The coccolithophore Emiliania huxleyi plays a pivotal role in the marine carbon cycle. However, we have only limited understanding of how its life cycle and bacterial interactions affect the production and composition of dissolved extracellular organic carbon and its transfer to the particulate pool. We traced the fate of photosynthetically fixed carbon during phosphate-limited stationary growth of non-axenic, calcifying E. huxleyi batch cultures, and more specifically the transfer of this carbon to bacteria and to dissolved high molecular weight neutral aldoses (HMW NAld) and extracellular particulate carbon. We then compared the dynamics of dissolved carbohydrates and transparent exopolymer particles (TEP) between cultures of non-axenic and axenic diploid E. huxleyi. In addition, we present the first data on extracellular organic carbon in (non-axenic) haploid E. huxleyi cultures. Bacteria enhanced the accumulation of dissolved polysaccharides and altered the composition of dissolved HMW NAld, while they also stimulated the formation of TEP containing high densities of charged polysaccharides in diploid E. huxleyi cultures. In haploid E. huxleyi cultures we found a more pronounced accumulation of dissolved carbohydrates, which had a different NAld composition than the diploid cultures. TEP formation was significantly lower than in the diploid cultures, despite the presence of bacteria. In diploid E. huxleyi cultures, we measured a high level of extracellular release of organic carbon (34–76%), retrieved mainly in the particulate pool instead of the dissolved pool. Enhanced formation of sticky TEP due to bacteria–alga interactions, in concert with the production of coccoliths, suggests that especially diploid E. huxleyi blooms increase the efficiency of export production in the ocean during dissolved phosphate-limited conditions.
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