Carbon dioxide and light are two major prerequisites of photosynthesis. Rising CO2 levels in oceanic surface waters in combination with ample light supply are therefore often considered stimulatory to marine primary production(1-3). Here we show that the combination of an increase in both CO2 and light exposure negatively impacts photosynthesis and growth of marine primary producers. When exposed to CO2 concentrations projected for the end of this century(4), natural phytoplankton assemblages of the South China Sea responded with decreased primary production and increased light stress at light intensities representative of the upper surface layer. The phytoplankton community shifted away from diatoms, the dominant phytoplankton group during our field campaigns. To examine the underlying mechanisms of the observed responses, we grew diatoms at different CO2 concentrations and under varying levels (5-100%) of solar radiation experienced by the phytoplankton at different depths of the euphotic zone. Above 22-36% of incident surface irradiance, growth rates in the high-CO2-grown cells were inversely related to light levels and exhibited reduced thresholds at which light becomes inhibitory. Future shoaling of upper-mixed-layer depths will expose phytoplankton to increased mean light intensities(5). In combination with rising CO2 levels, this may cause a widespread decline in marine primary production and a community shift away from diatoms, the main algal group that supports higher trophic levels and carbon export in the ocean.National Basic Research Program of China [2009CB421207, 2011CB200902]; National Natural Science Foundation of China [41120164007, 40930846]; Changjiang Scholars and Innovative Research Team project [IRT0941]; Ministry of Science and Technology [S2012GR0290]; United States National Science Foundation Division of Ocean Sciences [0942379, 0962309, 1043748]; German Ministry of Education and Research; 111 project; State Key Laboratory of Marine Environmental Science (Xiamen University); German Academic Exchange Service (DAAD

Ocean Acidification Multiple Driver Interactions Overall, most ocean-based global change biology studies have used single and/or double stressors in laboratory tests. This overview examines the combined effects of OA with other features such as warming, solar UV radiation, and deoxygenation, focusing on primary producers.

CO₂/pH perturbation experiments were carried out under two different pCO₂ levels (39.3 and 101.3 Pa) to evaluate effects of CO₂-induced ocean acidification on the marine diatom Phaeodactylum tricornutum. After acclimation (>20 generations) to ambient and elevated CO₂ conditions (with corresponding pH values of 8.15 and 7.80, respectively), growth and photosynthetic carbon fixation rates of high CO₂ grown cells were enhanced by 5% and 12%, respectively, and dark respiration stimulated by 34% compared to cells grown at ambient CO₂. The K_1/2 (dissolved inorganic carbon, DIC) for carbon fixation increased by 20% under the low pH and high CO₂ condition, reflecting a decreased photosynthetic affinity for HCO₃⁻ or/and CO₂ and down-regulated carbon concentrating mechanism (CCM). In the high CO₂ grown cells, the electron transport rate from photosystem II (PSII) was photoinhibited to a greater extent at high levels of photosynthetically active radiation, while non-photochemical quenching was reduced compared to low CO₂ grown cells. This was probably due to the down-regulation of CCM, which serves as a sink for excessive energy. Increasing seawater pCO₂ and decreasing pH associated with atmospheric CO₂ rise may enhance diatom growth, down-regulate their CCM, and enhanced their photo-inhibition and dark respiration. The balance between these positive and negative effects on diatom productivity will be a key factor in determining the net effect of rising atmospheric CO₂ on ocean primary production

Leafy thalli of the red alga Porphyra yezoensis Ueda, initiated from conchospores released from free-living conchocelis, were cultured using aeration with high CO 2 . It was found that the higher the CO 2 concentration, the faster the growth of the thalli. Aeration with elevated CO 2 lowered pH in dark, but raised pH remarkably in light with the thalli, because the photosynthetic conversion of HCO-to OH-and CO 2 proceeded much faster than the dissociation of hydrated CO 2 releasing H +. Photosynthesis of the alga was found to be enhanced in the seawater of elevated dissolved inorganic carbon (DIC, CO 2 + HC O-+ CO-). It is concluded that the increased pH in the light resulted in the increase of DIC in the culture media, thus enhancing photosynthesis and growth. The relevance of the results to removal of atmospheric CO 2 by marine algae is discussed.

We carried out experiments during an expedition (14 August to 14 September, 2007) that covered up to 250,000 km(2) to investigate the effects of solar UV radiation (UVR, 280-400 nm) on the photosynthetic carbon fixation of tropical phytoplankton assemblages in surface seawater of the South China Sea. From coastal to pelagic surface seawaters, UV-B (280-315 nm) caused similar inhibition, while UV-A (315-400 nm) induced photosynthetic inhibition increased from coastal to offshore waters. UV-B resulted in an inhibition by up to 27% and UV-A by up to 29%. Under reduced levels of solar radiation with heavy overcast, UV-A resulted in enhanced photosynthetic carbon fixation by up to 25% in coastal waters where microplankton was abundant. However, such a positive impact was not observed in the offshore waters where piconanoplankton was more abundant. The daily integrated inhibition of UV-A reached 4.3% and 13.2%, and that of UV-B reached 16.5% and 13.5%, in the coastal and offshore waters, respectively.