Coccolithophores are a group of marine phytoplankton species which are able to precipitate CaCO3 in the form of small calcitic scales (coccoliths) surrounding the organic part of the cell. They contribute about 1–10 % to marine primary production (Poulton et al., 2007) and approximately 50 % to pelagic deep-ocean CaCO3 sediments (Broecker and Clark, 2009). Blooms of coccolithophores can cover up to 8 million km2 of the Earth's surface (Moore et al., 2012) and are considered to be important drivers of biogeochemical cycling (Rost and Riebesell, 2004).

Despite intense research on coccolithophore calcification and its biogeochemical relevance during the last decade, it is still an unresolved question why coccolithophores calcify (Young, 1994; Raven and Crawfurd, 2012). One hypothesis is that the layer of coccoliths surrounding the cell (coccosphere) protects the organism from excess light and UV radiation. This notion is supported by the exceptionally high light tolerance of the surface layer dwelling species Emiliania huxleyi (Nanninga and Tyrell, 1996; Ragni et al., 2008; Gao et al., 2009; Loebl et al., 2010).

Physiological studies investigating the light tolerance of E. huxleyi showed that the radiation wavelength matters in this context. The coccosphere does not seem to constitute a protection against very high intensities of photosynthetically active radiation (PAR) since noncalcifying E. huxleyi cells are as resistant to photoinhibition as their calcifying counterparts (Nanninga and Tyrrell, 1996). This is in clear contrast to the influence of stressful ultraviolet radiation (UVR) on the cells where results from different physiological experiments support a protective role of the coccoliths (Gao et al., 2009, 2012; Guan and Gao, 2010). Protection from UVR or high light exposures by coccoliths may either work by physically shading intracellular organelles or by strongly scattering light, which is certainly a feature of coccolithophore blooms (Balch et al., 1996; Voss et al., 1998). The underlying mechanisms, however, are not well understood and warrant further investigations.

UVR strongly contributes to photoinhibition of photosystem II (e.g., Hakala-Yatkin et al., 2010) and effectively inhibits repair processes (Ragni et al., 2008). Therefore, it is likely that the coccoliths protect PSII repair from UV inhibition. In this study we explore in more detail how different PAR and UV radiation (280–400 nm) treatments affect calcified and naked E. huxleyi cells. Specifically we address the question of whether the coccosphere of E. huxleyi helps the cells to withstand stressful levels of PAR and/or UV radiation and whether calcification influences photochemical performance.

The coccolith layer of E. huxleyi absorbed both visible and UV radiation. It reduced the transmission of visible radiation (400–700 nm) by 7.5 %, that of UV-A (315–400 nm) by 14.1 % and that of UV-B by 18.4 % (280–315 nm) relative to decalcified cells and by 6.5 % for PAR, 6.6 % for UV-A and 5.1 % for UV-B, relative to naked cells (Fig. 1). The specific growth rate of calcifying E. huxleyi strain (PML B92/11) was about 2 times higher than that of the naked strain (CCMP 2090) (P < 0.05) when grown at 500 µmol photons m-2 s-1 of PAR under indoor conditions (Fig. 2a). Growth rates of both strains were significantly (P < 0.05) reduced when the cells were transferred outdoors and exposed to natural solar radiation. However, under outdoor conditions, growth rates of calcified cells were 3.5 times higher than those of the naked cells, indicating that the latter was more harmed by the solar exposure than the former (Fig. 2a). The cell diameter was not significantly different in the calcified cells between the indoor and outdoor conditions (P > 0.05), but an 18 % increase was found in the naked cells after they had grown under outdoor conditions for 7 days (P < 0.05) (Fig. 2b). The maximal quantum yield (Fv/Fm) decreased when the cells were transferred from indoor to the outdoor conditions, reflecting a harmful effect of solar radiation. The decrease in Fv/Fm, however, was much more pronounced in the naked cells (27 %) compared to calcified cells (11 %) (Fig. 2c).

Calcified cells had a significantly higher apparent light use efficiency (α), maximal electron transport rate (rETRmax) and light saturation parameters (Ik) compared to naked cells. The decalcified cells of the calcified strain showed a remarkable decrease in rETRmax (P < 0.05), and alpha and Ik also decreased but not statistically significantly (Fig. 3, Table 1). Increased actinic light levels (acclimating light during the fluorescence measurement) led to higher NPQ in both the calcified and naked strain (Fig. 4). Furthermore, calcified cells showed higher NPQ values compared to naked cells (p < 0.05).

When exposed to full-spectrum solar radiation, the quantum yield of calcified cells showed no significant change during the first 30 min (P > 0.05). After 30 min, the quantum yield quickly dropped from about 0.35 to 0.22 for ∼ 20 min (P < 0.05), followed by a slight recovery in the last 25 min. A similar trend was observed in the decalcified cells, with the key difference that the sharp decrease already happened during the first 10 min. The quantum yield of the naked cells decreased constantly for the first 50 min and remained at the low level thereafter (Fig. 5).

