The growing global demand for raw materials and energy has led to a multitude of pressing environmental issues that impact land and marine ecosystems. The use of fossil fuels (such as oil, coal and natural gas) to generate electricity and as fuel for transportation, and deforestation has led to the ever-increasing release into the atmosphere of greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄). CO₂ produced from human activities is known as anthropogenic CO₂ which is derived from the Greek "anthropos" (human being), and "genos" (produced from).

As a result of the increase in greenhouse gases, and strong evidence of human-related warming over the last century, there is profound concern about future rapid climate change. BIOS scientists are actively involved in research aimed at improving our understanding of how marine ecosystems will respond to these changes.

A lesser-known issue, but of equal concern is the impact of human activities on the chemistry of seawater through a process termed ocean acidification. Ocean acidification results primarily from the release of anthropogenic CO₂ to the atmosphere, but acid rain, another product of human activities can also contribute. Professor Fred Mackenzie, former deputy Director at BIOS in the 1960's and long-term contributor to research and education at BIOS, describes the process of ocean acidification and details the history of study in his Meridian article. In simple terms, the ocean absorbs nearly all of the anthropogenic CO₂ released to the atmosphere. As a consequence, surface seawater CO₂ concentrations increase while the pH of seawater decreases. The decrease in pH means that seawater gradually becomes more acidic. Off the reefs of Bermuda in the North Atlantic Ocean, the Bermuda Atlantic Time-series (BATS) and Hydrostation 'S' (HYDRO) ocean time-series observations have been taken continuously for 54 years. Since 1983, we've also observed changes in ocean pH conditions, the longest continuous study of anywhere in the world. Conducted by BIOS scientists including Drs. Tony Knap, Rod Johnson, Mike Lomas, and myself, this BATS program not only enhances our understanding of ocean processes and climate, but documents the reality of ocean acidification.

Acid rain can also contribute to ocean acidification, particularly in coastal marine environments. Long-term records of rainfall, acidic rain and air quality on the island of Bermuda initiated by former President of BIOS (Prof. James Galloway, VIMS) and continued by BIOS scientist Dr. Andrew Peters, document the local impact of atmospheric acidity delivered to the ocean.

As a consequence of the uptake of anthropogenic CO₂, surface ocean pH is predicted to decrease by 0.3-0.5 units over the next century and beyond. BIOS Postdoctoral Fellow, Dr. Natalie Goodkin, describes the global impact of ocean acidification and the research efforts aimed at understanding the future consequences of this phenomenon.

Ocean acidification is predicted to have far reaching consequences for marine calcifying organisms and coral reef ecosystems from both an ecological and geochemical aspect. Unless global anthropogenic CO₂ emissions are significantly reduced, rising seawater CO2 and ocean acidification will result in a decrease in the surface seawater carbonate ion concentration [CO₃^2- ] and the saturation state (Ω) with respect to carbonate minerals such as, calcite (CaCO₃), Mg-calcite (Ca_1-xMg_xCO₃) and aragonite (CaCO₃). The carbonate mineral saturation state is a direct function of [CO₃^2- ] and is calculated from the ion concentration product divided by the stoichiometric solubility product (e.g., aragonite saturation state, Ω_arag = [Ca²⁺] [CO₃^2- ]/Ksp*, arag). A large number of marine organisms in both high and low latitude oceans such as corals, coralline algae, pteropods, coccolithophorids, mollusks, echinoderms, and foraminifera produce tests, shells or skeletons made of carbonate minerals. BIOS Postdoctoral Fellow, Dr. Andreas Andersson, describes the impact of ocean acidification on organisms that produce shells or skeletons made of carbonate minerals. Experimental studies have shown that the ability and the rate at which these organisms calcify decrease as a result of ocean acidification and decreasing seawater [CO₃^2-]. Recent evidence also indicates that reproduction and the recruitment success of marine calcifiers such as coral reefs are negatively affected by ocean acidification. Furthermore, it is likely that dissolution of carbonate sediments and structures will increase as the seawater saturation state with respect to these minerals decrease.

For coral reefs, the impact of ocean acidification is thought to be highly negative, with predictions of the loss of most coral reefs by the next century. In the following Meridian articles, BIOS Scientist, Dr. Ross Jones, describes the impact of ocean acidification on coral reefs and the contribution of the Marine Environmental Program (MEP) to this understanding, while BIOS Scientist, Dr. Samantha DuPutron, discusses the impact of ocean acidification on the recruitment success of coral species in Bermuda. The carbonate mineral framework of coral reefs is the unique result of a complex symbiosis of coral animal and photosynthetic marine plant termed zooxanthellae. BIOS Postdoctoral Fellow, Dr. Alex Venn, describes how ocean acidification potentially disrupts the biochemistry of coral reefs, and how molecular biology tools are useful for investigating this process.

This special issue of Meridian highlights the research activities focused on aspects of ocean acidification conducted by BIOS scientists. Nearly all of our scientists contribute directly or indirectly to ocean acidification research through studies of ocean and atmospheric sciences, marine biology and marine molecular biology. Dr. Peter Sedwick has concluded that ocean acidification has the potential for changing the cycling of iron in seawater, iron being a critically important nutritive element of most marine phytoplankton. Dr. Maureen Conte's record of the transport of carbonate minerals produced by marine plants (from the surface of the ocean to the deep at the Ocean Flux Program (OFP) site off Bermuda) has great value for documenting change in marine plant ecosystems that are responding to climate change and ocean acidification.

The response of marine calcifying organisms and ecosystems to ocean acidification is a problem that requires immediate attention. At BIOS, we have a dedicated group of scientists studying this problem and the ability to access the coral reefs of Bermuda and the offshore environment of the North Atlantic Ocean with the R/V Atlantic Explorer. Our efforts are bearing fruit as the following articles in this issue of the Meridian make clear. We hope that you, the reader, will benefit from the efforts of our research, and will help us to raise the level of awareness to the additional impacts of ocean acidification on marine ecosystems from anthropogenic CO2 emissions. We welcome your support.