Macromolecular dynamics of the photosynthetic system over a seasonal developmental progression in Spartina alterniflora

Spartina alterniflora Loisel. is a dominant primary producer in salt marsh intertidal zones along the Bay of Fundy of New Brunswick and Nova Scotia, Canada, where it has a 5-month growing season from May to September. We quantified key subunits of the S. alterniflora photosynthetic system (chlorophyll, PsbA, PsaC, AtpB, RbcL) over a seasonal developmental progression to determine resource allocations and estimate capacities for key subprocesses of photosynthesis catalyzed by light-harvesting antennae, photosystems I and II (PSI and PSII, respectively), ATP (adenosine triphosphate) synthase, and RuBisCO (ribulose bisphosphate carboxylase–oxygenase). Early in the season, all of the macromolecules peaked in concentration around day 159, but then declined as leaves elongated. Later in the season, just after flowering, chlorophyll b, PsbA, PsaC, and AtpB peaked again before declining at the end of the season. RbcL, however, did not exhibit a second late-season peak, but continually declined from the early-season peak. Estimates of metabolic flux per PSII and per RuBisCO were closely parallel until late in the season, indicating coordinated regulation of catalytic turnover per complex. Early in the season, carbon fixation per RuBisCO peaked near the expected turnover rate (kcat) for a C4 plant, but then declined. The effective absorbance cross sections of PSII varied seasonally, reflecting significant regulation of antenna performance across the season.