Photosynthetic Characteristics of Smaller and Larger Cell Size-Fractioned Phytoplankton Assemblies in the Daya Bay, Northern South China Sea

Cell size of phytoplankton is known to influence their physiologies and, consequently, marine primary production. To characterize the cell size-dependent photophysiology of phytoplankton, we comparably explored the photosynthetic characteristics of piconano- (<20 µm) and micro-phytoplankton cell assemblies (>20 µm) in the Daya Bay, northern South China Sea, using a 36-h in situ high-temporal-resolution experiment. During the experimental periods, the phytoplankton biomass (Chl a) in the surface water ranged from 0.92 to 5.13 μg L⁻¹, which was lower than that in bottom layer (i.e., 1.83–6.84 μg L⁻¹). Piconano-Chl a accounted for 72% (mean value) of the total Chl a, with no significant difference between the surface and bottom layers. The maximum photochemical quantum yield (F_V/F_M) of Photosystem II (PS II) and functional absorption cross-section of PS II photochemistry (σ_{PS II}) of both piconano- and micro-cells assemblies varied inversely with solar radiation, but this occurred to a lesser extent in the former than in the latter ones. The σ_{PS II} of piconano- and micro-cell assemblies showed a similar change pattern to the F_V/F_M in daytime, but not in nighttime. Moreover, the fluorescence light curve (FLC)-derived light utilization efficiency (α) displayed the same daily change pattern as the F_V/F_M, and the saturation irradiance (E_K) and maximal rETR (rETR_max) mirrored the change in the solar radiation. The F_V/F_M and σ_{PS II} of the piconano-cells were higher than their micro-counterparts under high solar light; while the E_K and rETR_max were lower, no matter in what light regimes. In addition, our results indicate that the F_V/F_M of the micro-cell assembly varied quicker in regard to Chl a change than that of the piconano-cell assembly, indicating the larger phytoplankton cells are more suitable to grow than the smaller ones in the Daya Bay through timely modulating the PS II activity.