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

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Abstract
The impact of blue-green light incubation on short-term diurnal, daily, and integrated water column estimates of whole water (> 0.2 .mu.m) 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 upper water column of the open sea. Diurnal variations in size-fractioned (0.2-0.6 .mu.m, 0.6-1 .mu.m, and 1-5 .mu.m) blue-green vs white light photosynthesis-irradiance (P-I) curves, chlorophyll (Chl) and phycoerythrin (PE) concentrations, and cell abundance of PE-rich cyanobacterial Synechococcus spp. and Chl-fluorescing algae, were measured within samples from the surface, PE maximum, Chl maximum, and the base of the euphotic zone. Synechococcus sp. dominated ultraphytoplankton communities down to the light depths of the PE maximum (3 to 7% surface illumination, Io), with maxima in cell abundance routinely located at light depths .gtoreq. 50% Io. Blue-green and white light incubation conditions generally did not affect light-saturated rates of photosynthesis (Pmax) but blue-green light routinely did provide much higher estimates of light-limited rates of photosynthesis (alpha). For size-fractioned subpopulations dominated by Synechococcus spp., blue-green light values of alpha were .gtoreq. 5-fold greater than white light esitmates. Compared to white light estimates, blue-green light estimates of total (> 0.2 .mu.m) daily integrated water column primary producitvity were 6 to 13% higher, while the contribution of Synechococcus spp. to overall primary production rose from between 57 and 61% to between 73 and 84%. From the surface down to about 5% Io, the PE content of Synechococcus cells increased with decreasing light and/or increasing inorganic nitrogen availability. Increases in Synechococcus PE/cell occurred in direct proportion to blue-green light measurements of photosynthetic quantum efficiency, further indicating that these cyanobacteria are physiologically well suited to harvest photosynthetically utilizable light throughout a large portion of the euphotic zone.