Soft boiled eggs

Reef-building animals will struggle to reproduce as the climate changes

Ocean acidification – the result of elevated atmospheric carbon dioxide levels – will affect the sex lives of many marine organisms, according to a study published recently in Current Biology.

Coral calcification responds to seawater acidification: a working hypothesis towards a physiological mechanism

The decrease in the saturation state of seawater, Ω, following seawater acidification, is believed to be the main factor leading to a decrease in the calcification of marine organisms. To provide a physiological explanation for this phenomenon, the effect of seawater acidification was studied on the calcification and photosynthesis of the scleractinian tropical coral Stylophora pistillata. Coral nubbins were incubated for 8 days at three different pH (7.6, 8.0, and 8.2). To differentiate between the effects of the various components of the carbonate chemistry (pH, CO32−, HCO3−, CO2, Ω), tanks were also maintained under similar pH, but with 2-mM HCO3− added to the seawater. The addition of 2-mM bicarbonate significantly increased the photosynthesis in S. pistillata, suggesting carbon-limited conditions. Conversely, photosynthesis was insensitive to changes in pH and pCO2. Seawater acidification decreased coral calcification by ca. 0.1-mg CaCO3 g−1 d−1 for a decrease of 0.1 pH units. This correlation suggested that seawater acidification affected coral calcification by decreasing the availability of the CO32− substrate for calcification. However, the decrease in coral calcification could also be attributed either to a decrease in extra- or intracellular pH or to a change in the buffering capacity of the medium, impairing supply of CO32− from HCO3−.

Effect of aragonite saturation state on settlement and post-settlement growth of Porites astreoides larvae

In response to the increases in pCO2 projected in the 21st century, adult coral growth and calcification are expected to decrease significantly. However, no published studies have investigated the effect of elevated pCO2 on earlier life history stages of corals. Porites astreoides larvae were collected from reefs in Key Largo, Florida, USA, settled and reared in controlled saturation state seawater. Three saturation states were obtained, using 1 M HCl additions, corresponding to present (380 ppm) and projected pCO2 scenarios for the years 2065 (560 ppm) and 2100 (720 ppm). The effect of saturation state on settlement and post-settlement growth was evaluated. Saturation state had no significant effect on percent settlement; however, skeletal extension rate was positively correlated with saturation state, with ~50% and 78% reductions in growth at the mid and high pCO2 treatments compared to controls, respectively.

Ocean acidification and calcifying reef organisms: a mesocosm investigation

A long-term (10 months) controlled experiment was conducted to test the impact of increased partial pressure of carbon dioxide (pCO2) on common calcifying coral reef organisms. The experiment was conducted in replicate continuous flow coral reef mesocosms flushed with unfiltered sea water from Kaneohe Bay, Oahu, Hawaii. Mesocosms were located in full sunlight and experienced diurnal and seasonal fluctuations in temperature and sea water chemistry characteristic of the adjacent reef flat. Treatment mesocosms were manipulated to simulate an increase in pCO2 to levels expected in this century [midday pCO2 levels exceeding control mesocosms by 365 ± 130 μatm (mean ± sd)]. Acidification had a profound impact on the development and growth of crustose coralline algae (CCA) populations. During the experiment, CCA developed 25% cover in the control mesocosms and only 4% in the acidified mesocosms, representing an 86% relative reduction. Free-living associations of CCA known as rhodoliths living in the control mesocosms grew at a rate of 0.6 g buoyant weight year−1 while those in the acidified experimental treatment decreased in weight at a rate of 0.9 g buoyant weight year−1, representing a 250% difference. CCA play an important role in the growth and stabilization of carbonate reefs, so future changes of this magnitude could greatly impact coral reefs throughout the world. Coral calcification decreased between 15% and 20% under acidified conditions. Linear extension decreased by 14% under acidified conditions in one experiment. Larvae of the coral Pocillopora damicornis were able to recruit under the acidified conditions. In addition, there was no significant difference in production of gametes by the coral Montipora capitata after 6 months of exposure to the treatments.

Mass extinctions and ocean acidification: biological constraints on geological dilemmas

The five mass extinction events that the earth has so far experienced have impacted coral reefs as much or more than any other major ecosystem. Each has left the Earth without living reefs for at least four million years, intervals so great that they are commonly referred to as ‘reef gaps’ (geological intervals where there are no remnants of what might have been living reefs). The causes attributed to each mass extinction are reviewed and summarised. When these causes and the reef gaps that follow them are examined in the light of the biology of extant corals and their Pleistocene history, most can be discarded. Causes are divided into (1) those which are independent of the carbon cycle: direct physical destruction from bolides, ‘nuclear winters’ induced by dust clouds, sea-level changes, loss of area during sea-level regressions, loss of biodiversity, low and high temperatures, salinity, diseases and toxins and extraterrestrial events and (2) those linked to the carbon cycle: acid rain, hydrogen sulphide, oxygen and anoxia, methane, carbon dioxide, changes in ocean chemistry and pH. By process of elimination, primary causes of mass extinctions are linked in various ways to the carbon cycle in general and ocean chemistry in particular with clear association with atmospheric carbon dioxide levels. The prospect of ocean acidification is potentially the most serious of all predicted outcomes of anthropogenic carbon dioxide increase. This study concludes that acidification has the potential to trigger a sixth mass extinction event and to do so independently of anthropogenic extinctions that are currently taking place.

Theme section on ‘‘Ocean Acidification and Coral Reefs’’

Much can change in 4 years. Ocean acidification, defined as the reduction of the pH of the world’s oceans, was ranked 36th of 40 potential threats to the future of coral reefs by reef scientists in 2004 (Kleypas and Eakin 2007). Since then, a substantial body of evidence has emerged showing that ocean acidification may have more profound implications for the future of coral reefs than previously thought (e.g., Orr et al. 2005; Raven et al. 2005; Hoegh-Guldberg et al. 2007), and some researchers now believe that the implications of ocean acidification for many marine species may ultimately be even greater than that of warming temperatures (Harley et al. 2006; Veron 2008a).

Ocean Acidification: Impacts From the Coast to Open Ocean Based Upon Laboratory Studies, Proxy Data and Instrumental Records (Session at the AGU Fall Meeting)

A session on “Ocean Acidification: Impacts From the Coast to Open Ocean Based Upon Laboratory Studies, Proxy Data and Instrumental Records” will be organized at the AGU Fall Meeting, 15-19 Dec in San Francisco. For further details, please contact the session organizers.