What are coccolithophores? Coccolithophores are single-celled algae belonging to the phytoplankton, formally classified in the class Prymnesiophyceae. Like any other phytoplankton, coccolithophores live in large numbers throughout the upper surface layers of the ocean. They surround themselves with tiny, often disc-shaped platelets, known as coccoliths, which are made of carbonate. The terms “calcareous nannoplankton” or “calcareous nannofossils” include coccoliths and coccospheres of haptophyte algae and the associated nannoliths, which are of unknown provenance. As the groups name suggests calcareous nannofossils are small, generally less than 30 µm across (coccoliths are usually 2 to 10 µm). Due to their microscopic size and wide distribution, calcareous nannofossils have become very popular for solving various stratigraphic problems. First recorded occurrences of are from the late Triassic (Carnian). Today coccolithophores are important phytoplankton in the oceans, are found in marine deposits often in vast numbers (even forming chalk cliffs), and are used as sensitive indicator of environmental changes.
This group of phytoplankton play an important role in the carbon cycle because they remove CO2 from the atmosphere. The chemical reaction that makes the coccolith also generates carbon dioxide. While much of the gas is sucked back in by the coccoliths some of it escapes into the atmosphere. In the short term that greenhouse gases could cause the upper layers of the ocean to become more temperate and stagnant. Scanning electron microscopy has become widely available and greatly enhanced the study of nannofossils. Much of the work on fine structure and formation of coccoliths has been made possible by scanning electron microscopes. Since calcite crystals forming coccoliths often have differently oriented optic axes, distinctive extinction patterns can be used for identification under crossed nicols of a polarising microscope.
Coccolithophore species Emiliania huxleyi bloom from space (source: www.nasa.gov) and under scanning electron microscope
Biogeography and ecology of coccolithophores Coccolithophores rely on photosynthesis as their prime nutritional mode restricting them to the photic zone of the oceans. Within the uppermost water column, there are a number of environmental parameters that are known to affect communities in general, on both spatial and temporal scales. Better understanding of coccolithophore ecology at species-level and of their quantitative biogeography will help deciphering their role in global warming and ocean acidification. Therefore, we mainly aim studying the distribution of species relative to physico-chemical parameters in various parts of the oceans.
Distribution of Syracosphaera spp. in the pacific Southern Ocean (Saavedra et al., 2014)
Coccolithophore fluxes from sediment traps Coccolithophore flux is investigated to quantify coccolith and coccolith-carbonate export and to determine possible trends in species composition related to different productivity conditions or seasonal succession. It is furthermore intended to determine the extent to which coccolithophorid carbonate accumulation rate is affected by species composition. Present-day flux can be compared to the records of coccolith and carbonate accumulation in sediments underneath the mooring locations.
Coccolith flux in sediment traps CI 3-6 (Sprengel et al., 2000)
Micro evolution and size variations of coccolithophores Variation in the size and shape of individual coccoliths are found even within a single species of coccolithophores. This variation is often related to differing environment factors. We therefore study coccoliths morphometrically in plankton and sediment trap samples as well as in surface sediments, in order to characterize morphotypes present and to map out their distribution relative to ecological variables. Furthermore, we intend to reconstruct the microevolutionary development of morphotypes, using geological sample sets.
Size variations in Calcidiscus and Umbilicosphaera taxa (Baumann et al., 2016)
Transformation from living communities to fossil assemblages A prerequisite for the application of coccolithophores and their remains (organic and inorganic) in paleoceanographic reconstructions or paleoenvironmental studies is the knowledge of their ecology, taxonomy, biogeography and oceanographic significance, but also of taphonomical processes which alter the assemblages during sedimentation through the water column and during accumulation on the seafloor. We obtain such essential information on the final burial of coccoliths from the comparison of living coccolithophore assemblages with surface sediment assemblages from the same region.
