Diurnal light utilization efficiency of phytoplankton is decreased by elevated CO2 concentration: a mesocosm experiment

The effects of ocean acidification on marine primary production are of general concern in light of increases in oceanic CO2 uptake. However, little is known regarding the effects of elevated CO2 on the physiological performance of phytoplankton in eutrophic waters during the daytime. A mesocosm study was performed to investigate the diurnal photosynthetic behaviour of phytoplankton under ambient (400 μatm; LC) and elevated (1000 μatm;HC) CO2 concentrations. Elevated CO2 accelerated the relative electron transport rate (rETR) of phytoplankton and increased the efficiency of photosystem II (PSII) open reaction centers to capture light (Fv’ / Fm’), especially after peak irradiance at noon. However, increased light from PSII was not efficiently utilized by photosynthetic carbon fixation but dissipated by other photoprotective pathways. Dissipation helps to retard the photoinhibition rate in the morning and accelerate the PSII activity recovery rate in the afternoon. Under such circumstances, phytoplankton in HC mesocosms decreased their pigment contents and functional absorption cross-section of PSII (?PSII) to reduce light capture and prevent further potential photoinhibion. In general, these intracellular adjustments eventually lead to the total amount of fixed carbon in HC mesocosms decreasing by 31 % per day. Cellular particulate organic carbon (POC) and nitrogen (PON) content was also 26.8 % and 26.4 % lower in HC mesocosms, respectively. These results implied that elevated CO2 decreases diurnal light utilization efficiency in eutrophic water.

Xing T., Wu Y., Ding J. & Gao K., in press. Diurnal light utilization efficiency of phytoplankton is decreased by elevated CO2 concentration: a mesocosm experiment. Fundamental and Applied Limnology. Article (subscription required).


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