No effect of the radiation treatment (P, PA and PAB radiation) on the quantum yield of calcified cells was observed after the cells grown under indoor conditions were transferred to outdoor solar radiation for 1 h exposure (very cloudy day; average PAR, UV-A and UV-B were 481 µmol photons m-2 s-1, 22.1 and 0.7 W m-2, respectively) (P > 0.05). The quantum yield was significantly higher in the naked cells, however, when they were exposed to UV-A radiation (PA vs. P treatment, P < 0.05; Fig. 6a).

Similar responses were observed when the same test was done on a sunny day with an average PAR, UV-A and UV-B of 1605 µmol photons m-2 s-1 and 69 and 2.4 W m-2, respectively. Here, the quantum yield of the calcified cells showed no significant difference between the different light treatments, but it decreased significantly under the PAB treatment compared to P treatments in the naked cells (P < 0.05) (Fig. 6b).

Various hypotheses were proposed for the possible functions of coccoliths, but none of them is supported by sufficient evidence (Young, 1994; Raven and Crawfurd, 2012). One important function of coccoliths for surface-dwelling species such as E. huxleyi could be the protection against high photon flux densities, especially UV radiation (Berge, 1962; Young, 1994; Gao et al., 2009).

Some of our results support this hypothesis. The growth rate of the calcified cells of E. huxleyi grown under indoor conditions was about 2 times higher than that of naked cells. This difference came out even more strongly, with growth rates 3.5 times higher in calcified versus naked cells, when the cells were exposed to full-spectrum solar radiation (Fig. 2a). This could potentially be attributed to the screening of PAR, UV-A and UV-B by coccoliths. Although the daytime PAR of solar radiation was reduced to about half of the light level of the indoor test, noontime PAR levels were higher than 500 µmol photons m-2 s-1, and the presence of UV could lead to more harm to the naked cells. Light protection by coccoliths is further supported by the Fv/Fm measurements. The maximum photochemical efficiency of PSII was only slightly reduced in calcified cells but significantly decreased in naked cells when they were exposed to natural solar PAR and UV radiation (Fig. 2c). Furthermore, the photochemical performance of decalcified cells decreased significantly faster and more strongly with time compared to calcified cells (Fig. 5).

The diameter of calcified cells did not significantly change when they were exposed to the full spectrum of solar radiation. The diameter of the naked cells, however, increased significantly (Fig. 2b). Perhaps, the naked cells experienced more DNA damage and so did not enter the S phase regularly (Buma et al., 2000). Alternatively, it may reflect a strategy to acclimatize to stressful solar UV radiation since it is well known that smaller cells are usually more sensitive to UV than their larger counterparts (Garcia-Pichel, 1994; Laurion and Vincent, 1998). Some field and laboratory studies showed increased cell size with increased UV exposure (Buma et al., 2000), which can be interpreted as an adaptive or acclimation mechanism for protecting the cells against UV radiation. Furthermore, the naked cells might also employ other strategies, such as synthesizing UV screening compounds to ameliorate UV stress, because the naked strain had a lower UV transmittance than the decalcified strain.

Several studies found that coccoliths do not protect E. huxleyi from excess PAR (Nanninga and Tyrrell, 1996; Houdan et al., 2005; Trimborn et al., 2007). However, UV radiation was not considered in these experiments. Our results showed that the naked cells were more sensitive to full-spectrum solar radiation than calcified cells, and even in the same strain, the photochemical performance of decalcified cells decreased significantly when comparing the calcified cells. This suggests that coccoliths efficiently protect the cells from solar UV radiation.

On the other hand, E. huxleyi appears to be more sensitive to UV-B irradiances than other phytoplankton species, and its growth rate and physiological performances were highly inhibited by UV radiation (Peletier et al., 1996; Buma et al., 2000; Xu et al., 2011). However, competition tests for community changes are rare, and longer-term experiments with less extreme UVR would be more ecologically and evolutionarily relevant (Raven and Crawfurd, 2012). In our work, UVR had no significant effect on the quantum yield of calcified cells regardless of high- or low-light conditions but it showed inhibition in naked cells when they were exposed to high-solar light (Fig. 6a, b). This provides further evidence for protection by coccoliths against UV radiation.

On the cloudy day, no significant difference was observed among the treatments for the calcified cells; on the sunny day, under the fluctuating light (data not shown) calcified cells manage to refurbish damage to their photosynthetic apparatus by balancing damage and repair (Gao et al., 2007; Ragni et al., 2008; Loebl et al., 2010). For the naked cells, on the other hand, UV damage was not effectively repaired, leading to the observed negative effect on photosynthetic performance.

It has to be noted that our experimental data are based on only two strains of a naked and calcified E. huxleyi. However, similar trends in photophysiology between naked and decalcified cells in comparison to calcified cells suggest that the coccoliths of E. huxleyi play an important role in protecting this species against harmful solar radiation, especially UV-A and UV-B. Furthermore, the reported absence of photoinhibition in this alga at high light levels also appears to be connected to the coccosphere of E. huxleyi or its calcification process. In view of ongoing ocean change, the projected shoaling of the upper mixed layer (UML) caused by global warming and progressive ocean acidification that reduces the thickness or the number of coccoliths per cell (Gao et al., 2009; De Bodt et al., 2010) could reduce E. huxleyi growth rates within the UML due to increased UVR exposure.