Blue light effects on light-limited rates of photosynthesis: relationship to pigmentation and productivity estimates for Synechococcus populations from the Sargasso Sea

Abstract

The impact of blue-green light incubation on short-term diurnal, daily, and integrated water column estimates of whole w ater (> 0.2 pm) and Synechococcus-specific photosynthesis was assessed throughout the euphotic zone at 2 stations in the Sargasso Sea. Replicate samples were incubated under both tungsten white light and broad band blue-green light, where the latter simulated light quality within the upp e r w ater column of the open sea. Diurnal variations in size-fractioned (0.2-0.6 pm, 0.6-1 pm, and 1-5 pm) blue-green vs white light photosynthesis-irradiance (P-I) curves, chlorophyll (Chi) a n d ph y c o ery th rin (PE) c oncentrations, a n d cell a b u n d a n c e of PE-rich c y an o b a cte ria l Synechococcus spp. and Chl-fluorescing algae, were m easured within sam ples from the surface, PE maximum, Chi maximum, a nd the base of the euphotic zone. S yn ech o co ccu s spp. dom inated u ltra­ p h y to p la n k to n com m u n itie s do w n to the light d e p th s of the PE m a x im u m (3 to 7 % su rface illumination, 10), with m ax im a in cell a b u n d a n c e routinely loca ted at light d e p th s > 5 0 % I0. B l u e -g r e e n a n d white light in cu b a tio n conditions g e n e ra lly did not affect lig h t - s a t u r a t e d ra te s of p h o to sy n th e s is (Plndx) but blu e-g reen light routinely did provide m uch higher estim ates of light-limited rates of photosynthesis (alpha). For size-fractioned subpopulations dominated by Synechococcus spp., blue-green light values of alpha were > 5-fold greater than white light estimates. C om pared to white light estimates, blueg reen light estim ates of total (> 0.2 pm) daily integrated w ater column primary productivity w ere 6 to 13 % higher, while the con trib utio n of S y n e c h o c o c c u s spp. to overall p rim a ry p r o d u c tio n rose from b e tw e e n 57 a n d 61 % to b e t w e e n 73 a n d 84 %. From the surfa ce d o w n to a b o u t 5 % I0, the PE c o n te n t of Synechococcus cells increased with decreasing light and/or increasing inorganic nitrogen availability. Increases in S ynech oco ccu s PE/cell occurred in direct proportion to blu e-g re en light m easurem ents of photosynthetic quantum efficiency, further indicating that these cyanobacteria are physiologically well suited to harvest photosynthetieally utilizeable light throughout a large portion of the euphotic zone.