The Negative Effects Of Ocean Acidification On Coralline Algae

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Annual global carbon dioxide emissions increased by over 20% between 1997 and 2009 from approximately 24 billion tons to over 30 tons of carbon dioxide (CO2) per year (Friedlingstein et al., 2010). The implications of this are vast and are by no means limited to global warming. For many millions of year prior to industrialization, ocean pH levels remained stable; however, high levels of anthropogenic carbon emissions in the near 200 years since the industrial revolution, have caused fundamental chemical changes to occur in marine water that have driven (and continue to drive) marine water pH levels downward – a process referred to as ocean acidification (Orr et al., 2005). In this paper, I will present a discussion of ocean acidification. Specifically, I will provide an overview of the chemical underpinnings of ocean acidification and discuss the effects of ocean acidification on marine organisms. Ocean acidification refers to the phenomenon of a decrease in marine water pH, caused primarily by an increase in atmospheric CO2 levels (Orr et al., 2005). In essence, increasing atmospheric CO2 levels cause greater amounts of CO2 to be absorbed by the ocean. This is explained by Henry’s law, which states that the amount of dissolved gas in a liquid is directly related to the partial pressure of that gas above the surface of the liquid (Henry, 1803). This absorption process is chemically changing the oceans, in particular increasing its acidity. As carbon dioxide dissolves in seawater, it undergoes a series of reactions producing H+ and various negatively charged forms of dissolved carbon, as shown in the chemical equations below. Carbon dioxide dissolved in seawater first reacts with water molecules (H2O) to form carbonic acid (H2CO3) (Equation 1). Carbonic acid, then splits up into its constituents, releasing a hydronium ion (H3O+) and bicarbonate molecule (HCO3-) – thus decreasing pH levels (Equation 2). As described by Le Chatelier’s principle, an increase in acidity (i.e. an increase in H3O+ concentration) drives the reaction of some carbonate ions (CO32-) to react and become bicarbonate (Equation 3). As such, the net effect of the dissolution of CO2 in marine water is an increase in of H3O+, H2CO3 and HCO3- concentrations, and decreasing concentrations of CO32-. Since CO2 is absorbed at the surface, it is the surface waters that are most affected by ocean acidification. Additionally, polar marine waters are more highly susceptible to acidification; lower temperatures near the poles facilitates an increase in CO2 solubility (Henry, 1803; Guinotte & Fabry, 2008). Since the late 18th century, the average surface pH of the ocean has declined 0.1 pH units from 8.2 to 8.1 (Logan, 2010). Although this change seems minute and insignificant, because pH is measured on a logarithmic scale, this change is equivalent to a near 30% increase in acidity (Raven et al., 2000). Furthermore, ocean pH is projected to decline by another 0.3 to 0.4 pH units by the end of the century – a change equivalent to an increase in ocean acidity of 100-150% (Orr et al., 2005). A change this large in the ocean’s pH could have severe consequences for marine organisms that form biogenic calcium carbonate, such as coralline algae, coccolithophores, bryozoans, mollusks, and echinoderms (Guinotte & Fabry, 2008). More specifically, the reduction in carbonate ion concentration …show more content…
As such, they are considered to be among the species most sensitive to ocean acidification. The negative effects of ocean acidification on coralline algae include reduced growth rates and structural integrity (Ragazolla et al., 2012). Ragazolla and colleagues (2012) noted a significant downward trend in coralline algae (Lithothamnion glaciale) growth rates with increased carbon dioxide levels. Furthermore, the skeletal integrity of Lithothamnion glaciale was reduced in acidified conditions (Ragazolla et al., 2012). This has the consequence of increasing coralline algae vulnerability to boring. Additionally, this bears negative consequences for other organisms, as coralline algae play a critical role in ecosystem engineering; they are vital in coral reef cementing, and are an important food source for several mollusk species (Guiontter & Fabry, 2008). Moreover, coralline algae provide critical habitat-specific settlement cues for coral larvae (Tebben et al., 2015). A number of studies have shown that ocean acidification reduces coral recruitment has deleterious effects on coral settlement rates (Kuffner et al., 2008; Doropoulos et al.,

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