light, simulating the light environment of shallow aquatic habitats, while low Ci cells increased their PSII pool to buffer the light fluctuations. Generally, similar rates of macromolecular and physiological acclimation occurred in both cell types when expressed on a generational time scale normalised to growth rates, but on a chronological timescale, acclimation was slow in low Ci cells. The macromolecular and functional reorganisations in low Ci cells confer the advantage of photosynthetic competence at low Ci, but this gain occurred at the expense of slower acclimation relative to rates of environmental change.