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Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-diplontic life cycle. The haplo-diplontic life cycle allows coccolithophores to divide in both life cycle phases and potentially expands coccolithophore niche volume. Research has, however, to date largely overlooked the life cycle of coccolithophores and has instead focused on the diploid life cycle phase of coccolithophores. Through the synthesis and analysis of global scanning electron microscopy (SEM) coccolithophore abundance data (n=2534), we find that calcified haploid coccolithophores generally constitute a minor component of the total coccolithophore abundance (≈ 2 %–15 % depending on season). However, using case studies in the Atlantic Ocean and Mediterranean Sea, we show that, depending on environmental conditions, calcifying haploid coccolithophores can be significant contributors to the coccolithophore standing stock (up to ≈30 %). Furthermore, using hypervolumes to quantify the niche of coccolithophores, we illustrate that the haploid and diploid life cycle phases inhabit contrasting niches and that on average this allows coccolithophores to expand their niche by ≈18.8 %, with a range of 3 %–76 % for individual species. Our results highlight that future coccolithophore research should consider both life cycle stages, as omission of the haploid life cycle phase in current research limits our understanding of coccolithophore ecology. Our results furthermore suggest a different response to nutrient limitation and stratification, which may be of relevance for further climate scenarios. Our compilation highlights the spatial and temporal sparsity of SEM measurements and the need for new molecular techniques to identify uncalcified haploid coccolithophores. Our work also emphasizes the need for further work on the carbonate chemistry niche of the coccolithophore life cycle.

Two years-data (May 2011–February 2013) obtained from a monthly sampling carried out at the coastal long term Ecological Research station C1-LTER in the Gulf of Trieste (northern Adriatic Sea) were analysed to describe the seasonal dynamics and diversity of coccolithophore assemblages and to assess their relationship with environmental forcing. Coccolithophores represented 10.7% of the total Utermöhl phytoplankton that were mainly dominated by small (

Silicoflagellate populations and fluxes were analyzed from water samples and sediment traps in several basins of the eastern Mediterranean and Black Sea, to show species distribution in response to ecological factors and compared with the Quaternary records of the region. In the eastern Mediterranean, Dictyocha stapedia was the most abundant species, represented by two varieties: var. stapedia, smaller and with an apex spine, that dominated in the pelagic settings, and var. aspinosa, larger and lacking an apex spine, that dominated in the north Adriatic Sea. Dictyocha aculeata was the second most abundant species, Octactis pulchra was seasonally dominant in the Adriatic Sea. Stephanocha speculum occurred sporadically in the Adriatic Sea with 6-sided normal-sized specimens, but was nearly absent from pelagic assemblages. In the Black Sea, assemblages and fluxes were characterized by larger 7-sided skeletons of D. speculum with apex spines and a lower abundance of O. pulchra.