Light-Modulated Responses of Growth and Photosynthetic Performance to Ocean Acidification in the Model Diatom Phaeodactylum tricornutum

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.

Two Southern Ocean diatoms are more sensitive to ocean acidification and changes in irradiance than the prymnesiophyte Phaeocystis antarctica

“…The negative impact of OA even under low-light conditions on the photosynthetic capacity of C. debilis is surprising and has not yet been reported before. One may speculate whether the CCM of C. debilis was downregulated under OA to a lesser extent in the low light treatment compared with that under high light, as previously observed for the temperate diatom P. tricornutum (Li et al 2014), implying higher energetic costs for CCM operation under these conditions. Overall, it is notable that the Southern Ocean phytoplankton species tested so far (our study, Hoogstraten et al 2012a) did not show any OA-dependent stimulation in growth or POC fixation under low light, casting doubt on the beneficial effects of elevated pCO 2 for Southern Ocean phytoplankton species under these conditions.…”

“…In this case, the OA-dependent downregulation of the CO 2 concentrating mechanism (CCM), which elevates the concentration of CO 2 at the site of fixation by Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), implies lower energy expenditures for CCM operation and therefore enables enhanced carbon fixation rates and/or growth particularly under low light conditions (Hopkinson et al 2011). In line with this, several temperate diatom species displayed stimulation in growth under OA and low light (Li and Campbell 2013, Li et al 2014, McCarthy et al 2012. Such responses were, however, not reported for the Antarctic diatoms P. alata (Hoogstraten et al 2012a) and C. brevis (Boelen et al 2011), which both exhibited no OA-dependent change in growth rates in response to limiting or saturating irradiance levels.…”

“…Under OA combined with low light, temperate diatom species often exhibit an OA-dependent stimulation in growth (McCarthy et al 2012, Li and Campbell 2013, Li et al 2014). This phenomenon was ascribed to increased diffusive CO 2 uptake under OA counteracting the decreased capacity and/or affinity for inorganic carbon uptake (Kranz et al 2010, Rokitta and Rost 2012, Beardall and Raven 2013) caused by energy limitation under low light (Giordano et al 2005, Young and Beardall 2005).…”

Increasing copper alters cellular elemental composition (Mo and P) of marine diatom

“…Culturing was carried out in filtered seawater (0.2μm polycarbonate filters) taken from the South China Sea (116 E, 18 N) with Aquil medium enrichment. Culture solutions for cultures were carefully boiled to eliminate any possibility of bacterial contamination (e.g., Gao et al, 2012;Li, Gao, Villafañe, & Helbling, 2012;Li, Xu, & Gao, 2014). The background level of dissolved Cu in the seawater was verified to have a concentration of <1 nmol/L using the method of Wang et al (2012).…”

Ocean acidification stimulates particulate organic carbon accumulation in two Antarctic diatom species under moderate and high natural solar radiation

“…, Rokitta and Rost , Li and Campbell , Li et al. ). Under cold temperatures, however, such energy savings were not found to give much benefit to Antarctic phytoplankton (Kranz et al.…”