Salim M Al-Moghrabi

Increased seasonality in Middle East temperatures during the last interglacial period

The last interglacial period (about 125,000 years ago) is thought to have been at least as warm as the present climate. Owing to changes in the Earths orbit around the Sun, it is thought that insolation in the Northern Hemisphere varied more strongly than today on seasonal timescales, which would have led to corresponding changes in the seasonal temperature cycle. Here we present seasonally resolved proxy records using corals from the northernmost Red Sea, which record climate during the last interglacial period, the late Holocene epoch and the present. We find an increased seasonality in the temperature recorded in the last interglacial coral. Today, climate in the northern Red Sea is sensitive to the North Atlantic Oscillation, a climate oscillation that strongly influences winter temperatures and precipitation in the North Atlantic region. From our coral records and simulations with a coupled atmosphere–ocean circulation model, we conclude that a tendency towards the high-index state of the North Atlantic Oscillation during the last interglacial period, which is consistent with European proxy records, contributed to the larger amplitude of the seasonal cycle in the Middle East.

Inorganic carbon uptake for photosynthesis by the symbiotic coral/dinoflagellate association : I. Photosynthetic performances of symbionts and dependence on sea water bicarbonate

The aim of this and the accompanying paper is to investigate the mechanisms of dissolved inorganic carbon (DIC) uptake by the scleractinian coral Galaxea fascicularis, and its delivery to the endosymbiotic photosynthetic dinoflagellates (zooxanthellae). For this purpose, a comparison was made between the photosynthetic performance of zooxanthellae in intact symbiosis within microcolonies of Galaxea fascicularis, freshly isolated zooxanthellae (FIZ) and cultured zooxanthellae (CZ) under different conditions. Discrimination between CO2 or HCO3− uptake was achieved by several means including changes in DIC concentration, pH variations, pharmacology or modifications of ion concentration in seawater. In this paper, the photosynthesis/irradiance curves of G. fascicularis microcolonies, FIZ and CZ are presented. It is shown that zooxanthellae inside their host have lower photosynthetic performance than isolated zooxanthellae. Light saturation (Ik) occurred at higher irradiance in the intact association than in isolated symbionts. Light utilization efficiency (α) was minimum in the intact association and increased in FIZ and CZ. G. fascicularis microcolonies, FIZ and CZ were tested for their ability to utilize HCO3− as a source of DIC for photosynthesis. Two main approaches were used, the first consisting of changing the bicarbonate concentration by adding HCC3− to bicarbonate-free artificial seawater at constant pH, and the second of modifying the pH of the seawater in a closed or open system. At saturating light intensity, the DIC concentration saturating for photosynthesis is no more than that of normal sea water. At pH 8.2, a half-maximal rate of photosynthetic O2 evolution is achieved at 408, 71 and 178 μM HCO3− for coral, FIZ and CZ respectively. The photosynthetic O2 production with constant inorganic carbon but varying pH reached an optimum at pH 8 to 9 suggesting that HCO3− is the main species taken up initially. FIZ and CZ possess the ability to utilize both CO2 and HCO3− as substrates for transport. The rate of non-enzymatic dehydration of HCO3− exceeds the rate of photosynthesis in coral and FIZ, but not in CZ. The results presented in this paper suggest that G. fascicularis microcolonies are able to take up bicarbonate to supply symbiont photosynthesis, although zooxanthellae in hospite seems DIC-limited. FIZ seem to absorb CO2 and HCO3− indiscriminately while CZ use HCO3−.

Microsensor study of photosynthesis and calcification in the scleractinian coral, Galaxea fascicularis: active internal carbon cycle

Abstract Sources of inorganic carbon (Ci) for photosynthesis and calcification and the mechanisms involved in their uptake in scleractinian corals were investigated in microcolonies of Galaxea fascicularis . Direct measurements of Ca 2+ , pH and O 2 on the surface and inside the polyps coelenteron were made with microsensors. Gross photosynthesis (Pg) and net photosynthesis (Pn) were measured on the surface. Light respiration (LR) was calculated from Pg and Pn. The effect of light/dark and dark/light switches on Ca 2+ and pH dynamics on the surface and inside the coelenteron were followed. To evaluate the different sources of Ci for photosynthesis and calcification, Ci-free seawater and 6-Ethoxyzolamide and Acetazolamide, inhibitors for carbonic anhydrase (CA) were used. In normal seawater, Pg was about seven times higher than Pn, the LR was ca. 80–90% of the Pg. Thus, most of the O 2 produced in Pg are immediately consumed in respiration, indicating the presence of a highly active internal C-cycle. As the internal C-cycle is highly active, a large part of the Ci for calcification will have passed through the metabolism of the symbiont. The high LR provides ATP for energy requiring processes in light. Ci for photosynthesis and calcification can come from seawater in the form of free Ci, respiration of photosynthates (internal C-cycle) or respiration of the ingested plankton. These sources form a common carbon pool (C-pool) that is used for the different processes. In Ci-free seawater, Pg decreased by about 12.5%, indicating that most of the photosynthetically fixed Ci can temporarily be supplied from internal sources. The initial decalcification, observed directly upon the switch to Ci-free seawater, showed that the Ca-pools in the coral are exchangeable. Part of the Pg in Ci-free seawater may depend on this decalcification for its Ci supply. Three localities of CA were defined. One on the surface facing seawater and one on endodermal cells facing the coelenteron, while the third is intracellular. The inhibition of CA decreased Pg by about 30%, while it increased the concentration of Ca 2+ as a result of a decrease in its precipitation. The reduction of photosynthesis and calcification by CA inhibition demonstrated that both processes need the enzyme for the supply of Ci. The pH on the surface and inside the coelenteron decreased upon 6-Ethoxyzolamide addition indicating a role of CA in pH control.

Spatial distribution of calcification and photosynthesis in the scleractinian coral Galaxea fascicularis

The spatial heterogeneity of photosynthesis and calcification of single polyps of the coral Galaxea fascicularis was investigated. Photosynthesis was investigated with oxygen microsensors. The highest rates of gross photosynthesis (Pg) were 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. Calcification was investigated by radioactive tracers. The incorporation pattern of 45Ca and 14C in the corallites was imaged with use of a Micro-Imager. The β-images obtained showed that the incorporation of the radioactive tracers coincided with the Pg distribution pattern with the highest incorporation rates found in the corallite septa. Thus, the high growth rate of the septa 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. This further emphasizes the close relation between calcification and photosynthesis.