Summary
Much of the global interest in coral reefs is because they are major features on the earth’s surface of high ecological and economical importance. The scleractinian corals are the main components of the coral reefs. In the past, calcification by corals and other photosynthesizing organisms played a major role in converting most of the carbon in the biosphere into limestone. A functional linkage between calcification and photosynthesis must be involved, since photosynthesizing organisms account for most of the calcification. Thus, global carbon cycles and sedimentary geology cannot be understood without addressing how calcification and photosynthesis are linked.
It has been discovered that scleractinian corals calcify at a faster rate in light and that algal symbiosis is necessary for rapid calcification rates exhibited by corals. Since the discovery of this phenomenon 50 years ago, scientists were trying to understand the mechanism of calcification and the role of light and the symbiont in its enhancement in corals. It is still not very well understood.
This thesis addressed this problem. Since carbon is a common substrate for calcification and photosynthesis, the second chapter in this thesis studied the sources of carbon for the two processes and its pathways in the coral. The title of this study is “Microsensor Study of Photosynthesis and Calcification in the Scleractinian Coral, Galaxea fascicularis: active internal carbon cycle”. In this study, small colonies of the scleractinian coral, Galaxea fascicularis were used. Microsensors for O2, Ca2+ and pH together with use of specific metabolic inhibitors and carbon free seawater were applied. Gross photosynthesis (Pg) and net photosynthesis (Pn) were measured on the surface of the polyp. Light respiration (LR) was calculated from Pg and Pn. The Ca2+ and pH dynamics on the surface and inside the polyp’s coelenteron were compared for the first time. The effect of light/dark and dark/light switches on Ca2+ and pH dynamics on the surface and inside the coelenteron were followed.
The results obtained showed that the coral has a Pg rate that is much higher than the Pn rate and that up to 90% of the oxygen produced by photosynthesis is consumed in metabolic respiration of the symbiont and the coral host. Thus, photosynthesis and respiration form an internal carbon cycle within the coral. As the internal C-cycle is highly active, a large part of the Ci for calcification will pass through the metabolism of the symbiont. The cycle provides ATP for energy requiring processes in light. The inorganic carbon for photosynthesis and calcification can come from seawater (free Ci) and from respiration of plankton and photosynthates. Carbon from the different pools (e.g. tissue, skeleton, photosynthates, and the dissolved organic and inorganic carbon in seawater and in coral) can easily be exchanged. Thus it is very difficult to follow the fate of carbon from one pool. Three localities of the enzyme Carbonic Anhydrase were defined. One on the surface facing seawater and one on endodermal cells facing the coelenteron, while the third is intracellular. The enzyme in the different localities helps in the uptake of inorganic carbon for photosynthesis and calcification and is also functioning in internal pH regulation.
In the second study entitled “Mechanism of calcification and its relation to photosynthesis and respiration in the scleractinian coral Galaxea fascicularis”, the mechanism of calcification and its relation to photosynthesis and respiration in, Galaxea fascicularis, was studied with microsensors for Ca2+, pH and O2 (Chapter 3). In this study, the energy budget of the coral in light and dark conditions was compared. The results showed that the coral has a higher energy production in light than in dark as it was deduced from the higher rate of respiration and ATP content in light when compared to dark.
The direct measurements of Ca2+ and pH dynamics on the surface, inside the polyp’s coelenteron and under the calicoblastic layer showed that the Ca2+ concentrations decreased in light on the surface and in the coelenteron compartments and increased upon switching light off. Under the calicoblastic layer the opposite was observed. In light, the level of Ca2+ was lower on the surface than in seawater, and even lower inside the coelenteron. Under the calicoblastic layer, the concentration of calcium was ca. 0.5-1 mM higher than seawater. Thus Ca2+ can diffuse from seawater to the coelenteron, but metabolic energy is needed for the transport across the calicoblastic layer to the skeleton. The pH under the calicoblastic layer was alkaline compared to the surface and inside the coelenteron. Because of this, the aragonite saturation state under the calicoblastic layer has increased from ca. 3.2 in the dark to ca. 25 in the light, creating an excellent environment for calcification in light.
When Ruthenium Red (specific inhibitor of Ca-ATPase) was added, Ca2+ and pH dynamics were inhibited under the calicoblastic layer. This indicated that Ca-ATPase transports Ca2+ against its gradient in exchange for H+ at the calicoblastic layer. Addition of DCMU (PSII inhibitor) completely inhibited photosynthesis. The calcium dynamics under the calicoblastic layer continued, however, they were less regular. The initial rates were maintained. Thus it was concluded that light and not energy generation triggers calcium uptake, however energy (mainly supplied from respiration of photosynthates) is also needed.
In the third study (Chapter 4), the interaction between photosynthesis and calcification was studied at a microscale levels with microsensors and micro-autoradiography. The title of this study is “Spatial distribution of calcification and photosynthesis in the scleractinian coral Galaxea fascicularis”. The results of this study showed that the highest rates of gross photosynthesis (Pg) are found on the tissue covering the septa, the tentacles and the tissues surrounding the mouth opening of the polyp. Lower rates were found on the tissues of the wall and the coenosarc. Incubation of the coral colonies with radioactive tracers for calcium ions and carbonate showed that the distribution of calcification on the polyp surface coincides with the distribution of photosynthesis. Thus, the high growth rate of the polyp septa, which showed highest rate of tracer incorporation, is supported by the high rates of Pg by the symbiont in the adjacent tissues. The total incorporation rates were higher in light than in dark, however, the distribution pattern of the radioisotope incorporation was not affected by illumination indicating that it is a morphogenetically controlled process. This further emphasizes the close relation between calcification and photosynthesis.
The incorporation of 14C from HCO3- and glucose in the coral skeleton again demonstrated that both organic and inorganic carbon sources are potentially used in coral calcification. It was also shown that the coral tissue incorporates the two tracers in light preferentially over the skeleton while 45Ca is not incorporated at all in the tissue. This underlines the conclusion from Chapter 2 that the pools exchange.
The use of killed colonies in the incubation experiments helped in estimating isotopic exchange rate and better estimating the actual calcification rates occurring in corals. The use of novel micro-analytical techniques (microsensors and β-imaging) allowed settling a classical scientific debate on the coupling of photosynthesis and calcification in a convincing way.