Rising CO2 and increased light exposure synergistically reduce marine primary productivity

Abstract

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

Resistance of Arctic phytoplankton to ocean acidification and enhanced irradiance

“…Furthermore, whereas previous studies have demonstrated interactions between OA and high light, causing amplified high-light stress under shortterm exposure (Gao et al 2012b;Hoppe et al 2015), we did not observe such effects. In fact, prior to the dilution, we even observed decreasing electron transport and enhanced energy conversion efficiency (P max -ETR RCII and P max -J C /n PSII ; Table 4) with increasing pCO 2 under LL, hinting towards potentially beneficial OA effects under low irradiances (Rokitta and Rost 2012).…”

Changes in Bioenergetics Associated with Ocean Acidification and Climate Changes

“…), elevated CO 2 remarkably enhanced their growth [5,6], with their CO 2 concentrating mechanisms (CCMs) being down-regulated. In diatoms, similar trend has been reported under laboratory conditions (low light and constant temperature), however, under high levels of sunlight, the diatoms grow much slower under the OA treatment compared to ambient CO 2 condition [7]. Mechanistically, downregulation of CCMs under elevated pCO 2 can save energy for active uptake of inorganic C, the saved energy could be employed to stimulate growth under light-limiting condition and to exacerbate light stress under over-saturating light levels, leading to up-regulated photorespiration.…”

“…Mechanistically, downregulation of CCMs under elevated pCO 2 can save energy for active uptake of inorganic C, the saved energy could be employed to stimulate growth under light-limiting condition and to exacerbate light stress under over-saturating light levels, leading to up-regulated photorespiration. That is, the changed carbonate chemistry due to OA is supposed to increase light stress, as indicated in enhanced nonphotochemical quenching (NPQ) in surface phytoplankton assemblages in the South China Sea [7]. Nevertheless, increased NPQ under OA do not always couple with decreased growth or photosynthetic carbon fixation [6].…”

Ocean Acidification Alters the Photosynthetic Responses of a Coccolithophorid to Fluctuating Ultraviolet and Visible Radiation

“…It is only under ideal conditions that there is a strong linear correlation between PSII photochemistry and photosynthetic carbon fixation (Genty et al, 1989), although in situ photochemical yield showed a good correlation with primary productivity (Kolber and Falkowski, 1993;Suggett et al, 2001). Increased carbon loss by photorespiration or respiration was found under high irradiance in HC-acclimated phytoplankton (Gao et al, 2012b;Yang and Gao, 2012). An increase in the water-water cycle (Asada, 1999) could also drain electrons from PSII without contributing to carbon fixation.…”