Photoacclimation involves the modification of components of the light and dark reactions to optimize photosynthesis following changes in available light. All of the energy required for photosynthesis comes from linear electron transport through PSII and PSI and is dependent upon the amount of light harvested by PSII relative to PSI (a* PSII and a* PSI ). The amount of light harvested is determined by the effective absorption cross-sections (r PSII , r PSI ) and cellular contents of the PSII and PSI reaction center complexes (RCII, RCI). Here, we examine the effective absorption cross-sections and reaction center contents for calcifying (B11) and noncalcifying (B92) strains of the globally important coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler when grown under various photon flux densities (PFDs). The two strains displayed different ''strategies'' of acclimation. As growth PFD increased, B11 preferentially changed r and the cellular content of chl a per cell over PSU ''size'' (the total cellular chl a content associated with the reaction center complexes); strain B92 preferentially changed PSU size over the cellular content of reaction complexes. Neither strategy was specifically consistent with the majority of previous studies from other microalgal species. For both strains, cellular light absorption for PSII and PSI was maintained close to unity across the range of growth PFDs since changes of r PSII and r PSI were reciprocated by those of RCIIs and RCIs per cell. Our results demonstrate a significant adaptive flexibility of E. huxleyi to photoacclimate. Finally, we calculated the amount of chl a associated with either photosystem to consider our interpretations of photoacclimation based on conventional determinations of PSU size.

Phytoplankton play a key role in determining the partitioning of CO 2 between the atmosphere and the ocean on seasonal, interannual, and millennial time scales. The magnitude of biological draw-down of atmospheric CO 2 and C storage in the oceans is affected by concurrent changes in other environmental factors, like nutrient supply. Furthermore, variations in carbon-to-nitrogen (C:N) and carbonto-phosphorus (C:P) assimilation ratios modify the oceanic CO 2 storage capacity. Here we show that increased atmospheric CO 2 concentration enhances CO 2 fixation into organic matter by a noncalcifying strain of Emiliania huxleyi (Lohmann) Hay & Mohler only under certain conditions, namely high light and nutrient limitation. Enhanced organic matter production was accompanied by marked deviations of the C:N:P ratio from the canonical stoichiometry of marine particulate matter of 106:16:1 (C:N:P) known as the Redfield ratio. Increased cell organic carbon content, C:N, and C:P were observed at high light when growth was either nitrogen or phosphorus limited. Elevated CO 2 led to further increases in the particulate C:N and C:P ratios. Enhanced CO 2 uptake by phytoplankton such as E. huxleyi, in response to elevated atmospheric CO 2 , could increase carbon storage in the nitrogen-limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO 2 levels.

The critical nitrogen-to-phosphorous ratio (N : P) defines the transition between N and P limitation of growth rate and is not a biological constant. To test the effect of environmental conditions on the critical N : P, we cultured the diatom Chaetoceros muelleri in chemostats with inflow nitrate : phosphate ratios ranging from 5 to 90 mol N (mol P)Ϫ1 at two photon flux densities (PFDs; 50 and 700 mol photons m Ϫ2 s Ϫ1). The nitrate : phosphate ratio marking the transition between N and P limitation increased from between 15-30 mol N : mol P at the high PFD to 45 mol N : mol P at the low PFD. The particulate ratio marking this transition increased from 16-23 mol N : mol P at low PFD to 35 mol N : mol P at high PFD. Cell phosphorus and RNA contents decreased with increasing N : P ratio up to the critical N : P ratio for each PFD, above which they remained stable. In contrast, cell dry weight, chlorophyll a, C, N, and protein were not influenced by nitrate : phosphate in the inflow medium, although they were influenced by PFD. Total protein per RNA increased with increasing N : P ratio at the low light conditions, suggesting increased ratio of protein synthesis per RNA. Our results showed the effect of PFD, growth rate, or both on the critical N : P ratio. Agreement was found in the assessment of the transition between N and P limitation on the basis of nutrient enrichment bioassays, cellular elemental (N and P) quotas, and cell RNA content. Our results are consistent with theoretical predictions of higher N requirements under low light conditions due to the coupling of the photosynthetic mechanism with N uptake. In contrast, P-rich cellular components, such as RNA, were dependent on P availability rather than light.

The response of the coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler to acute exposure to high photon flux densities (PFD) was examined in terms of PSII photoinhibition, photoprotection, and photorepair. The time and light dependencies of these processes were characterized as a function of the photoacclimation state of the alga. Low-light (LL) acclimated cells displayed a higher degree of photoinhibition, measured as decline in Fv /Fm , than high-light (HL) acclimated cells. However, HL cultures were more susceptible to photodamage but also more capable of compensating for it by performing a faster repair cycle. The relation between gross photoinhibition (observed in the presence of an inhibitor of repair) and PFD to which the algae were exposed deviated from linearity at high PFD, which calls into question the universality of current concepts of photoinhibition in mechanistic models. The light dependence of the de-epoxidation state (DPS) of the xanthophyll cycle (XC) pigments on the timescale of hours was the same in cells acclimated to LL and HL. However, HL cells were more efficient in realizing nonphotochemical quenching (NPQ) on short timescales, most likely due to a larger XC pool. LL cells displayed an increase in the PSII effective cross-section (σPSII ) as a result of photoinhibition, which was observed also in HL cells when net photoinhibition was induced by blocking the D1 repair cycle. The link between σPSII and photoinhibition suggests that the population of PSII reaction centers (RCIIs) of E. huxleyi shares a common antenna, according to a "lake" organization of the light-harvesting complex.

There is still considerable uncertainty about the relationship between calcification and photosynthesis. It has been suggested that since calcification in coccolithophorids is an intracellular process that releases CO2 , it enhances photosynthesis in a manner analogous to a carbon-concentrating mechanism (CCM). The ubiquitous, bloom-forming, and numerically abundant coccolithophorid Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler was studied in nutrient-replete, pH and [CO2 ] controlled, continuous cultures (turbidostats) under a range of [Ca(2+) ] from 0 to 9 mM. We examined the long-term, fully acclimated photosynthesis-light responses and analyzed the crystalline structure of the coccoliths using SEM. The E. huxleyi cells completely lost their coccosphere when grown in 0 [Ca(2+) ], while thin, undercalcified and brittle coccoliths were evident at 1 mM [Ca(2+) ]. Coccoliths showed increasing levels of calcification with increasing [Ca(2+) ]. More robust coccoliths were noted, with no discernable differences in coccolith morphology when the cells were grown in either 5 or 9 mM (ambient seawater) [Ca(2+) ]. In contrast to calcification, photosynthesis was not affected by the [Ca(2+) ] in the media. Cells showed no correlation of their light-dependent O2 evolution with [Ca(2+) ], and in all [Ca(2+) ]-containing turbidostats, there were no significant differences in growth rate. The results show unequivocally that as a process, photosynthesis in E. huxleyi is mechanistically independent from calcification.