The Impacts of Ocean Acidification on Marine Biodiversity: Coral Reefs Are Not the Whole Story

          Since the beginning of the industrial revolution, the ocean has absorbed about 1/3 of CO₂ emitted to the atmosphere from human activities. The ocean plays a vital role to alleviate global warming caused by this greenhouse gas. However, as CO₂ concentrations increase continuously in atmosphere, the absorptive amount of CO₂ in ocean goes up dramatically. When CO₂ dissolves in seawater, it forms carbonic acid and increases the acidity of ocean. Ocean acidification and its effects on marine biodiversity have been largely overlooked by mainstream medias during the debate of climate change. In addition, the concern about ocean acidification has often focused on its negative effects on coral reefs. However, the broader impacts of acidic sea on marine organisms and ecosystems may be much more significant. 

          The basic chemistry of how an ocean becomes acidic is well understood. When CO₂ dissolves in

http://www.pmel.noaa.gov/co2/story/Ocean+Acidification

seawater, it forms carbonic acid, which dissociates to bicarbonate and hydrogen ions. Bicarbonate ions can further dissociate to carbonate and hydrogen ions. According to NOAA, the more CO₂ in the atmosphere, the more CO₂ is in the ocean. The increasing quantities of CO₂ in seawater will drive the equilibrium of this chemical process to produce more hydrogen ions and greatly decrease the pH in the ocean. This changing chemistry in the ocean will put a great threat on marine plants or animals, whose shells or skeletons are made up of calcium carbonate (CaCO₃), since CaCO₃ dissolves at certain lower pH level. What is more, it is much more difficult for organisms to construct calcium carbonate shells under acidic condition. Currently, the ocean tends to be corrosive to numerous marine organisms with calcium carbonate covers.

           The impacts of acidic ocean on coral reefs have been studied thoroughly. Corals are composed of coral polyps, which are covered by calcium carbonate secreted over time. Corals live with algae (Zooxanthellae) symbiotically, relying on the nutrients photosynthesized by the algae and provide stable habitats for the algae. Corals face two threats from acidification: First, coral reefs are very sensitive to temperature change. A 1-2℃ warmer maximum summer temperature will lead to a phenomenon called “bleaching” which refers to the expelling of symbiotic algae by coral which makes itself become translucent(Jones et al., 2008). Secondly, increasing acidity decreases calcification rates of coral reefs, which makes it grow slower and vulnerable to erosion. Both threats lead host coral to decline or even die. However, the threats to coral reefs are not the whole story. Coral reef ecosystems are one of the most biologically diverse hotspots. They are also called the rainforests of the sea, since they cover only 2% of global surface but provide habitat for almost 25% of all marine species and contain almost 40% of the whole world’s productivity (John et al.). Many species such as sponges, mollusks, crustaceans are interconnected with coral reefs. Any fluctuation-declining of corals will influence the abundance of other species, especially who rely on coral for food and habitat. 

https://web.duke.edu/nicholas/bio217/aer9/effects.htm 

       Ocean acidification will trigger a chain reaction and alter the marine food web. Calcifying organisms that are protected by carbonate shell or skeleton span the entire marine food web. Specifically, many microscopic phytoplankton and zooplankton species form the base of this food web. Phytoplankton have calcium carbonate shells outside to protect themselves from predators such as ciliate protozoa. In recent studies, almost all calcifying organisms decrease their ability to construct shells under acidic seawater (Endo et al.,2013). These two photos compare the calcifying rate of one phytoplankton called Gephyrocapsa under current CO₂ conditions and the expected higher CO₂ conditions by the end of this century. Under the higher CO₂ conditions, the calcifying rate decreases significantly by about 25% and shows clear structural damage (Endo et al.,2013).

 Reduced calcification of marine phytoplankton in response to increased atmospheric CO2(left:current; right:2100),     
                                                                                         Nature 407, 364-367

Without protective shells, phytoplankton will lose their physical function and be easily captured by predators. Ciliate protozoa gorging on these weekened phytoplankton will flourish at the expense of loss of other organisms in the food chain (Endo et al.,2013). What is more, as the primary producer of marine system, unprotected phytoplankton will lower the nutritional content available for organisms higher up in the food chain due to their loss of organic carbon cover. Reduced primary production quality leads to lower nutritional availability for zooplankton, thereby lowering growth and reproduction rates of other organisms in the food chain as there is less energy input for higher organisms (Potera, 2010). 

