Effects of Acidified Seawater on the Skeletal Structure of a Scleractinian
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Zoological Studies 51(8): 1319-1331 (2012) Effects of Acidified Seawater on the Skeletal Structure of a Scleractinian Coral from Evidence Identified by SEM Isani Chan1, Shao-Hung Peng1,2, Ching-Fong Chang3,4, Jia-Jang Hung2, and Jiang-Shiou Hwang1,4,* 1Institute of Marine Biology, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung 202, Taiwan 2Institute of Marine Geology and Chemistry, Asian-Pacific Ocean Research Center, National Sun Yat-Sen University, Kaohsiung 804, Taiwan 3Department of Aquaculture, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung 202, Taiwan 4Center of Excellence for the Oceans, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung 202, Taiwan (Accepted August 22, 2012) Isani Chan, Shao-Hung Peng, Ching-Fong Chang, Jia-Jang Hung, and Jiang-Shiou Hwang (2012) Effects of acidified seawater on the skeletal structure of a scleractinian coral from evidence identified by SEM. Zoological Studies 51(8): 1319-1331. This study investigated the response of the scleractinian coral Acropora valida to acidified seawater from the Kueishan I. hydrothermal fields off northeastern Taiwan. Acropora valida was exposed to seawater with various pH values, and modification of the skeletal structure was examined and recorded through scanning electron microscopy (SEM). The SEM images displayed various degrees of erosion of the vertical rods and radial bars, and considerable and significant acidified seawater effects on the fusiform crystals and small spherulitic tuft clusters of aragonite as the pH decreased in the experimental treatments. The results demonstrated that a low pH may inhibit coral calcium carbonate formation, implying that the pH may be the limiting factor in the coral's distribution around the Kueishan I. hydrothermal fields. Morphological changes in scleractinian coral skeletal structures with erosion of CaCO3 crystals could also be explained by the saturation state (Ω) of the experimental seawater with respect to aragonite under various pH conditions. Because ocean acidification is an urgent global environmental issue, the implication is that Kueishan I. hydrothermal vents have a considerable influence on the spatial distribution of corals around Kueishan I. This was demonstrated by the use of coral, A. valida, as a model species in the present study. http://zoolstud.sinica.edu.tw/Journals/51.8/1319.pdf Key words: Scleractinian coral, SEM, pH, Ocean acidification, Kueishantao hydrothermal vents. F inding hydrothermal plumes on the and wide ranges of compositions of fluids, gases, seafloor along the Galapagos Ridge in 1976 and substantially low pH values that constitute a considerably changed our understanding of previously unknown ecosystem and support high hydrothermal vent biology and ecology; in biomass with low species diversity (Dando et al. particular, vent ecosystems are recognized as 1995, Cowen et al. 2002, Hwang and Lee 2003, extreme environments that can support various Hwang et al. 2008, Ki et al. 2009). Research on life forms in highly unstable physicochemical shallow hydrothermal vents has evaluated various conditions in the surrounding ecosystems (Lyon aspects of hydrothermal fluids, water parameters, et al. 1977, Karl 1995, Dando et al. 2000, Kelly et and the abundances, distributions, and adaptations al. 2001, Luther et al. 2001, Hwang et al. 2008, of marine organisms in vent ecosystems (Dando Ka and Hwang 2011). Hydrothermal systems are and Leahy 1993, Pichler and Veizer 1999 2004, characterized by unexpectedly high temperatures, Selkoe et al. 2008, Ka and Hwang 2011, Peng * To whom correspondence and reprint requests should be addressed. Jiang-Shiou Hwang and Jia-Jang Hung contributed equally to this work. Tel: 886-935289642. Fax: 886-2-24629464. E-mail:[email protected]; [email protected] 1319 1320 Chan et al. – Acidified Seawater Effect on Coral et al. 2011). The shallow-water hydrothermal responses to lowered pH by scleractinian coral vents off Kueishan I. in northeastern Taiwan are vertical rods, radial bars, fusiform crystals, and recognized as a part of the Okinawa Arc; the small spherulitic tuft clusters of aragonite remain island is located on a tectonic conjunction of the relatively unknown. The range of pH tolerance extension of the fault system of Taiwan and the and effects on the calcium carbonate crystal southern rifting end of the Okinawa Trough (Lee et structure of corals must be examined because al. 1980, Sibuet et al. 1998, Hwang and Lee 2003, of ocean acidification. Kueishan I. hydrothermal Liu et al. 2008). The hydrothermal vents discharge fields consist of a yellow vent and a white vents, highly acidic and sulfur-rich fluids and gases which discharge highly acidic fluid (low pH), may composed of carbon dioxide, sulfur dioxide, and contribute to the abundance and distribution of hydrogen sulfide. The highest temperature of the corals around Kueishan I. This study focused Kueishan I. hydrothermal fields is approximately on understanding the following: (i) the effect