Abstract
The bloom-forming coccolithophore Emiliania huxleyi (Haptophyta) is a dominant marine phytoplankton, cells of which are covered with calcareous plates (coccoliths). E. huxleyi produces unique lipids of C37–C40 long-chain ketones (alkenones) with two to four trans-unsaturated bonds, β-glucan (but not α-glucan) and acid polysaccharide (AP) associated with the morphogenesis of CaCO3 crystals in coccoliths. Despite such unique features, there is no detailed information on the patterns of carbon allocation into these compounds. Therefore, we performed quantitative estimation of carbon flow into various macromolecular products by conducting 14C-radiotracer experiments using NaH14CO3 as a substrate. Photosynthetic 14C incorporation into low molecular-mass compounds (LMC), extracellular AP, alkenones, and total lipids except alkenones was estimated to be 35, 13, 17, and 25 % of total 14C fixation in logarithmic growth phase cells and 33, 19, 18, and 18 % in stationary growth phase cells, respectively. However, less than 1 % of 14C was incorporated into β-glucan in both cells. 14C-mannitol occupied ca. 5 % of total fixed 14C as the most dominant LMC product. Levels of all 14C compounds decreased in the dark. Therefore, alkenones and LMC (including mannitol), but not β-glucan, function in carbon/energy storage in E. huxleyi, irrespective of the growth phase. Compared with other algae, the low carbon flux into β-glucan is a unique feature of carbon metabolism in E. huxelyi.Electronic supplementary materialThe online version of this article (doi:10.1007/s10126-015-9632-1) contains supplementary material, which is available to authorized users.
Highlights
Coccolithophores play an important role in global oceanic carbon cycling due to their worldwide distribution and capacity to produce huge blooms (Tyrrell and Merico 2004; Harada et al 2012; Read et al 2013)
The common coccolithophore, Emiliania huxleyi, possesses a chloroplast-localized pyruvate carboxylase (PYC) (Tsuji et al 2012) and an enzyme set corresponding to the ornithine–urea cycle (Allen et al 2011)
The organism used in this study was the coccolithophore E. huxleyi NIES 837 (Haptophyta), which was isolated from the Great Barrier Reef in 1990
Summary
Coccolithophores play an important role in global oceanic carbon cycling due to their worldwide distribution and capacity to produce huge blooms (Tyrrell and Merico 2004; Harada et al 2012; Read et al 2013). Coccolithophores belong to the Haptophyta, which evolved through the secondary endosymbiosis of red algae into a non-photosynthetic eukaryotic host. During this process, many genes were transferred from the endosymbiont to the host cell. Secondary algae including coccolithophores contain mosaic genomes consisting of genes from endosymbionts and host non-photosynthetic protists (Read et al 2013). According to recent genomic studies, the newly acquired genes from host protists contribute to the carbon metabolism of secondary algae. The carbon metabolism of E. huxleyi is distinct from that of primary
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