                                   

                                                  http://seagrant.uaf.edu/nosb/papers/2009/kodiak-smart.php

Pteropods (a kind of zooplankton), also called “sea butterflies” are important prey for many commercial fish species such as the North Pacific salmon. A recent study shows that when a peteropod’s shell is placed in seawater with the pH predicted for the year 2100, the shell dissolves and disappears after just 45 days (Holland, 2007). Without a protective shell, other non-calcifying organisms will out-compete the peteropods for food and lead to change of prey-predator relationships. Some species may go extinct without enough nutrition, others may become flourishing and invasive in local environments. Current research shows that ocean acidification even negatively affects commercial fisheries and shellfish industries. Due to lack of nutrition, North Pacific salmons are smaller in size, and due to erosion of acidic sea, west coast oysters have failed in both aquaculture and natural development (Holland, 2007). The change of the marine food chain will eventually affect the food security of humanity.

         Ocean acidification not only indirectly affects food web of marine organisms, but also directly affects their physiological function. Acidic seawater disrupts normal biological mechanisms that are vital for marine species survival and causes internal damage. For marine animals in general, such as invertebrates or fish, lower pH may result in acidosis or acidification of body fluids. These internal damages will lead to reduced resistance to diseases, metabolic disorders, decreased reproductive rates and depression in physical activity (Poertner etal.,2005). For instance, squids require large amounts of

          

                Photo credit link                                              Photo credit link

energy when they swim. Their energy-demanding way of survival requires abundant supply of oxygen in the blood. However acidosis negatively affects the transport of oxygen and leads to behavior depression of squid. In another study from the University of Bristol, ocean acidification can cause deafness in juvenile clownfish. Baby clownfish use both hearing and olfactory senses to detect predator-rich coral reefs in order to avoid capture. The researchers put clownfish in current seawater (390 ppmCO₂) and seawater with higher CO₂ (900ppmCO₂) to mimic emission levels projected by 2100 if the emission rate is constant (Mundaya, 2009). In both the hearing and olfactory sense tests, clownfish can detect and subsequently swim away from the predator-rich coral reefs in current seawater, but baby fish in 2100 seawater have no response to predator noise and cannot tell which direction they should swim (Mundaya, 2009). According to this study, if we continue to emit CO₂ at current rate, the entire sensory system of clownfish will lose effectiveness, making them more susceptible to predators and face possible extinction. These disrupted physiological processes may occur in other marine organisms and lead to a subsequent loss of biodiversity.

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        Ocean acidification has great impacts on marine biodiversity. These potential effects are not only limited in erosion of coral reefs by dissolving their calcium carbonate shells, but have broader impacts on disruption of biodiversity in the ocean. Acidic oceans will directly alter the physiological function of certain marine species, disrupt their sensory systems and depress behaviors; It will indirectly influence the food resources in the ocean by altering marine food chain. However, these known impacts are just one piece of the big puzzle of ocean acidification effects. More experiment-related research is needed in order to fill the knowledge gap. At the same time, everybody should be aware of the severe results that acidic ocean will bring and change our lifestyle to reduce CO₂ emission daily. Go green with our transportation by using buses or bikes; Turn off the lights when leaving an empty room; Save water by taking short showers instead of baths… So start small and start now!

Citations:

Popular Media:

1. John Sibbick, Ocean Acidification

http://palaeo.gly.bris.ac.uk/ocean/Biology.html

2. Jennifer S. Holland (November 2007) Acid Threat. National Geographic.

http://ngm.nationalgeographic.com/2007/11/marine-miniatures/acid-threat-text

3. https://web.duke.edu/nicholas/bio217/spring2010/chang/fishbiology.html

Primary Literature:

1.  Endo, H, T Yoshimura, T Kataoka, K Suzuki, BV ELSEVIER SCIENCE, and Koji Suzuki.  
"Effects of CO2 and Iron Availability on Phytoplankton and Eubacterial Community Compositions 
in the Northwest Subarctic Pacific."Journal of Experimental Marine Biology and Ecology, 439 
(2013): 160-175.

2.  Jones, A, R Berkelmans, M. J. H. van Oppen, J Mieog, and W Sinclair. "A Community Change   
in the Algal Endosymbionts of a Scleractinian Coral Following a Natural Bleaching Event: Field
Evidence of Acclimatization." Proceedings: Biological Sciences, 275.1641 (2008): 1359-1365.

3. Mundaya, Philip L. "Ocean acidification impairs olfactory discrimination and homing ability of a marine fish." Proceedings of the National Academy of Science. 106.6 (2009): 1848–1852 .

4.  Potera, Carol. "MARINE and COASTAL SCIENCE: Will Ocean Acidification Erode the Base of the Food Web?."Environmental Health Perspectives, 118.4 (2010): A157.

5.  Poertner, H.O., M. Langenbuch, and B. Michaelidis “ Synergistic effects of temperature extremes, hypoxia, and increases in Co2 on marine animals” From earth history to global change, Journal of Geophysical Research-Oceans (2005): 110, C09S10, doi:10.1029/2004JC002561.