Corals and foraminifera

Calcium carbonate of foraminiferal shells is precipitated in a complex physiological process. We study the mechanisms of calcification with Ca2+, CO2, O2 , and pH microsensors. It is suggested that the metabolic processes like photosynthesis and respiration affect the calcite deposition in symbiont-bearing foraminifera (i.e. species harbouring autotrophic microalgae). Also the hydrodynamic conditions affect the physico-chemical microenvironment close to the foraminiferal shell.
Stephanie Köhler-Rink is involved in the G.I.F. project "Calcification mechanisms in foraminifera: new experimental approaches to develop better understanding of paleoceanographic proxies". We use microelectrodes for CO2, O2, pH, Ca2+ and CO32- to study the biomineralization process of symbiont-bearing benthic foraminifera and their shell-less specimens, so-called ameboids. Ameboids are spheres of foraminiferal cytoplasm formed by dissolving the calcite shell. These cytoplasmic spheres calcify immediately after transfer into seawater. They proved ideal model systems to study calcification mechanisms with microsensors and microscopic techniques (by our Israeli partners). We investigate the effects of seawater chemistry (pH, CO32-) and environmental changes like temperature and light intensity on biogenic calcification. Future studies will focus on the -imaging of radiotracer to study the two-dimensional microscopic distribution of -emitters (45Ca and 14C). This new technique will be applied to follow the effects of light and carbonate chemistry with high spatial resolution in ameboids and in foraminiferal shells.

Stefanie Köhler-Rink, Fuad Al-Horani, Miriam Weber, Raphaela Schoon

The studies on corals by Fuad Al-Horani have been finished, at least in our house. He has made important progress in the understanding of coral physiology. He showed that in corals there is an intense carbon cycle, gross photosynthesis being 5-10 times higher than net photosynthesis. Therefore respiration, and thus energy conservation in the form of ATP synthesis, is strongly stimulated in the light. This in itself could lead to the well-known light stimulation of calcification, an energy consuming process in corals. However, the regulation of calcification by light is a bit more complicated. He further measured the transport of calcium ions from the seawater, through the tissue and the calicoblastic layer (a high resistant membrane covering the skeleton) to the skeleton. Particularly the behaviour of calicoblastic layer proved essential for the understanding of calcification. In this membrane a Ca-H exchange mechanism is located, resulting in increasing pH and calcium concentration at the skeleton. This transportprotein is energized by ATP (fuelled by the carbon cycle), but directly activated by light. In the dark, calcification will instantly decrease, due to deactivation of the Ca and H+ transport, while the ATP levels remain high for several hours.

Stefanie Köhler-Rink, Fuad Al-Horani, Miriam Weber, Raphaela Schoon

It is well known that siltation destroys coral reefs. However, large variations are found between the amount of silt and the effect on corals. Miriam Weber found that different types of silt can have strong differences in effects. Particularly, nutrient rich sediments can kill corals in a very short time. She hypothesized that the killing is microbially mediated. Sterile silt does not have any effect, and she found evidence that sulfate reduction plays a role. She will investigate the mechanism of coral demise by a combination of microsensors and molecular techniques. With microsensors the microenvironment of the polyps under the silt layer will be measured, particularly sulfide development and photosynthesis will be measured. Miriam has planned laboratory measurements, allowing experimental incubations and in situ measurements (in Australia and Elba) for which diver-deployable equipment (amplifiers and data-acquisition) is designed and build. Samples will be investigated with molecular methods to assess the microbial community change upon exposure to silt.

Stefanie Köhler-Rink, Fuad Al-Horani, Miriam Weber, Raphaela Schoon