Coral Reef Calcification by Sara Krajewski on Prezi
As corals produce calcium carbonate they slowly add on to their existing reef structure allowing the reef size to grow.
23/04/2010 · Transcript of Coral Reef Calcification
Many marine species, from microscopic plankton to shellfish and coral reef builders, are referred to as calcifiers, species that use solid calcium carbonate (CaCO3) to construct their skeletons or shells. Seawater contains ample calcium but to use it and turn it into calcium carbonate, species have to bring it to specific sites in their bodies and raise the alkalinity (lower the acidity) at these sites to values higher than in other parts of the body or in ambient seawater. That takes energy. If high CO2 levels from outside penetrate the organism and alter internal acidity levels, keeping the alkalinity high takes even more energy. The more energy is needed for calcification, the less is available for other biological processes like growth or reproduction, reducing the organisms’ weight and overall competitiveness and viability. Exposure of external shells to more acidic water can affect their stability by weakening or actually dissolving carbonate structures. Some of these shells are shielded from direct contact with seawater by a special coating that the animal makes (as is the case in mussels). The increased energy needed for making the shells to begin with impairs the ability of organisms to protect and repair their dissolving shells. Presently, more acidic waters brought up from the deeper ocean to the surface by wind and currents off the Northwest coast of the United States are having this effect on oysters grown in aquaculture. Ocean acidification not only affects species producing calcified exoskeletons. It affects many more organisms either directly or indirectly and has the potential to disturb food webs and fisheries. Most organisms that have been investigated display greater sensitivity at extreme temperatures, so as ocean temperatures change, those species that are forced to exist at the edges of their thermal ranges will experience stronger effects of acidification.
As we can see, this enzyme could be useful in both zooxanthellae photosynthetic processes and coral skeleton calcification. Bicarbonate does not pass easily through cell walls but carbon dioxide does, hence an external CA could convert bicarbonate to carbon dioxide to allow easy passage. Once inside the cell, CA converts carbon dioxide back to bicarbonate to prevent back-diffusion. Bicarbonate Active Transport (BAT; see below) carries bicarbonate through the mesoglea where it once again is converted to carbon dioxide by CA. CO2 is then utilized by zooxanthellae. See Figure 2.
potentially compromising coral calcification ..
The figure below provides an overview of the experimental results. It immediately reveals that the effect of feeding on coral growth depends on ambient conditions, in terms of available light and oxygen. In light, coral calcification was only marginally affected by feeding. In fact, at an elevated oxygen saturation of 150%, corals calcified even faster. When the oxygen level got even higher (280%), however, calcification was negatively affected by feeding. In total darkness, this picture was very different. No matter how much oxygen we provided, corals showed very low calcification rates when they were actively feeding on zooplankton. At 150% saturation, we even measured a slight decalcification during feeding. Without zooplankton supplementation, on the other hand, dark calcification was quite high at normal oxygen levels. How can we explain these results?
Apparently, our theory that oxygen is limiting corals to calcify when fed in darkness was not supported by the data. Even at 280% oxygen saturation, there was no beneficial effect on dark calcification rates. Either oxygen was sufficiently available during feeding in darkness, or a different process was preventing corals from calcifying. After having refuted our own hypothesis, we decided to return to the available literature, and we found that corals can greatly increase their metabolic rates during and after feeding. When fed with nauplii, coral respiration rates quickly increase by approximately 2.5-fold (Szmant-Froelich and Pilson 1984), which means they start producing a lot of additional carbon dioxide (CO2) by metabolizing organic compounds. This may have two reasons; corals need to produce energy to fuel muscle contraction during feeding, and/or they start burning the organic nutrients gained from digested prey. As corals can quickly digest , i.e. within 3-6 hours (Hii et al. 2009; Wijgerde et al. 2011), this process may have occurred during the six-hour incubations. Could it be possible that these increased respiration rates caused the coral tissue to acidify? If that were true, it could severely affect calcification as this process is thought to be highly sensitive to tissue pH (Furla et al. 2000; Al-Horani et al. 2003).
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Nevertheless, some fundamental processes taking place in many marine organisms are substantially impacted by pH changes. One of these is calcification, and it is known that calcification in corals depends on pH, and that calcification falls as pH falls. Using these types of facts, along with the integrated experience of many hobbyists, we can develop some general guidelines about what is an acceptable pH range for reef aquaria, and make some determination as to what values are pushing the limits of acceptability.
Over the last decade, research has also shown that corals gain essential nutrients from plankton feeding (reviewed by Houlbrèque and Ferrier-Pagès 2009). Zooplankton, for example, although not abundant on reefs, is a major source of organic nitrogen and phosphorous. By using their tentacles, armed with powerful nematocytes and sticky mucus, corals capture small crustaceans such as copepods which are rich in proteins and fatty acids. Zooplankton feeding is especially relevant in aquaria, as high prey concentrations can be reached easily by dosing live cultures to the water. When corals are regularly fed with rotifers (), brine shrimp () or copepods (), their growth can be greatly enhanced. This includes both soft tissue synthesis and calcification, the production of the coral exoskeleton. However, research has also revealed that the short- and long-term effects of zooplankton feeding on coral growth are inconsistent.
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Our results indicate that coral growth in the wild may be much more variable than previously thought, as zooplankton feeding and oxygen have profound effects on calcification. We do not yet know, however, how high coral feeding rates have to get to actually disrupt calcification. The prey concentrations we used are only found under aquaculture conditions, although on reefs, corals can feed on bacteria and phytoplankton as well, which seem to be more abundant in the wild compared to filtered aquarium water (Feldman et al. 2011). Future research will have to determine the dose-response relationship between feeding and dark calcification rates. As nocturnal feeding may negatively affect coral growth, it seems puzzling that in the wild, many coral species expand their tentacles to feed at night. A possible explanation for this phenomenon is that at night, zooplankton concentrations are significantly higher than during the day (Holzman et al. 2005; Yahel et al. 2005a,b). The concentration of copepods, for example, can increase fivefold at night (Yahel et al. 2005a). As plankton is a source of essential nutrients, the inhibition of calcification during the night may be outweighed by the nutrition corals gain from feeding.
References - Environmental Measurement Systems
Hypothesized effects of feeding under light and dark conditions. Feeding increases metabolic rates, CO2 production, and as a result proton production in coral tissue. In light, these protons are neutralized by photosynthetically generated hydroxide ions. In darkness, protons accumulate in coral tissue. This temporarily slows down calcification, as this process is sensitive to decreased pH.
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A Pulse Amplitude Modulated (PAM) fluorometer (Junior PAM, Heinz Walz GmbH, Effeltrich, Germany) with a fiber optic cord measured the relative rate of photosynthesis (relative electron transport rate, or rETR) of the coral's zooxanthellae under conditions of low, natural, slightly elevated, and high alkalinity. Alkalinity concentrations were adjusted upwards with a commercially available buffer (Reef Builder, Seachem Laboratories, Madison, Georgia, USA.) Alkalinity was measured with a colorimeter (Alkalinity Checker HI755, Hanna Instruments, Woonsocket, Rhode Island, USA.) and doubled checked initially by titration to a pH of 4.2 through use of a commercially available titrant (1.6N sulfuric acid) and a digital titrator (Hach, Loveland, Colorado, USA.)
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