Species-specific differential dissolution morphology of selected coccolithophore species: an experimental study

Coccolith dissolution in the water column is an important process in the marine carbon cycle. Identifying dissolution in water column samples has been difficult due to a lack of experimental reference datasets showing dissolution morphologies. We conducted a laboratory CaCO3 dissolution experiment to detect differential dissolution morphologies of three selected coccolithophore (abundant marine calcareous phytoplankton) species, Coccolithus braarudii, Helicosphaera carteri, and Scyphosphaera apsteinii. These species were selected because they are ecologically and biogeochemically important (significant contributors to CaCO3 production) and have been less studied than Gephyrocapsa. Muroliths of S. apsteinii dissolve faster than lopadoliths, which in turn dissolve as fast as H. carteri but faster than C. braarudii. In S. apsteinii lopadoliths, dissolution rate depends on the crystallographic orientation of the crystals. Comparison with field samples shows that experimental data are helpful when interpreting field samples. For example, we identify dissolution in water and sediment samples reported in the literature. In C. braarudii dissolution reveals a nanostructure on the proximal side of the distal shield, an observation that has implications for coccolith biomineralization models, which do not currently account for the formation of such a structure. This nanostructure features “units” of ca. 50–100 nm and resembles the nanostructure well known from extracellular calcifiers such as molluscs and foraminifera. Whether this resemblance is underpinned by a similar formation mechanism remains unknown, but we think this unlikely.