Abstract
Ocean acidification (OA) due to atmospheric CO2 rise is expected to influence marine primary productivity. In order to investigate the interactive effects of OA and light changes on diatoms, we grew Phaeodactylum tricornutum, under ambient (390 ppmv; LC) and elevated CO2 (1000 ppmv; HC) conditions for 80 generations, and measured its physiological performance under different light levels (60 µmol m−2 s−1, LL; 200 µmol m−2 s−1, ML; 460 µmol m−2 s−1, HL) for another 25 generations. The specific growth rate of the HC-grown cells was higher (about 12–18%) than that of the LC-grown ones, with the highest under the ML level. With increasing light levels, the effective photochemical yield of PSII (Fv′/Fm′) decreased, but was enhanced by the elevated CO2, especially under the HL level. The cells acclimated to the HC condition showed a higher recovery rate of their photochemical yield of PSII compared to the LC-grown cells. For the HC-grown cells, dissolved inorganic carbon or CO2 levels for half saturation of photosynthesis (K1/2 DIC or K1/2 CO2) increased by 11, 55 and 32%, under the LL, ML and HL levels, reflecting a light dependent down-regulation of carbon concentrating mechanisms (CCMs). The linkage between higher level of the CCMs down-regulation and higher growth rate at ML under OA supports the theory that the saved energy from CCMs down-regulation adds on to enhance the growth of the diatom.
Highlights
Atmospheric CO2 concentration is expected to reach 800– 1000 ppmv by the end of this century due to relentless consumptions of fossil fuels and exacerbated deforestation [1]; at the same time the oceans are taking up CO2 from the atmosphere at a rate of about 1 million tons per hour, leading to ocean acidification (OA) [2]
Growth rate of P. tricornutum was enhanced by the elevated CO2 concentration under photosynthesis-limited, halfsaturated and saturated light levels, being consistent with previous studies [11,12,36,37]
This result is consistent with our previous results obtained under low sunlight levels, but contradictory to that under high sunlight levels with daytime average PAR over 200 mmol m22 s21 (Table 4), that led to inhibited growth rate under the OA condition [12]
Summary
Atmospheric CO2 concentration is expected to reach 800– 1000 ppmv by the end of this century due to relentless consumptions of fossil fuels and exacerbated deforestation [1]; at the same time the oceans are taking up CO2 from the atmosphere at a rate of about 1 million tons per hour, leading to ocean acidification (OA) [2]. Elevated CO2 concentrations are shown to enhance [8,9,10,11,12,13,14], have no effect [8,12,20,21] or even inhibit [10,12,14,20,22] growth rates of diatom species. Elevated CO2 in the ocean increases its availability to photosynthetic organisms, the reduced pH can influence the acid-base balance of cells [23]. The elevated CO2 and reduced pH levels can interact with solar radiation and temperature, showing synergistic, antagonistic or balanced effects [24]. The mechanisms involved in the responses to OA of diatoms need to be further explored
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