A natural pH gradient caused by marine CO2 seeps off Vulcano Island (Italy) was used to assess the effects of ocean acidification on coccolithophores, which are abundant planktonic unicellular calcifiers. Such seeps are used as natural laboratories to study the effects of ocean acidification on marine ecosystems, since they cause longterm changes in seawater carbonate chemistry and pH, exposing the organisms to elevated CO2 concentrations and therefore mimicking future scenarios. Previous work at CO2 seeps has focused exclusively on benthic organisms. Here we show progressive depletion of 27 coccolithophore species, in terms of cell concentrations and diversity, along a calcite saturation gradient from Ωcalcite6.4 to 1. Water 98848 Nouméa, New Caledonia, France. changes correspond to increases of both [CO2] and [H ], and decreases of pH, [CO32-], and of carbonate saturation states (). This process, called ocean acidification, has raised concerns about long-term effects on marine ecosystems (Caldeira and Wickett, 2003; Kroecker et al., 2013a). High CO2 concentrations affect the metabolism, growth, calcification, and behavior of many marine organisms (Rodolfo-Metalpa et al., 2011; Kroeker et al., 2013a). Some photosynthetic organisms benefit from increased levels of CO2, although many calcified species are adversely affected by corrosion or competition from non-calcified forms (Porzio et al., 2011; Johnson et al., 2013; Kroeker et al., 2013b). calcite collected close to the main CO2 seeps had the highest concentrations of malformed Emiliania huxleyi. These observations add to a growing body of evidence that ocean acidification may benefit some algae but will likely cause marine biodiversity loss, especially by impacting calcifying species, which are affected as carbonate saturation falls. © 2014 Marine Biological Laboratory.