of 116°C; however, it can reach up to 126°C after acidified seawater on the skeletal structure of an earthquake (Hwang and Lee 2003, Chen et al. the coral after exposure to the hydrothermal vent 2005). In addition, shallow hydrothermal vents water; and (ii) the possible effects of hydrothermal also exhibit extreme environmental characteristics; vents on the distribution of corals around Kueishan however, relatively few studies of the ecology and I. We used the scleractinian coral, Acropora valida biology of vent organisms have been conducted in (Dana, 1846), as a model species to examine the the Kueishan I. hydrothermal vent ecosystem (Jeng effect of acidified seawater on the skeletal structure et al. 2004, Hwang et al. 2008, Ki et al. 2009, Ka of the coral. The detailed structure of the coral and Hwang 2011, Peng et al. 2011). The discovery skeleton after exposure to acidified vent waters of a coral reef community on the opposite side of was examined by scanning electron microscopy Kueishan I., 4 km from the vent side, has intrigued (SEM). researchers for a long time, particularly the manner in which the coral community flourishes near the vent plumes (Hwang and Lee 2003). Scleractinian MATERIALS AND METHODS coral is formed from calcium carbonate (CaCO3), a unique underwater structure. However, coral reefs Acropora valida is classified into the are fragile in extreme vent ecosystems, and are Animalia, phylum Cnidaria, class Anthozoa, order especially sensitive to high water temperatures and Scleractinia, family Acroporidae, and genus low pH values from the vent plumes. Therefore, Acropora. Colonies have a wide range of forms researching the effects of acidified seawater on the from compact bushes to tables, and axial corallites skeletal structure of scleractinian corals was the are small. Radial corallites are a mixture of sizes, primary objective of the present study. and are strongly appressed and swollen with Recently, the world’s coral reef ecosystems small openings. The color of A. valida is usually have encountered increasing threats from cream, brown, or yellow, and occasionally brown anthropogenic pressures (Marubini and Atkinson with purple branch tips and cream radial corallites. 1999, Jokiel et al. 2008, Holcomb et al. 2009, Habitats cover a wide range of reef environments Suwa et al. 2010, Tseng et al. 2011, Chan et al. and rocky foreshores (Almany et al. 2009, Brusca 2012) and loss of biodiversity (Dai et al. 2002, and Brusca 2009). Wilkinson 2004, Anthony et al. 2007). Hence, corals experience considerable threats from Coral and water collection polluted seawaters (Tseng et al. 2011, Chan et al. 2012). The increase in ocean acidification Colonies of A. valida were collected from a and climate change caused by rising atmospheric fringing reef off Kueishan I. (Fig. 1). Specimens carbon dioxide (CO2) concentrations threaten were placed in a temperature-insulated box marine organisms (Caldiera and Wickett 2003, containing seawater obtained from the Aquaculture Hoegh-Guldberg et al. 2007), including coral reef Department of National Taiwan Ocean Univ., ecosystems. Consequently, a decrease in ocean and immediately transported to the laboratory. pH contributes to decalcification caused by lowered Experimental seawater was collected from surface pH levels in several marine calcifying organisms water of the hydrothermal fields off Kueishan I. (Caldiera and Wickett 2003, Orr et al. 2005, (121°57'E, 24°50'N). Seawater samples collected Marubini et al. 2008), such as corals (Suwa et al. from the hydrothermal vents (121°57'E, 24°50'N) 2010). However, morphological and physiological were also analyzed in situ for various chemical Zoological Studies 51(8): 1319-1331 (2012) 1321 parameters, such as temperature, pH, and total tissue and polyps) or recently dead (white freshly dissolved sulfide (Fig. 2). The temperature was exposed skeleton) (Williams et al. 2008). Alkalinity measured with a thermocouple device (European was determined with Gran’s (1952) titration method Union banned mercury-in-glass thermometer), pH according to a method by Cai et al. (2010). The was measured with a portable pH meter (www. saturation state of aragonite (Ω) was calculated up-aqua.com), and total dissolved sulfide was according to a method of Lewis and Wallace (1998). measured according to a method described by The saturation state was defined as the [Ca2+] 2- 2+ 2- Cline (1969). [CO3 ]/kp’, where [Ca ] and [CO3 ] are respective concentrations of calcium and carbonate ions Experiments in seawater, and kp’ is the conditional solubility product of aragonite in seawater. Thirty-three colonies of A. valida were equally distributed in 11 aquarium tanks, which contained SEM control and experimental treatments. The coral colonies were exposed to various pH values as Materials for SEM (Chan et al. 2012) were experimental conditions (pH 8.0, 7.0, 6.0, 5.0, obtained from skeletons of A. valida. Samples 4.0, 3.0, and 2.0) and a control condition (pH 8.2) were cleaned with sodium hydroxide to remove for 7 d at 25°C. The experimental treatments tissue from within the skeletal structure. The A.