The role of coccoliths in protecting &amp;lt;i&amp;gt;Emiliania huxleyi&amp;lt;/i&amp;gt; against stressful light and UV radiation

Juntian Xu,Ulf Riebesell,Wenyan Zhao,Kai G Schulz,Kunshan Gao,Lennart T Bach

doi:10.5194/bg-13-4637-2016

Abstract

Abstract. Coccolithophores are a group of phytoplankton species which cover themselves with small scales (coccoliths) made of calcium carbonate (CaCO3). The reason why coccolithophores form these calcite platelets has been a matter of debate for decades but has remained elusive so far. One hypothesis is that they play a role in light or UV protection, especially in surface dwelling species like Emiliania huxleyi, which can tolerate exceptionally high levels of solar radiation. In this study, we tested this hypothesis by culturing a calcified and a naked strain under different light conditions with and without UV radiation. The coccoliths of E. huxleyi 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 (280–315 nm) by 18.4 %. Growth rates of the calcified strain (PML B92/11) were about 2 times higher than those of the naked strain (CCMP 2090) under indoor constant light levels in the absence of UV radiation. When exposed to outdoor conditions (fluctuating sunlight with UV radiation), growth rates of calcified cells were almost 3.5 times higher compared to naked cells. Furthermore, the relative electron transport rate was 114 % higher and non-photochemical quenching (NPQ) was 281 % higher in the calcified compared to the naked strain, implying higher energy transfer associated with higher NPQ in the presence of calcification. When exposed to natural solar radiation including UV radiation, the maximal quantum yield of photosystem II was only slightly reduced in the calcified strain but strongly reduced in the naked strain. Our results reveal an important role of coccoliths in mitigating light and UV stress in E. huxleyi.

Highlights

Coccolithophores are a group of marine phytoplankton species which are able to precipitate CaCO3 in the form of small calcitic scales surrounding the organic part of the cell
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 photosynthetically active radiation (PAR), 6.6 % for UV-A and 5.1 % for UV-B, relative to naked cells (Fig. 1)
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)

Summary

Introduction

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). 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)

Methods

Results

Conclusion