Untitled - 中国植物生理与分子生物学学会

Ocean acidification enhances UVR-induced inhibition of
photosynthesis and calcification of a calcifying phytoplankton
Kunshan Gao 1 * ,Zuoxi Ruan 2 ,Virginia E. Villafañe 3 , E. Walter Helbling 3
1 State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, 361005,
China; 2 Institute of Marine Biology, Science Center, Shantou University, Shantou, 515063, China; 3
Estación de Fotobiología Playa Unión, Playa Unión & Consejo Nacional de Promoción Científica y
Tecnológica (CONICET), Rawson, Chubut, Argentina (corresponding author; ksgao@xmu.edu.cn)
CO 2 , as a greenhouse gas, is absorbed by and released from seawater, depending on the
difference of its partial pressure between the air and the seawater. Increased atmospheric CO 2
concentration due to industrial activities is known to have enhanced the process of CO 2 dissolution
into seawater from air. However, such a capacity of seawater to sink CO 2 has been greatly reduced,
while CO 2 make the seawater less alkaline (ocean acidification). Ocean acidification, by 2100, will
reduce the pH of surface ocean by 0.3-0.4 unit, an increase of [H + ] by 1-1.5 times. Such a pH change
definitely alters many chemical processes in seawater, and therefore affects biological activities. It
has been suggested to negatively affect biological calcification that is important for corals, mussels
and calcifying marine plants (oralline algae and coccolithophores).
Biological production in surface ocean functions as a biological pump for the ocean to sink CO 2
from the atmosphere. However, how photosynthetic and other metabolic processes would be affected
by ocean acidification associated with increased atmospheric CO 2 concentration remains almost
unknown due to lack of experimental data. We aimed to investigate how photosynthesis and
calcification of calcifying algae respond to solar UVR under acidified conditions when their
calcification process is inhibited, with a hypothesis that calcified structures may play a protective
role in shielding off harmful UV radiation.
Calcification of Emiliania huxleyi is known to be negatively affected by elevated CO 2
concentration. However, it is unknown if the reduced coccolith formation would affect the cell’s
response to solar radiation since the coccoliths may shield off PAR (400-700 nm) as well as UVR
(280-400 nm). We cultured the cells at pH 8.2, 7.9, and 7.6 (corresponding pCO 2 350, 780, and 1460
ppm) and examined their photosynthesis and calcification under different radiation treatments with
or without UVR. Under the acidified conditions, rate of calcification was reduced and the coccolith
layer became significantly thinner in the cells of Emiliania huxleyi grown under the acidified
conditions, though their photosynthetic carbon fixation was not affected. The cells with less amount
of coccoliths showed higher inhibition caused by UVR for both photosynthesis and calcification.
UVR-induced inhibition increased from about 45% at pH 8.2 to about 90% at pH 7.6 for
acidification, but only from about 5% at pH 8.2 to about 15% at pH 7.6, leading a tremendous
reduction of the C/P ratio. The coccolith layer for the cells grown at pH 8.2 can shield off about 25%
UVR, suggesting that the coccoliths play a protective role in the cellular physiological processes
against harmful UVR. Analysis of the optical transmission between the cells with or without
coccoliths indicated that the cells with coccoliths received about 20-25% less UVR (300-400 nm),
10% less blue light, and 15-22% less other part of the PAR.
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Keywords:Ocean acidification; UVR; photosynthesis; calcification of a calcifying phytoplankton
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