Abstract
The impact of blue-green light incubation on short-term diurnal, daily, and integrated water column estimates of whole water (> 0.2 pm) and Synechococcus-sp ecific 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 upper water 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-i rradiance (P-I) curves, chlorophyll (Chi) and phycoerythrin (PE) concentrations, and cell abundance of PE-rich cyanobacterial S y n e c h o c o c c u s spp. and Chl-fluorescing algae, were m easured within samples from the surface, PE maximum, Chi maximum, and the base of the euphotic zone. S y n e c h o c o c c u s spp. dominated ultra phytoplankton communities down to the light depths of the PE maximum (3 to 7 % surface illumination, 10), with maxima in cell abundance routinely located at light depths > 5 0 % I0. Blue-green and white light incubation conditions generally did not affect light-saturated rates of photosynthesis (Plndx) but blue-green light routinely did provide much higher estimates of light-limited rates of photosynthesi s (alpha). For size-fractioned subpopulations dominated by S y n e c h o c o c c u s spp., blue-green light values of alpha were > 5-fold greater than white light estimates. Compared to white light estimates, blue- green light estimates of total (> 0.2 pm) daily integrated water column primary productivity were 6 to 13 % higher, while the contribution of S y n e c h o c o c c u s spp. to overall primary production rose from between 57 and 61 % to between 73 and 84 %. From the surface down to about 5 % I0, the PE content of S y n e c h o c o c c u s cells increased with decreasing light and/or increasing inorganic nitrogen availability. Increases in S y n e c h o c o c c u s PE/cell occurred in direct proportion to blue-green light m easurements of photosynthetic quantum efficiency, further indicating that these cyanobacteria are physiologically well suited to harvest photosynthetieal ly utilizeable light throughout a large portion of the euphotic zone.
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