Photoacclimation to abrupt changes in light intensity by Phaeodactylum tricornutum and Emiliania huxleyi: the role of calcification

Abstract

Phytoplankton experience strong and abrupt variations in light intensity. How cells cope with these changes influences their competitiveness in a highly dynamical environment. While a considerable amount of work has focused on photoacclimation, it is still unknown whether processes specific of phytoplankton groups (e.g. calcification and silicification) influence their response to changing light. Here we show that the diatom Phaeodactylum tricornutum and the coccolithophore Emiliania huxleyi respond to an abrupt increase in irradiance by increasing carbon fixation rates, decreasing light absorption through the decrease of light-harvesting pigments and increasing energy dissipation through the xanthophyll cycle. In addition, E. huxleyi rapidly increases calcium carbonate precipitation in response to elevated light intensity, thereby providing an additional sink for excess energy. Differences between the 2 species also emerge with regard to the magnitude and timing of their individual responses. While E. huxleyi show a pronounced decrease in chlorophyll a and fucoxanthin cellular contents following increased light intensity, P. tricornutum has a faster increase in diadinoxanthin quota, a slower decrease in F v /F m (ratio of variable to maximum fluorescence) and a stronger increase in organic carbon fixation rate during the first 10 min. Our findings provide further evidence of species-specific responses to abrupt changes in light intensity, which may partly depend on the phytoplankton functional groups, with coccolithophores having a supplementary path (calcification) for the rapid dissipation of excess energy produced after an abrupt increase in light intensity. These differences might influence competition between coexisting species and may therefore have consequences at the community level.

A data–model synthesis to explain variability in calcification observed during a CO<sub>2</sub> perturbation mesocosm experiment

“…The reason for a sharp increase in the molar PIC : Cassimilation ratio at low growth rates in our model is because of a down regulation of LHC. Such model behaviour is in agreement with the interpretation of Barcelos e Ramos et al (2012), who describe calcification as a process into which the coccolithophores can channel excess energy. In order to maximise (optimise) growth rate under nutrient-depleted and high-light conditions, the model allocates more resources and energy to support nutrient acquisition than to the LHC (indicated by low f 0 LHC cocco values).…”

“…The molar ratio (f PIC ) is assumed to increase when the fraction of resources allocated to the LHC of a cell (f 0 LHC cocco ) decreases. According to our model approach, the process of calcification can be interpreted as an additional pathway for dissipating excess energy (Barcelos e Ramos et al, 2012), as is the case under high-light conditions when chlorophyll a synthesis rates diminish (induced by a reduction of f 0 LHC cocco ). On the one hand, PIC formation becomes enhanced under high-light conditions, while fewer resources become allocated to LHC.…”

The modulating effect of light intensity on the response of the coccolithophore Gephyrocapsa oceanica to ocean acidification

“…1B, 3B,D). Light availability is likely to affect the supply of inorganic carbon to photosynthesis, calcification and growth in general (Zondervan et al 2002;Barcelos e Ramos et al 2012;Rokitta and Rost 2012).…”

“…Coccolithophores play an important role in the marine carbon cycle through the fixation of inorganic carbon via photosynthesis, as well as the precipitation of calcium carbonate (Rost and Riebesell 2004). Coccolith formation has been suggested to reduce the risk of photo-damage of coccolithophores under high light conditions either by shading the cells like a sunshade (Braarud et al 1952) or by contributing to excess energy dissipation (Barcelos e Ramos et al 2012;Xu and Gao 2012). Declining pH generally reduces calcification rates (Bach et al 2011;Riebesell and Tortell 2011), which may then put the cells at higher risk to suffer from photo-inhibition.…”

Environmental controls on coccolithophore calcification

“…However, Nielsen (1995) found a higher light-saturated rate of photosynthesis at a given inorganic carbon concentration in highly calcified cells than in cells with little calcification. Alternative evidence suggests that increased calcification in Emiliania huxleyi can increase photochemical quenching of excess excitation energy following a steep increase in PAR (Barcelos e Ramos et al 2012). Coccoliths may also be able to focus PAR to the plastids in deeper waters where photosynthesis is PAR-limited (Nanninga & Tyrrell 1996, Raven & Waite 2004.…”