Ocean acidification interacts with variable light to decrease growth but increase particulate organic nitrogen production in a diatom

Highlights

• Variable light decreased growth rate and pigmentation contents in both LC and HC.

• Cells grown under variable light appeared more tolerant of high light.

• HC and varying light decreased carbon fixation rate but increased POC and PON.

• HC and varying light lead to less primary productivity but more PON per biomass.

Abstract

Phytoplankton in the upper oceans are exposed to changing light levels due to mixing, diurnal solar cycles and weather conditions. Consequently, effects of ocean acidification are superimposed upon responses to variable light levels. We therefore grew a model diatom Thalassiosira pseudonana under either constant or variable light but at the same daily photon dose, with current low (400 μatm, LC) and future high CO2 (1000 μatm, HC) treatments. Variable light, compared with the constant light regime, decreased the growth rate, Chl a, Chl c, and carotenoid contents under both LC and HC conditions. Cells grown under variable light appeared more tolerant of high light as indicated by higher maximum relative electron transport rate and saturation light. Light variation interacted with high CO2/lowered pH to decrease the carbon fixation rate, but increased particulate organic carbon (POC) and particularly nitrogen (PON) per cell, which drove a decrease in C/N ratio, reflecting changes in the efficiency of energy transfer from photo-chemistry to net biomass production. Our results imply that elevated pCO2 under varying light conditions can lead to less primary productivity but more PON per biomass of the diatom, which might improve the food quality of diatoms and thereby influence biogeochemical nitrogen cycles.

Li W., Wang T., Campbell D. A. & Gao K., in press. Ocean acidification interacts with variable light to decrease growth but increase particulate organic nitrogen production in a diatom. Marine Environmental Research. Article (subscription required).

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OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

OUP book