Coccolithophores are globally important marine calcifying phytoplankton. They contribute to the organic carbon pump through the primary production and the ballast of organic matter, and to the carbonate pump through the production of calcium carbonate. Here we compiled all available scanning electron microscopy (SEM) coccolithophore abundance observations. Taxa were standardized following NannoTax3 to a species level where possible. Subspecies (e.a. C. leptoporus subsp. leptoporus and C. leptoporus subsp. quadriperforatus) were grouped as single species. The database contains 2556 abundance observations from 35 different publications. The data span the period of 1993-2017, with observations from all ocean basins and all seasons, and at depths ranging from the surface to 5000 m. We limited our compilation to SEM observations (or observations which further identified samples with SEM) because SEM provides greater detail of coccolithophore diversity than more commonly used polarized light microscopy. Although this limits the number of observations, this allows for a more in-depth analysis of coccolithophore ecology, such as the ecological significance of the coccolithophore life cycle.

We investigated the effects of an increase in dissolved CO2 on the microbial communities of the Mediterranean Sea during two mesocosm experiments in two contrasting seasons: winter, at the peak of the annual phytoplankton bloom, and summer, under low nutrient conditions. The experiments included treatments with acidification and nutrient addition, and combinations of the two. We followed the effects of ocean acidification (OA) on the abundance of the main groups of microorganisms (diatoms, dinoflagellates, nanoeukaryotes, picoeukaryotes, cyanobacteria, and heterotrophic bacteria) and on bacterial activity, leucine incorporation, and extracellular enzyme activity. Our results showed a clear stimulation effect of OA on the abundance of small phytoplankton (pico- and nanoeukaryotes), independently of the season and nutrient availability. A large number of the measured variables showed significant positive effects of acidification in summer compared with winter, when the effects were sometimes negative. Effects of OA were more conspicuous when nutrient concentrations were low. Our results therefore suggest that microbial communities in oligotrophic waters are considerably affected by OA, whereas microbes in more productive waters are less affected. The overall enhancing effect of acidification on eukaryotic pico- and nanophytoplankton, in comparison with the non-significant or even negative response to nutrient-rich conditions of larger groups and autotrophic prokaryotes, suggests a shift towards medium-sized producers in a future acidified ocean.