Effects of changing seawater temperature on photosynthesis and calcification in the scleractinian coral <i>Galaxea fascicularis</i>, measured with O₂, Ca²⁺ and pH microsensors

Abstract

SUMMARY: Single polyps of Galaxea fascicularis were fixed to glass vials with underwater epoxy resin. After regeneration into microcolonies they were used for microsensor measurements of photosynthesis and calcification under different incubating temperatures. Gross photosynthesis was found highest at temperatures of 23 and 26ºC (ca. 0.022 mole O 2 m -3 s -1 ), close to the ambient temperature (i.e. 26ºC). At 35°C, gross photosynthesis was irreversibly inhibited as the microcolonies bleached. The net photosynthesis rapidly decreased with temperature and became negative at temperatures higher than 29ºC. Profiles of O 2 and Ca 2+ showed a strong effect of temperature on them. The concentrations of Ca 2+ measured on the polyp surface also showed temperature dependence of Ca 2+ uptake. In the dark and below 29ºC, the surface Ca 2+ concentration was temperature independent. During illumination, the surface Ca 2+ concentration showed a dip at 26 ºC (ca. 8.7 mM), indicating maximum uptake rates at ambient temperature. However, at 32ºC and higher, Ca 2+ was slightly higher at the tissue surface than in the seawater, in both light and dark, resulting from calcium dissolution. The surface pH in light increased gradually from 8.3 to 8.6 with increasing temperature to 23ºC, and thereafter remained constant to 29ºC. At 32 ºC, the pH became slightly acidic compared with the water phase, probably due to a decrease in the uptake of CO 2 by photosynthesis. The largest difference in pH between light and dark incubations was at temperatures between 23 and 29ºC (7.5-7.7 in dark versus 8.6-8.7 in light), which indicate high rates of photosynthesis and respiration in this temperature range . It is concluded that photosynthetic activity in the coral is maintained up to rather high temperatures (32ºC), but corals at super-optimum temperatures (above 26ºC) consume more O 2 than they produce, decalcify and produce CO 2 .Keywords: temperature, coral, gross photosynthesis, net photosynthesis, calcification, Galaxea fascicularis, microsensors.. RESUMEN: EFECTOS DEL CAMBIO EN LA TEMPERATURA DEL AGUA EN LA FOTOSÍNTESIS Y LA CALCIFICACIÓN DEL CORAL ESCLE-RACTINIARIO GALAXEA FASCICULARIS MEDIDOS CON MICROSENSORES DEO 2 , Ca 2+ Y pH. -Se fijaron pólipos de Galaxea fascicularis sobre viales de cristal con resina epoxy. Una vez generaron pequeñas colonias se utilizaron como sensores para medir los cambios en la fotosíntesis y calsificación a diferentes temperaturas. Los valores de fotosíntesis total (bruta) fueron más elevados a temperaturas entre 23 y 26°C (ca. 0,022 mole O 2 m -3 s -1 ), similares a los del ambiente (26°C). A valores de 35°C, la fotosíntesis total se inhibió irreversiblemente y las pequeñas colonias se blanquearon. La fotosíntesis neta disminuyó rápi-damente con la temperatura hasta llegar a valores negativos en temperaturas mayores de 29°C. Los perfiles de O 2 y Ca 2+ mostraron un elevado efecto de la temperatura. Las concentraciones de Ca 2+ medidas en la superficie de los pólipos también mostraron una influencia en la incorpo...

Marine projections of warming and ocean acidification in the Australasian region

“…Studies have shown that while calcification is enhanced as temperature increases, once an optimum value (typically a few degrees below the seasonal maximum temperature) is exceeded, calcification rates rapidly decline (Al-Horani, 2005;Cantin and Lough, 2014). If this warming continues it will ultimately lead to corals ejecting their symbiont dinoflagellates (zooxanthellae), a process known as coral bleaching, which in many cases can lead to coral death (McClanahan et al, 2007).…”

Modelling coral calcification accounting for the impacts of coral bleaching and ocean acidification

“…Fig. 4 shows the fit of the adapted range (α) functional form to the experimental measurements of calcification rate (Al-Horani, 2005). Other researchers have modelled the temperature response of corals with cubic polynomials (e.g.…”

“…Experimental studies have shown that maximum calcification rates occur at an optimum temperature that is 2-3 • C below the maximum temperature (Marshall and Clode, 2004) and that their calcification decreases more rapidly for warming above the optimum than for cooling. If the warming continues, the calcification rate continues to decline, ceasing at a temperature that is typically a few degrees above the seasonal maximum (Al-Horani, 2005;Cooper et al, 2008;Cantin et al, 2010). If the temperature exceeds this threshold, the corals lose their dinoflagellate symbiont, zooxanthellae, in a process known as thermal bleaching.…”

“…The dependence of calcification rate on the seawater carbonate system has been investigated (Erez et al, 2011;Pandolfi et al, 2011;Schneider and Erez, 2006;Putron et al, 2010;Shamberger et al, 2011;Jokiel et al, 2014), but a full understanding of the process is yet to be achieved. Experiments have shown that corals ingest seawater and are able to manipulate its carbonate chemistry within the subcalicoblastic medium to up-regulate the aragonite saturation state well above the surrounding seawater to enhance the precipitation of aragonite crystals (Al-Horani, 2005;Venn et al, 2013). Recent evidence indicates that, rather than the decrease in aragonite saturation states, it could be change in external seawater pH due to ocean acidification that disrupts the pH balance in the underlying calcifying tissue (calicoblastic epithelium) of reef-building corals, which in turn reduces the rate of calcification (Venn et al, 2013).…”

Diversity, Distribution and Abundance of Scleractinian Coral in District-based Marine Protected Area Olele, Bone Bolango, Gorontalo - Indonesia

“…When this happens, some oxygen ceases to play a normal role in photosynthesis and becomes chemically active as oxygen 'radicals' causing cellular distress. As oxygen radicals become toxic, the zooxanthellae that produce them are expelled by the corals even though this action too puts the corals at risk [32,33] In the reef slope zone, the light was limiting factor for corals as optimal light intensity decreases. Light controls productivity, physiology, and ecology of corals.…”